Saturday, September 30, 2017

Decoupling of Magnetic Fields in Collapsing Protostellar Envelopes and Disk Formation and Fragmentation

Decoupling of Magnetic Fields in Collapsing Protostellar Envelopes and Disk Formation and Fragmentation

Authors:


Zhao et al

Abstract:
Efficient magnetic braking is a formidable obstacle to the formation of rotationally supported disks (RSDs) around protostars in magnetized dense cores. We have previously shown, through 2D (axisymmetric) non-ideal MHD simulations, that removing very small grains (VSGs: ~10 \AA to few 100 \AA) can greatly enhance ambipolar diffusion and enable the formation of RSDs. Here we extend the simulations of disk formation enabled by VSG removal to 3D. We find that the key to this scenario of disk formation is that the infall speed of ions (and thus the magnetic field lines tied to them) almost vanishes in the 102-103 AU-scale "pseudo-disk" where the field lines are most severely pinched and most of protostellar envelope mass infall occurs. As a result, the bulk neutral envelope matter can collapse without dragging much magnetic flux into the disk-forming region, which lowers the magnetic braking efficiency. We find that the initial disks enabled by VSG removal tend to be Toomre-unstable, which leads to the formation of prominent spiral structures that function as centrifugal barriers. The piling-up of infall material near the centrifugal barrier often produces dense fragments of tens of Jupiter masses, especially in cores that are not too strongly magnetized. Some fragments accrete onto the central stellar object, producing bursts in mass accretion rate. Others are longer lived, although whether they can survive long-term to produce multiple systems remains to be ascertained. Our results highlight the importance of dust grain evolution in determining the formation and properties of protostellar disks and potentially multiple systems.

Improved Constraints on the Disk Around MWC 349A from the 23-Meter LBTI

Improved Constraints on the Disk Around MWC 349A from the 23-Meter LBTI 
Authors:

Sallum et al

Abstract:
We present new spatially resolved observations of MWC 349A from the Large Binocular Telescope Interferometer (LBTI), a 23-meter baseline interferometer made up of two, co-mounted 8-meter telescopes. MWC 349A is a B[e] star with an unknown evolutionary state. Proposed scenarios range from a young stellar object (YSO), to a B[e] supergiant, to a tight binary system. Radio continuum and recombination line observations of this source revealed a sub-arcsecond bipolar outflow surrounding a ~100 mas circumstellar disk. Followup infrared studies detected the disk, and suggested that it may have skew and an inner clearing. Our new infrared interferometric observations, which have more than twice the resolution of previously-published datasets, support the presence of both skew and a compact infrared excess. They rule out inner clearings with radii greater than ~14 mas. We show the improvements in disk parameter constraints provided by LBTI, and discuss the inferred disk parameters in the context of the posited evolutionary states for MWC 349A.

An Incipient Debris Disk in the Chamaeleon I Cloud

An Incipient Debris Disk in the Chamaeleon I Cloud

Authors:


Espaillat et al

Abstract:
The point at which a protoplanetary disk becomes a debris disk is difficult to identify. To better understand this, here we study the ∼40~AU separation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type for T~54~A (which dominates the emission of the system) and an age of ∼2~Myr. However, the dust disk properties of T~54 are consistent with those of debris disks seen around older and earlier-type stars. At the same time, T~54 has evidence of gas remaining in the disk as indicated by [Ne II], [Ne III], and [O I] line detections. We model the spectral energy distribution of T~54 and estimate that ∼3×10−3 Earth-masses of small dust grains (<0 .25="" along="" an="" are="" at="" circumbinary="" disk="" earth-masses="" in="" larger="" least="" m="" of="" optically="" present="" thin="" with="">10~μm) grains within a circumprimary disk. Assuming a solar-like mixture, we use Ne line luminosities to place a minimum limit on the gas mass of the disk (∼3×10−4 Earth-masses) and derive a gas-to-dust mass ratio of ∼0.1. We do not detect substantial accretion, but we do see Hα in emission in one epoch, suggestive that there may be intermittent dumping of small amounts of matter onto the star. Considering the low dust mass, the presence of gas, and young age of T~54, we conclude that this system is on the bridge between the protoplanetary and debris disk stages.

Friday, September 29, 2017

Auroral Radio Emission from Ultracool Dwarfs: a Jovian Model

Auroral Radio Emission from Ultracool Dwarfs: a Jovian Model

Authors:


Turnpenney et al

Abstract:
A number of fast-rotating ultra cool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron cyclotron maser instability, a phenomenon that occurs in the solar system at planets with strong auroral emission. In this paper we examine magnetosphere-ionosphere coupling currents in UCDs, adopting processes used in models of Jovian emission. We consider the angular velocity gradient arising from a steady outward flux of angular momentum from an internal plasma source, as analogous to the jovian main oval current system, as well as the interaction of a rotating magnetosphere with the external medium. Both of these mechanisms are seen in the solar system to be responsible for the production of radio emission. We present the results of an investigation over a range of relevant plasma and magnetosphere-ionosphere coupling parameters to determine regimes consistent with observed UCD radio luminosities. Both processes are able to explain observed UCD luminosities with ionospheric Pedersen conductances of ∼1 − 2 mho, either for a closed magnetosphere with a plasma mass outflow rate of ∼105 kg s−1, i.e. a factor of ∼100 larger than that observed at Jupiter’s moon Io, or for a dwarf with an open magnetosphere moving through the interstellar medium at ∼50 km s−1 and a plasma mass outflow rate of ∼1000 kg s−1. The radio luminosity resulting from these mechanisms have opposing dependencies on the magnetic field strength, a point which may be used to discriminate between the two models as more data become available.

Ultracool dwarf benchmarks with Gaia primaries

Ultracool dwarf benchmarks with Gaia primaries

Authors:


Marocco et al

Abstract:
We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and present the results of an initial search for metal-rich/metal-poor systems. A simulated population of resolved ultracool dwarf companions to Gaia primary stars is generated and assessed. Of order ∼24,000 companions should be identifiable outside of the Galactic plane (|b| > 10 deg) with large-scale ground- and space-based surveys including late M, L, T, and Y types. Our simulated companion parameter space covers 0.02 ≤ M/M⊙ ≤ 0.1, 0.1 ≤ age/Gyr ≤ 14, and −2.5 ≤ [Fe/H] ≤ 0.5, with systems required to have a false alarm probability less than 10−4, based on projected separation and expected constraints on common-distance, common-proper motion, and/or common-radial velocity. Within this bulk population we identify smaller target subsets of rarer systems whose collective properties still span the full parameter space of the population, as well as systems containing primary stars that are good age calibrators. Our simulation analysis leads to a series of recommendations for candidate selection and observational follow-up that could identify ∼500 diverse Gaia benchmarks. As a test of the veracity of our methodology and simulations, our initial search uses UKIDSS and SDSS to select secondaries, with the parameters of primaries taken from Tycho-2, RAVE, LAMOST and TGAS. We identify and follow-up 13 new benchmarks. These include M8-L2 companions, with metallicity constraints ranging in quality, but robust in the range −0.39 ≤ [Fe/H] ≤ +0.36, and with projected physical separation in the range 0.6 less than s/kau less than 76. Going forward, Gaia offers a very high yield of benchmark systems, from which diverse sub-samples may be able to calibrate a range of foundational ultracool/sub-stellar theory and observation.

A new statistical method for characterizing the atmospheres applied to brown dwarf GJ 758B

A new statistical method for characterizing the atmospheres of extrasolar planets

Authors:


Henderson et al

Abstract:

By detecting light from extrasolar planets,we can measure their compositions and bulk physical properties. The technologies used to make these measurements are still in their infancy, and a lack of self-consistency suggests that previous observations have underestimated their systemic errors.We demonstrate a statistical method, newly applied to exoplanet characterization, which uses a Bayesian formalism to account for underestimated errorbars. We use this method to compare photometry of a substellar companion, GJ 758b, with custom atmospheric models. Our method produces a probability distribution of atmospheric model parameters including temperature, gravity, cloud model (fsed), and chemical abundance for GJ 758b. This distribution is less sensitive to highly variant data, and appropriately reflects a greater uncertainty on parameter fits.

Thursday, September 28, 2017

EPIC 228735255b: an eccentric 6.57 day transiting hot Jupiter in Virgo

EPIC 228735255b - An eccentric 6.57 day transiting hot Jupiter in Virgo

Authors:


Giles et al

Abstract:

We present the discovery of EPIC 228735255b, a P= 6.57 days Jupiter-mass (MP=1.019±0.070 MJup) planet transiting a V=12.5 (G5-spectral type) star in an eccentric orbit (e=0.120+0.056−0.046) detected using a combination of K2 photometry and ground-based observations. With a radius of 1.095±0.018RJup the planet has a bulk density of 0.726±0.062ρJup. The host star has a [Fe/H] of 0.12±0.045, and from the K2 light curve we find a rotation period for the star of 16.3±0.1 days. This discovery is the 9th hot Jupiter from K2 and highlights K2's ability to detect transiting giant planets at periods slightly longer than traditional, ground-based surveys. This planet is slightly inflated, but much less than others with similar incident fluxes. These are of interest for investigating the inflation mechanism of hot Jupiters.

Ground-based photometry of the 21-day Neptune HD 106315c

Ground-based photometry of the 21-day Neptune HD106315c

Authors:

Lendl et al

Abstract:
Space-based transit surveys such as K2 and TESS allow the detection of small transiting planets with orbital periods beyond 10 days. Few of these warm Neptunes are currently known around stars bright enough to allow for detailed follow-up observations dedicated to their atmospheric characterization. The 21-day period and 3.95 R⊕ planet HD106315c has been discovered based on the observation of two of its transits by K2. We have observed HD106315 using the 1.2m Euler telescope equipped with the EulerCam camera on two instances to confirm the transit using broad band photometry and refine the planetary period. Based on two observed transits of HD106315c, we detect its ∼1 mmag transit and obtain a precise measurement of the planetary ephemerids, which are critical for planning further follow-up observations. We have used the attained precision together with the predicted yield from the TESS mission to evaluate the potential for ground-based confirmation of Neptune-sized planets found by TESS. We find that 1-meter-class telescopes on the ground equipped with precise photometers could substantially contribute to the follow-up of 162 TESS candidates orbiting stars with magnitudes of V≤14. Out of these, 74 planets orbit stars with V≤12 and 12 planets orbit V≤10, which makes these candidates high-priority objects for atmospheric characterization with high-end instrumentation.

HATS-36b and 24 other transiting/eclipsing systems from the HATSouth - K2 Campaign 7 program

HATS-36b and 24 other transiting/eclipsing systems from the HATSouth - K2 Campaign 7 program

Authors:


Bayliss et al

Abstract:
We report on the result of a campaign to monitor 25 HATSouth candidates using the K2 space telescope during Campaign 7 of the K2 mission. We discover HATS-36b (EPIC 215969174b), a hot Jupiter with a mass of 2.79±0.40 MJ and a radius of 1.263±0.045 RJ which transits a solar-type G0V star (V=14.386) in a 4.1752d period. We also refine the properties of three previously discovered HATSouth transiting planets (HATS-9b, HATS-11b, and HATS-12b) and search the K2 data for TTVs and additional transiting planets in these systems. In addition we also report on a further three systems that remain as Jupiter-radius transiting exoplanet candidates. These candidates do not have determined masses, however pass all of our other vetting observations. Finally we report on the 18 candidates which we are now able to classify as eclipsing binary or blended eclipsing binary systems based on a combination of the HATSouth data, the K2 data, and follow-up ground-based photometry and spectroscopy. These range in periods from 0.7 days to 16.7 days, and down to 1.5 mmag in eclipse depths. Our results show the power of combining ground-based imaging and spectroscopy with higher precision space-based photometry, and serve as an illustration as to what will be possible when combining ground-based observations with TESS data.

Wednesday, September 27, 2017

TW Hydrae is Forming a SubSaturn Gas Giant

Deep imaging search for planets forming in the TW Hya protoplanetary disk with the Keck/NIRC2 vortex coronagraph


Authors:


Ruane et al

Abstract:
Distinct gap features in the nearest protoplanetary disk, TW Hya (distance of 59.5±0.9 pc), may be signposts of ongoing planet formation. We performed long-exposure thermal infrared coronagraphic imaging observations to search for accreting planets especially within dust gaps previously detected in scattered light and submm-wave thermal emission. Three nights of observations with the Keck/NIRC2 vortex coronagraph in L′ (3.4-4.1μm) did not reveal any statistically significant point sources. We thereby set strict upper limits on the masses of non-accreting planets. In the four most prominent disk gaps at 24, 41, 47, and 88 au, we obtain upper mass limits of 1.6-2.3, 1.1-1.6, 1.1-1.5, and 1.0-1.2 Jupiter masses (MJ) assuming an age range of 7-10 Myr for TW Hya. These limits correspond to the contrast at 95\% completeness (true positive fraction of 0.95) with a 1\% chance of a false positive within 1′′ of the star. We also approximate an upper limit on the product of planet mass and planetary accretion rate of MpM˙≲10−8M2J/yr implying that any putative ∼0.1MJ planet, which could be responsible for opening the 24 au gap, is presently accreting at rates insufficient to build up a Jupiter mass within TW Hya's pre-main sequence lifetime.

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

Authors:


Moses et al

Abstract:
Ablation of interplanetary dust supplies oxygen to the upper atmospheres of Jupiter, Saturn, Uranus, and Neptune. Using recent dynamical model predictions for the dust influx rates to the giant planets (Poppe et al., 2016), we calculate the ablation profiles and investigate the subsequent coupled oxygen–hydrocarbon neutral photochemistry in the stratospheres of these planets. We find that dust grains from the Edgeworth–Kuiper Belt, Jupiter-family comets, and Oort-cloud comets supply an effective oxygen influx rate of 1.0View the MathML source O atoms cm−2 s−1 to Jupiter, 7.4View the MathML source cm−2 s−1 to Saturn, 8.9View the MathML source cm−2 s−1 to Uranus, and 7.5View the MathML source cm−2 s−1 to Neptune. The fate of the ablated oxygen depends in part on the molecular/atomic form of the initially delivered products, and on the altitude at which it was deposited. The dominant stratospheric products are CO, H2O, and CO2, which are relatively stable photochemically. Model-data comparisons suggest that interplanetary dust grains deliver an important component of the external oxygen to Jupiter and Uranus but fall far short of the amount needed to explain the CO abundance currently seen in the middle stratospheres of Saturn and Neptune. Our results are consistent with the theory that all of the giant planets have experienced large cometary impacts within the last few hundred years. Our results also suggest that the low background H2O abundance in Jupiter’s stratosphere is indicative of effective conversion of meteoric oxygen to CO during or immediately after the ablation process — photochemistry alone cannot efficiently convert the H2O into CO on the giant planets.

Kepler-448c & Kepler-693c: Two new Warm Jupiters in Eccentric Orbits

Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the Formation of Warm Jupiters

Author:


Masuda

Abstract:
I report the discovery of non-transiting close companions to two transiting warm Jupiters (WJs), Kepler-448/KOI-12b (orbital period P=17.9days, radius Rp=1.23+0.06−0.05RJup) and Kepler-693/KOI-824b (P=15.4days, Rp=0.91±0.05RJup), via dynamical modeling of their transit timing and duration variations (TTVs and TDVs). The companions have masses of 22+7−5MJup (Kepler-448c) and 150+60−40MJup (Kepler-693c), and both are on eccentric orbits (e=0.65+0.13−0.09 for Kepler-448c and e=0.47+0.11−0.06 for Kepler-693c) with periastron distances of 1.5au. Moderate eccentricities are detected for the inner orbits as well (e=0.34+0.08−0.07 for Kepler-448b and e=0.2+0.2−0.1 for Kepler-693b). In the Kepler-693 system, a large mutual inclination between the inner and outer orbits (53+7−9deg or 134+11−10deg) is also revealed by the TDVs. This is likely to induce a secular oscillation of the inner WJ's eccentricity that brings its periastron close enough to the host star for tidal star-planet interactions to be significant. In the Kepler-448 system, the mutual inclination is weakly constrained and such an eccentricity oscillation is possible for a fraction of the solutions. Thus these WJs may be undergoing tidal migration to become hot Jupiters (HJs), although the migration via this process from beyond the snow line is disfavored by the close-in and massive nature of the companions. This may indicate that WJs can be formed in situ and could even evolve into HJs via high-eccentricity migration inside the snow line.

Tuesday, September 26, 2017

Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b

Cloud formation in metal-rich atmospheres of hot super-Earths like 55 Cnc e and CoRot7b

Authors:


Mahapatra et al

Abstract:

Clouds form in the atmospheres of planets where they can determine the observable spectra, the albedo and phase curves. Cloud properties are determined by the local thermodynamical and chemical conditions of an atmospheric gas. A retrieval of gas abundances requires a comprehension of the cloud formation mechanisms under varying chemical conditions. With the aim of studying cloud formation in metal rich atmospheres, we explore the possibility of clouds in evaporating exoplanets like CoRoT-7b and 55 Cnc e in comparison to a generic set of solar abundances and the metal-rich gas giant HD149026b. We assess the impact of metal-rich, non-solar element abundances on the gas-phase chemistry, and apply our kinetic, non-equilibrium cloud formation model to study cloud structures and their details. We provide an overview of global cloud properties in terms of material compositions, maximum particle formation rates, and average cloud particle sizes for various sets of rocky element abundances. Our results suggest that the conditions on 55 Cnc e and HD149026b should allow the formation of mineral clouds in their atmosphere. The high temperatures on some hot-rocky super-Earths (e.g. the day-side of Corot-7b) result in an ionised atmospheric gas and they prevent gas condensation, making cloud formation unlikely on its day-side.

The Thermal Phase Curve Offset on Tidally and Nontidally Locked Exoplanets

The Thermal Phase Curve Offset on Tidally and Nontidally Locked Exoplanets: A Shallow Water Model

Authors:


Penn et al

Abstract:
Using a shallow water model with time-dependent forcing, we show that the peak of an exoplanet thermal phase curve is, in general, offset from the secondary eclipse when the planet is rotating. That is, the planetary hot spot is offset from the point of maximal heating (the substellar point) and may lead or lag the forcing; the extent and sign of the offset are functions of both the rotation rate and orbital period of the planet. We also find that the system reaches a steady state in the reference frame of the moving forcing. The model is an extension of the well-studied Matsuno–Gill model into a full spherical geometry and with a planetary-scale translating forcing representing the insolation received on an exoplanet from a host star. The speed of the gravity waves in the model is shown to be a key metric in evaluating the phase curve offset. If the velocity of the substellar point (relative to the planet's surface) exceeds that of the gravity waves, then the hot spot will lag the substellar point, as might be expected by consideration of forced gravity wave dynamics. However, when the substellar point is moving slower than the internal wave speed of the system, the hottest point may lead the passage of the forcing. We provide an interpretation of this result by consideration of the Rossby and Kelvin wave dynamics, as well as, in the very slowly rotating case, a one-dimensional model that yields an analytic solution. Finally, we consider the inverse problem of constraining planetary rotation rate from an observed phase curve.

Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short-period and Ultra-short-period Planets

Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short-period and Ultra-short-period Planets


Authors:


Lee et al

Abstract:
Sub-Neptunes around FGKM dwarfs are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. Both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be attributed to the truncation of protoplanetary disks by their host star magnetospheres at corotation. We demonstrate this by deriving planet occurrence rate profiles from empirical distributions of pre-main-sequence stellar rotation periods. Observed profiles are better reproduced when planets are distributed randomly in disks—as might be expected if planets formed in situ—rather than piled up near disk edges, as would be the case if they migrated in by disk torques. Planets can be brought from disk edges to ultra-short (less than 1 day) periods by asynchronous equilibrium tides raised on their stars. Tidal migration can account for how ultra-short-period planets are more widely spaced than their longer-period counterparts. Our picture provides a starting point for understanding why the sub-Neptune population drops at ~10 days regardless of whether the host star is of type FGK or early M. We predict planet occurrence rates around A stars to also break at short periods, but at ~1 day instead of ~10 days because A stars rotate faster than stars with lower masses (this prediction presumes that the planetesimal building blocks of planets can drift inside the dust sublimation radius).

Monday, September 25, 2017

Atmospheric Stripping Might Limit Habitability of Exoplanets

New models of massive stellar eruptions hint at an extra layer of complexity when considering whether an exoplanet may be habitable or not. Models developed for our own Sun have now been applied to cool stars favoured by exoplanet hunters, in research presented by Dr Christina Kay, of the NASA Goddard Flight Center, on Monday 3rd July at the National Astronomy Meeting at the University of Hull.

Link.

Enhanced interplanetary panspermia in the TRAPPIST-1 system

Enhanced interplanetary panspermia in the TRAPPIST-1 system

Authors:


Lingam et al

Abstract:
We present a simple model for estimating the probability of interplanetary panspermia in the recently discovered system of seven planets orbiting the ultracool dwarf star TRAPPIST-1 and find that panspermia is potentially orders of magnitude more likely to occur in the TRAPPIST-1 system compared with the Earth-to-Mars case. As a consequence, we argue that the probability of abiogenesis is enhanced on the TRAPPIST-1 planets compared with the solar system. By adopting models from theoretical ecology, we show that the number of species transferred and the number of life-bearing planets are also likely to be higher because of the increased rates of immigration. We propose observational metrics for evaluating whether life was initiated by panspermia on multiple planets in the TRAPPIST-1 system. These results are also applicable to habitable exoplanets and exomoons in other planetary systems.

The Potential Climates of the Trappist-1 System

Comparative Climates of TRAPPIST-1 planetary system: results from a simple climate-vegetation model

Authors:


Alberti et al

Abstract: 
The recent discovery of the planetary system hosted by the ultracool dwarf star TRAPPIST-1 could open new perspectives into the investigation of planetary climates of Earth-sized exoplanets, their atmospheres and their possible habitability. In this paper, we use a simple climate-vegetation energy-balance model to study the climate of the seven TRAPPIST-1 planets and the climate dependence on the global albedo, on the fraction of vegetation that could cover their surfaces and on the different greenhouse conditions. The model allows us to investigate whether liquid water could be maintained on the planetary surfaces (i.e., by defining a "surface water zone") in different planetary conditions, with or without the presence of greenhouse effect.

It is shown that planet TRAPPIST-1d seems to be the most stable from an Earth-like perspective, since it resides in the surface water zone for a wide range of reasonable values of the model parameters. Moreover, according to the model outer planets (f, g and h) cannot host liquid water on their surfaces, even for Earth-like conditions, entering a snowball state. Although very simple, the model allows to extract the main features of the TRAPPIST-1 planetary climates.

The Trappist-1 System may NOT be Habitable

The Threatening Environment of the TRAPPIST-1 Planets

Authors:


Garraffo

Abstract:
Recently, four additional Earth-mass planets were discovered orbiting the nearby ultracool M8 dwarf TRAPPIST-1, making a remarkable total of seven planets with equilibrium temperatures compatible with the presence of liquid water on their surface. Temperate terrestrial planets around an M-dwarf orbit close to their parent star, rendering their atmospheres vulnerable to erosion by the stellar wind and energetic electromagnetic and particle radiation. Here, we use state-of-the-art 3D magnetohydrodynamic models to simulate the wind around TRAPPIST-1 and study the conditions at each planetary orbit. All planets experience a stellar wind pressure between 103 and 105 times the solar wind pressure on Earth. All orbits pass through wind pressure changes of an order of magnitude and most planets spend a large fraction of their orbital period in the sub-Alfv\'enic regime. For plausible planetary magnetic field strengths, all magnetospheres are greatly compressed and undergo much more dynamic change than that of the Earth. The planetary magnetic fields connect with the stellar radial field over much of the planetary surface, allowing direct flow of stellar wind particles onto the planetary atmosphere. These conditions could result in strong atmospheric stripping and evaporation and should be taken into account for any realistic assessment of the evolution and habitability of the TRAPPIST-1 planets.

Sunday, September 24, 2017

Eccentricity excitation and merging of planetary embryos heated by pebble accretion

Eccentricity excitation and merging of planetary embryos heated by pebble accretion

Authors:


Chrenko et al

Abstract:
Context:

Planetary embryos can continue to grow by pebble accretion until they become giant planet cores. Simultaneously, these embryos mutually interact and also migrate due to torques arising from the protoplanetary disk.

Aims:

Our aim is to investigate how pebble accretion alters the orbital evolution of embryos undergoing the Type-I migration. In particular, we study whether they establish resonant chains, whether these chains are prone to instabilities and if giant planet cores form through embryo merging, thus occurring more rapidly than by pebble accretion alone.

Methods:

For the first time, we perform self-consistent global-scale radiative hydrodynamic simulations of a two-fluid protoplanetary disk consisting of gas and pebbles, the latter being accreted by embedded embryos. Accretion heating, along with other radiative processes, is accounted for to correctly model the Type-I migration.

Results:

We track the evolution of four super-Earth-like embryos, initially located in a region where the disk structure allows for a convergent migration. Generally, embryo merging is facilitated by rapidly increasing embryo masses and breaks the otherwise oligarchic growth. Moreover, we find that the orbital eccentricity of each embryo is considerably excited (≃0.03) due to the presence of an asymmetric underdense lobe of gas, a so-called `hot trail', produced by accretion heating of the embryo's vicinity. Eccentric orbits lead the embryos to frequent close encounters and make resonant locking more difficult.

Conclusions:

Embryo merging typically produces one massive core (≳10ME) in our simulations, orbiting near 10AU. Pebble accretion is naturally accompanied by occurrence of eccentric orbits which should be considered in future efforts to explain the structure of exoplanetary systems.

Survey of Cold Water Lines in Protoplanetary Disks: Indications of Systematic Volatile Depletion

Survey of Cold Water Lines in Protoplanetary Disks: Indications of Systematic Volatile Depletion

Authors:


Du et al

Abstract:
We performed very deep searches for 2 ground-state water transitions in 13 protoplanetary disks with the HIFI instrument on board the Herschel Space Observatory, with integration times up to 12 hr per line. We also searched for, with shallower integrations, two other water transitions that sample warmer gas. The detection rate is low, and the upper limits provided by the observations are generally much lower than predictions of thermo-chemical models with canonical inputs. One ground-state transition is newly detected in the stacked spectrum of AA Tau, DM Tau, LkCa 15, and MWC 480. We run a grid of models to show that the abundance of gas-phase oxygen needs to be reduced by a factor of at least $\sim 100$ to be consistent with the observational upper limits (and positive detections) if a dust-to-gas mass ratio of 0.01 were to be assumed. As a continuation of previous ideas, we propose that the underlying reason for the depletion of oxygen (hence the low detection rate) is the freeze-out of volatiles such as water and CO onto dust grains followed by grain growth and settling/migration, which permanently removes these gas-phase molecules from the emissive upper layers of the outer disk. Such depletion of volatiles is likely ubiquitous among different disks, though not necessarily to the same degree. The volatiles might be returned back to the gas phase in the inner disk ($\lesssim 15$ au), which is consistent with current constraints. Comparison with studies on disk dispersal due to photoevaporation indicates that the timescale for volatile depletion is shorter than that of photoevaporation.

Be discs in binary systems: II. Misaligned orbits

Be discs in binary systems: II. Misaligned orbits

Authors:


Cyr et al

Abstract:
We use a smoothed particle hydrodynamics (SPH) code to examine the effects of misaligned binary companions on Be star discs. We systematically vary the degree of misalignment between the disc and the binary orbit, as well as the disc viscosity and orbital period to study their effects on the density in the inner and outer parts of the disc. We find that varying the degree of misalignment, the viscosity, and the orbital period affects both the truncation radius and the density structure of the outer disc, while the inner disc remains mostly unaffected. We also investigate the tilting of the disc in the innermost part of the disc and find the tilt increases with radius until reaching a maximum around 5 stellar radii. The direction of the line of nodes, with respect to the equator of the central star, is found to be offset compared to the orbital line of nodes, and to vary periodically in time, with a period of half a orbital phase. We also compare the scale height of our discs with the analytical scale height of an isothermal disc, which increases with radius as r1.5. We find that this formula reproduces the scale height well for both aligned and misaligned systems but underestimates the scale height in regions of the disc where density enhancements develop.

Saturday, September 23, 2017

Anomalies in the Kepler Asteroseismic Legacy Project Data

Anomalies in the Kepler Asteroseismic Legacy Project Data. A re-analysis of 16 Cyg A&B, KIC8379927 and 6 solar-like stars


Author:


Roxburgh

Abstract:

I compare values of the frequencies, separation ratios, errors and covariance matrices from a new analysis of 9 solar-like stars with the Legacy project values reported by Lund et al and, for 16Cyg A&B and KIC8379927, with values derived by Davies et al. There is good agreement between my results (using Davies power spectra) and Davies's for these 3 stars, but no such agreement with the Legacy project results.

My frequencies differ from the Legacy values, there are inconsistencies in the Legacy frequency covariance matrices which are not positive definite, and the Legacy errors on separation ratios are up to 40 times larger than mine and the values and upper limits derived from the Legacy frequency covariances. There are similar anomalies for 6 other solar-like stars: frequencies and separation ratio errors disagree and 2 have non positive definite covariance matrices. There are inconsistencies in the covariance matrices of 27 the 66 stars in the full Legacy set and problems with the ratio errors for the vast majority of these stars.

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Authors:

Nakatani et al

Abstract:

Protoplanetary disks are thought to have lifetimes of 3−6 million years in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low metallicity environment. We perform a suite of radiation hydrodynamics simulations of photoevaporation of protoplanetary disks to study the disk structure and its long-term evolution of ∼10000 years, and the metallicity dependence of mass-loss rate. Our simulations follow hydrodynamics, extreme and far ultraviolet radiative transfer, and non-equilibrium chemistry in a self-consistent manner. Dust grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk gas metallicity over a wide range of 10−4Z⊙≤Z≤10 Z⊙. For our fiducial model with a 0.5 M⊙ central star with solar metallicity, the time-averaged photoevaporation rate is M˙ph=1.38×10−8M⊙yr−1. The photoevaporation rate is lower with higher metallicity in the range of 10−0.5Z⊙≲Z≲10 Z⊙, because dust shielding effectively prevents far-ultra violet (FUV) photons from penetrating into and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10−1Z⊙≲Z≲10−0.5Z⊙, because FUV photoelectric heating is not efficient any more to raise the gas temperature and to drive outflows. At 10−4Z⊙≤Z≲10−1Z⊙, HI photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with roughly a constant rate. The typical disk lifetime is shorter at Z=0.3 Z⊙ than at Z=Z⊙, being consistent with recent observations of the extreme outer galaxy.

Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

Authors:


Wang et al

Abstract:
Photoevaporation is an important dispersal mechanism for protoplanetary disks. We conduct hydrodynamic simulations coupled with ray-tracing radiative transfer and consistent thermochemistry to study photoevaporative winds driven by ultraviolet and X-ray radiation from the host star. Most models have a three-layer structure: a cold midplane, warm intermediate layer, and hot wind, the last having typical speeds ∼30 km s−1 and mass-loss rates ∼10−9 M⊙ yr−1 when driven primarily by ionizing UV radiation. Observable molecules including CO, OH and H2O re-form in the intermediate layer and survive at relatively high wind temperatures due to reactions being out of equilibrium. Mass-loss rates are sensitive to the intensity of radiation in energy bands that interact directly with hydrogen. Comparison with previous works shows that mass loss rates are also sensitive to the treatment of both the hydrodynamics and the thermochemistry. Divergent results concerning the efficiency of X-ray photoevaporation are traced in part to differing assumptions about dust and other coolants.

Friday, September 22, 2017

Optical and Near-infrared Spectra of σ Orionis Isolated Planetary-mass Objects

Optical and Near-infrared Spectra of σ Orionis Isolated Planetary-mass Objects

Authors:


Zapatero Osorio et al

Abstract:

We have obtained low-resolution optical (0.7–0.98 μm) and near-infrared (1.11–1.34 μm and 0.8–2.5 μm) spectra of 12 isolated planetary-mass candidates (J = 18.2–19.9 mag) of the 3 Myr σ Orionis star cluster with the aim of determining the spectroscopic properties of very young, substellar dwarfs and assembling a complete cluster mass function. We have classified our targets by visual comparison with high- and low-gravity standards and by measuring newly defined spectroscopic indices. We derived L0–L4.5 and M9–L2.5 using high- and low-gravity standards, respectively. Our targets reveal clear signposts of youth, thus corroborating their cluster membership and planetary masses (6–13 M Jup). These observations complete the σ Orionis mass function by spectroscopically confirming the planetary-mass domain to a confidence level of ~75%. The comparison of our spectra with BT-Settl solar metallicity model atmospheres yields a temperature scale of 2350–1800 K and a low surface gravity of log g ≈ 4.0 [cm s−2], as would be expected for young planetary-mass objects. We discuss the properties of the cluster's least-massive population as a function of spectral type. We have also obtained the first optical spectrum of S Ori 70, a T dwarf in the direction of σ Orionis. Our data provide reference optical and near-infrared spectra of very young L dwarfs and a mass function that may be used as templates for future studies of low-mass substellar objects and exoplanets. The extrapolation of the σ Orionis mass function to the solar neighborhood may indicate that isolated planetary-mass objects with temperatures of ~200–300 K and masses in the interval 6–13 M Jup may be as numerous as very low-mass stars.

Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

Post-main-sequence Evolution of Icy Minor Planets. II. Water Retention and White Dwarf Pollution around Massive Progenitor Stars

Authors:


Malamud et al

Abstract:
Most studies suggest that the pollution of white dwarf (WD) atmospheres arises from the accretion of minor planets, but the exact properties of polluting material, and in particular the evidence for water in some cases, are not yet understood. Here we study the water retention of small icy bodies in exo-solar planetary systems, as their respective host stars evolve through and off the main sequence and eventually become WDs. We explore, for the first time, a wide range of star masses and metallicities. We find that the mass of the WD progenitor star is of crucial importance for the retention of water, while its metallicity is relatively unimportant. We predict that minor planets around lower-mass WD progenitors would generally retain more water and would do so at closer distances from the WD than compared with high-mass progenitors. The dependence of water retention on progenitor mass and other parameters has direct implications for the origin of observed WD pollution, and we discuss how our results and predictions might be tested in the future as more observations of WDs with long cooling ages become available.

Is WISEP J060738.65+242953.4 Really A Magnetically-active, Pole-on L Dwarf?


Author:

Route

Abstract:

The interplay of rotation and manifested magnetic activity on ultracool dwarfs (UCDs) is of key importance for gathering clues as to the operation of the dynamos within these objects. A number of magnetized UCDs host kG-strength magnetic fields. It was recently reported that the L8 dwarf WISEP J060738.65+242953.4 is a radio-emitting UCD that is likely observed pole-on, due to its lack of photometric variability and narrow spectral lines. Follow-up radio observations at Arecibo Observatory, together with careful analysis of previously published details, however, suggest that the scientific and statistical significance of the radio and spectroscopic data have been overstated. If the UCD is observed along its aligned spin/magnetic axis, the absence of observed Hα activity may present challenges to the auroral model of UCD magnetism, although short or long-term cyclic magnetic activity may explain this behavior. Monte Carlo simulations presented here suggest that the source probably rotates with v sin i=6-12 km s−1, indicating that its inclination angle and rotational velocity are unexceptional and that its angular momentum has evolved as expected for brown dwarfs observed in ~1 Myr-old clusters. The discovery and verification of the most rapidly and slowest rotating brown dwarfs place valuable constraints on the angular momentum evolution and magnetic activity history of these objects.

Thursday, September 21, 2017

How Strong is hot Jupiter HD 209458b's Magnetic Field?

Effect of stellar wind-induced magnetic fields on planetary obstacles of non-magnetized hot Jupiters

Authors:


Erkaev et al

Abstract:


We investigate the interaction between the magnetized stellar wind plasma and the partially ionized hydrodynamic hydrogen outflow from the escaping upper atmosphere of non- or weakly magnetized hot Jupiters. We use the well-studied hot Jupiter HD 209458b as an example for similar exoplanets, assuming a negligible intrinsic magnetic moment. For this planet, the stellar wind plasma interaction forms an obstacle in the planet’s upper atmosphere, in which the position of the magnetopause is determined by the condition of pressure balance between the stellar wind and the expanded atmosphere, heated by the stellar extreme ultraviolet (EUV) radiation. We show that the neutral atmospheric atoms penetrate into the region dominated by the stellar wind, where they are ionized by photo-ionization and charge exchange, and then mixed with the stellar wind flow. Using a 3D magnetohydrodynamic (MHD) model, we show that an induced magnetic field forms in front of the planetary obstacle, which appears to be much stronger compared to those produced by the solar wind interaction with Venus and Mars. Depending on the stellar wind parameters, because of the induced magnetic field, the planetary obstacle can move up to ≈0.5–1 planetary radii closer to the planet. Finally, we discuss how estimations of the intrinsic magnetic moment of hot Jupiters can be inferred by coupling hydrodynamic upper planetary atmosphere and MHD stellar wind interaction models together with UV observations. In particular, we find that HD 209458b should likely have an intrinsic magnetic moment of 10–20% that of Jupiter.

GENESIS: New Self-Consistent Models of Exoplanetary Spectra

GENESIS: New Self-Consistent Models of Exoplanetary Spectra

Authors:


Gandhi et al

Abstract:
We are entering the era of high-precision and high-resolution spectroscopy of exoplanets. Such observations herald the need for robust self-consistent spectral models of exoplanetary atmospheres to investigate intricate atmospheric processes and to make observable predictions. Spectral models of plane-parallel exoplanetary atmospheres exist, mostly adapted from other astrophysical applications, with different levels of sophistication and accuracy. There is a growing need for a new generation of models custom-built for exoplanets and incorporating state-of-the-art numerical methods and opacities. The present work is a step in this direction. Here we introduce GENESIS, a plane-parallel, self-consistent, line-by-line exoplanetary atmospheric modelling code which includes (a) formal solution of radiative transfer using the Feautrier method, (b) radiative-convective equilibrium with temperature correction based on the Rybicki linearisation scheme, (c) latest absorption cross sections, and (d) internal flux and external irradiation, under the assumptions of hydrostatic equilibrium, local thermodynamic equilibrium and thermochemical equilibrium. We demonstrate the code here with cloud-free models of giant exoplanetary atmospheres over a range of equilibrium temperatures, metallicities, C/O ratios and spanning non-irradiated and irradiated planets, with and without thermal inversions. We provide the community with theoretical emergent spectra and pressure-temperature profiles over this range, along with those for several known hot Jupiters. The code can generate self-consistent spectra at high resolution and has the potential to be integrated into general circulation and non-equilibrium chemistry models as it is optimised for efficiency and convergence. GENESIS paves the way for high-fidelity remote sensing of exoplanetary atmospheres at high resolution with current and upcoming observations.

Transiting Exoplanet Monitoring Project (TEMP). II. Refined System Parameters and Transit Timing Analysis of HAT-P-33b

Transiting Exoplanet Monitoring Project (TEMP). II. Refined System Parameters and Transit Timing Analysis of HAT-P-33b

Authors:


Wang et al

Abstract:

We present ten R-band photometric observations of eight different transits of the hot Jupiter HAT-P-33b, which has been targeted by our Transiting Exoplanet Monitoring Project (TEMP). The data were obtained by two telescopes at the Xinglong Station of National Astronomical Observatories of China (NAOC) from 2013 December through 2016 January, and exhibit photometric scatter of 1.6−3.0mmag. After jointly analyzing the previously published photometric data, radial-velocity (RV) measurements, and our new light curves, we revisit the system parameters and orbital ephemeris for the HAT-P-33b system. Our results are consistent with the published values except for the planet-to-star radius ratio (RP/R∗), the ingress/egress duration (τ) and the total duration (T14), which together indicate a slightly shallower and shorter transit shape. Our results are based on more complete light curves, whereas the previously published work had only one complete transit light curve. No significant anomalies in Transit Timing Variations (TTVs) are found, and we place upper mass limits on potential perturbers, largely supplanting the loose constraints provided by the extant RV data. The TTV limits are stronger near mean-motion resonances, especially for the low-order commensurabilities. We can exclude the existence of a perturber with mass larger than 0.6, 0.3, 0.5, 0.5, and 0.3M⊕ near the 1:3, 1:2, 2:3, 3:2, and 2:1 resonances, respectively.

Wednesday, September 20, 2017

Stability of Multiplanetary Systems in Star Clusters

Stability of Multiplanetary Systems in Star Clusters

Authors:


Cai et al

Abstract:
Most stars form in star clusters and stellar associated. To understand the roles of star cluster environments in shaping the dynamical evolution of planetary systems, we carry out direct N-body simulations of four planetary systems models in three different star cluster environments with respectively N=2k, 8k and 32k stars. In each cluster, an ensemble of initially identical planetary systems are assigned to solar-type stars with ∼1M⊙ and evolved for 50~Myr. We found that following the depletion of protoplanetary disks, external perturbations and planet-planet interactions are two driving mechanisms responsible for the destabilization of planetary systems. The planet survival rate varies from ∼95% in the N=2k cluster to ∼60% in the N=32k cluster, which suggests that most planetary systems can indeed survive in low-mass clusters, except in the central regions. We also find that planet ejections through stellar encounters are cumulative processes, as only ∼3% of encounters are strong enough to excite the eccentricity by Δe≥0.5. Short-period planets can be perturbed through orbit crossings with long-period planets. When taking into account planet-planet interactions, the planet ejection rate nearly doubles, and therefore multiplicity contributes to the vulnerability of planetary systems. In each ensemble, ∼0.2% of planetary orbits become retrograde due to random directions of stellar encounters. Our results predict that young low-mass star clusters are promising sites for next-generation planet surveys, yet low planet detection rates are expected in dense globular clusters such as 47 Tuc. Nevertheless, planets in denser stellar environments are likely to have shorter orbital periods, which enhances their detectability.

Radiative Transfer for Exoplanet Atmospheres

Radiative Transfer for Exoplanet Atmospheres


Authors:

Heng et al

Abstract:

Remote sensing of the atmospheres of distant worlds motivates a firm understanding of radiative transfer. In this review, we provide a pedagogical cookbook that describes the principal ingredients needed to perform a radiative transfer calculation and predict the spectrum of an exoplanet atmosphere, including solving the radiative transfer equation, calculating opacities (and chemistry), iterating for radiative equilibrium (or not), and adapting the output of the calculations to the astronomical observations. A review of the state of the art is performed, focusing on selected milestone papers. Outstanding issues, including the need to understand aerosols or clouds and elucidating the assumptions and caveats behind inversion methods, are discussed. A checklist is provided to assist referees/reviewers in their scrutiny of works involving radiative transfer. A table summarizing the methodology employed by past studies is provided.

On the formation of multiple concentric rings and gaps in protoplanetary disks

On the formation of multiple concentric rings and gaps in protoplanetary disks

Authors:

Bae et al

Abstract:

As spiral waves driven by a planet in a gaseous disk steepen into a shock, they deposit angular momentum, opening a gap in the disk. This has been well studied using both linear theory and numerical simulations, but so far, only for the primary spiral arm -- the one directly attached to the planet. Using two-dimensional hydrodynamic simulations, we show that the secondary and tertiary arms driven by a planet can also open gaps as they steepen into shocks. The depths of the secondary/tertiary gaps in surface density grow with time in a low viscosity disk (α=5×10−5), so even low-mass planets (e.g., super-Earth or mini-Neptune) embedded in the disk can open multiple observable gaps, provided that sufficient time has passed. Applying our results to the HL Tau disk, we show that a single 30 Earth-mass planet embedded in the ring at 68.8 au (B5) can reasonably well reproduce the positions of the two major gaps at 13.2 and 32.3 au (D1 and D2), and roughly reproduce two other major gaps at 64.2 and 74.7 au (D5 and D6) seen in the mm continuum. The positions of secondary/tertiary gaps are found to be sensitive to the planetary mass and the disk temperature profile, so with accurate observational measurements on the temperature structure we can use the positions of multiple gaps to constrain the mass of the planet. We also comment on the gaps seen in the TW Hya and HD 163296 disk.

Tuesday, September 19, 2017

Detecting Exoplanets in S Type Systems Around Eclipsing Binary Stars

How eclipse time variations, eclipse duration variations and radial velocities can reveal S-type planets in close eclipsing binaries 

Authors:


Oshagh et al

Abstract:

While about a dozen transiting planets have been found in wide orbits around an inner, close stellar binary (so-called P-type planets), no planet has yet been detected orbiting only one star (a so-called S-type planet) in an eclipsing binary. This is despite a large number of eclipsing binary systems discovered with the Kepler telescope. Here we propose a new detection method for these S-type planets, which uses a correlation between the stellar radial velocities (RVs), eclipse timing variations (ETVs) and eclipse duration variations (EDVs). We test the capability of this technique by simulating a realistic benchmark system and demonstrate its detectability with existing high-accuracy RV and photometry instruments. We illustrate that with a small number of RV observations, the RV–ETV diagrams allows us to distinguish between prograde and retrograde planetary orbits and also the planetary mass can be estimated if the stellar cross-correlation functions can be disentangled. We also identify a new (though minimal) contribution of S-type planets to the Rossiter–McLaughlin effect in eclipsing stellar binaries. We finally explore possible detection of exomoons around transiting luminous giant planets and find that the precision required to detect moons in the RV curves of their host planets is of the order of cm s−1 and therefore not accessible with current instruments.

Dynamics and Collisional Evolution of Closely Packed Planetary Systems

Dynamics and Collisional Evolution of Closely Packed Planetary Systems


Authors:


Hwang et al

Abstract:
High-multiplicity Kepler systems (referred to as Kepler multis) are often tightly packed and may be on the verge of instability. Many systems of this type could have experienced past instabilities, where the compact orbits and often low densities make physical collisions likely outcomes. We use numerical simulations to study the dynamical instabilities and planet-planet interactions in a synthetically generated sample of closely-packed, high-multiplicity systems. We focus specifically on systems resembling Kepler-11, a Kepler multi with six planets, and run a suite of dynamical integrations, sampling the initial orbital parameters around the nominal values reported in Lissauer et al. (2011a), finding that most of the realizations are unstable, resulting in orbit crossings and, eventually, collisions and mergers. We study in detail the dependence of stability on the orbital parameters of the planets and planet-pair characteristics to identify possible precursors to instability, compare the systems that emerge from dynamical instabilities to the observed Kepler sample (after applying observational corrections), and propose possible observable signatures of these instabilities. We examine the characteristics of each planet-planet collision, categorizing collisions by the degree of contact and collision energy, and find that grazing collisions are more common than direct impacts. Since the structure of many planets found in Kepler multis is such that the mass is dominated by a rocky core, but the volume is dominated by a low-density gaseous envelope, the sticky-sphere approximation may not be valid, and we present hydrodynamic calculations of planet-planet collisions clearly deviating from this approximation. Finally, we rerun a subset of our dynamical calculations using instead a modified prescription to handle collisions, finding, in general, higher multiplicity remnant systems.

Exoplanet Predictions Based on Harmonic Orbit Resonances

Exoplanet Predictions Based on Harmonic Orbit Resonances

Authors:


Aschenwanden et al

Abstract:
The current exoplanet database includes 5454 confirmed planets and candidate planets observed with the KEPLER mission. We find 932 planet pairs from which we extract distance and orbital period ratios. While earlier studies used the Titius-Bode law or a generalized version with logarithmic spacing, which both lack a physical model, we employ here the theory of harmonic orbit resonances, which contains quantized ratios instead, to explain the observed planet distance ratios and to predict undetected exoplanets. We find that the most prevailing harmonic ratios are (2:1), (3:2), and (5:3), in 73\% of the cases, while alternative harmonic ratios of (5:4), (4:3), (5:2), (3:1) occur in 27\% of the other cases. Our orbital predictions includes 171 exoplanets, 2 Jupiter moons, one Saturn moon, 3 Uranus moons, and 4 Neptune moons. The accuracy of the predicted planet distances amounts to a few percent, which fits the data significantly better than the Titius-Bode law or a logarithmic spacing. This information may be useful for targeted exoplanet searches with Kepler data and to estimate the number of live-carrying planets in habitable zones.

Monday, September 18, 2017

Where's the Flux? The Official Page of the Investigations into Tabby's

There is an official Website for the investigations into Tabby's Star, the mysteriously fading star.

Stable habitable zones of single Jovian planet systems

Stable habitable zones of single Jovian planet systems

Authors:


Agnew et al

Abstract:

With continued improvement in telescope sensitivity and observational techniques, the search for rocky planets in stellar habitable zones is entering an exciting era. With so many exoplanetary systems available for follow-up observations to find potentially habitable planets, one needs to prioritise the ever-growing list of candidates. We aim to determine which of the known planetary systems are dynamically capable of hosting rocky planets in their habitable zones, with the goal of helping to focus future planet search programs. We perform an extensive suite of numerical simulations to identify regions in the habitable zones of single Jovian planet systems where Earth mass planets could maintain stable orbits, specifically focusing on the systems in the Catalog of Earth-like Exoplanet Survey Targets (CELESTA). We find that small, Earth-mass planets can maintain stable orbits in cases where the habitable zone is largely, or partially, unperturbed by a nearby Jovian, and that mutual gravitational interactions and resonant mechanisms are capable of producing stable orbits even in habitable zones that are significantly or completely disrupted by a Jovian. Our results yield a list of 13 single Jovian planet systems in CELESTA that are not only capable of supporting an Earth-mass planet on stable orbits in their habitable zone, but for which we are also able to constrain the orbits of the Earth-mass planet such that the induced radial velocity signals would be detectable with next generation instruments.

The O2 A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

The O2 A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets


Authors:


Fauchez et al

Abstract:

Earth-like, potentially habitable exoplanets are prime targets in the search for extraterrestrial life. Information about their atmospheres and surfaces can be derived by analyzing the light of the parent star reflected by the planet. We investigate the influence of the surface albedo A s, the optical thickness b cloud, the altitude of water clouds, and the mixing ratio of biosignature O2 on the strength of the O2 A-band (around 760 nm) in the flux and polarization spectra of starlight reflected by Earth-like exoplanets. Our computations for horizontally homogeneous planets show that small mixing ratios (η < 0.4) will yield moderately deep bands in flux and moderate-to-small band strengths in polarization, and that clouds will usually decrease the band depth in flux and the band strength in polarization. However, cloud influence will be strongly dependent on properties such as optical thickness, top altitude, particle phase, coverage fraction, and horizontal distribution. Depending on the surface albedo and cloud properties, different O2 mixing ratios η can give similar absorption-band depths in flux and band strengths in polarization, especially if the clouds have moderate-to-high optical thicknesses. Measuring both the flux and the polarization is essential to reduce the degeneracies, although it will not solve them, especially not for horizontally inhomogeneous planets. Observations at a wide range of phase angles and with a high temporal resolution could help to derive cloud properties and, once those are known, the mixing ratio of O2 or any other absorbing gas.

Geophysical tests for habitability in ice-covered ocean worlds

Geophysical tests for habitability in ice-covered ocean worlds

Authors:

Vance et al

Abstract:
Geophysical measurements can reveal the structure of icy ocean worlds and cycling of volatiles. The associated density, temperature, sound speed, and electrical conductivity of such worlds thus characterizes their habitability. To explore the variability and correlation of these parameters, and to provide tools for planning and data analyses, we develop 1-D calculations of internal structure, which use available constraints on the thermodynamics of aqueous MgSO4, NaCl (as seawater), and NH3, water ices, and silicate content. Limits in available thermodynamic data narrow the parameter space that can be explored: insufficient coverage in pressure, temperature, and composition for end-member salinities of MgSO4 and NaCl, and for relevant water ices; and a dearth of suitable data for aqueous mixtures of Na-Mg-Cl-SO4-NH3. For Europa, ocean compositions that are oxidized and dominated by MgSO4, vs reduced (NaCl), illustrate these gaps, but also show the potential for diagnostic and measurable combinations of geophysical parameters. The low-density rocky core of Enceladus may comprise hydrated minerals, or anydrous minerals with high porosity comparable to Earth's upper mantle. Titan's ocean must be dense, but not necessarily saline, as previously noted, and may have little or no high-pressure ice at its base. Ganymede's silicious interior is deepest among all known ocean worlds, and may contain multiple phases of high-pressure ice, which will become buoyant if the ocean is sufficiently salty. Callisto's likely near-eutectic ocean cannot be adequately modeled using available data. Callisto may also lack high-pressure ices, but this cannot be confirmed due to uncertainty in its moment of inertia.

Sunday, September 17, 2017

An ALMA Survey of CO isotopologue emission from Protoplanetary Disks in Chamaeleon I

An ALMA Survey of CO isotopologue emission from Protoplanetary Disks in Chamaeleon I

Authors:


Long et al

Abstract:

The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey 13CO and C18O J = 3−2 line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from 0.03 -- 2 M⊙ in the nearby Chamaeleon I star-forming region. We detect 13CO emission from 17 sources and C18O from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical ISM CO-to-H2 ratios of 10−4, the resulting gas masses are implausibly low, with an average gas mass of ∼ 0.05 MJup as inferred from the average flux of stacked 13CO lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.

Radially resolved simulations of collapsing pebble clouds in protoplanetary discs

Radially resolved simulations of collapsing pebble clouds in protoplanetary discs


Authors:


Jansson et al

Abstract:

We study the collapse of pebble clouds with a statistical model to find the internal structure of comet-sized planetesimals. Pebble-pebble collisions occur during the collapse and the outcome of these collisions affect the resulting structure of the planetesimal. We expand our previous models by allowing the individual pebble sub-clouds to contract at different rates and by including the effect of gas drag on the contraction speed and in energy dissipation. Our results yield comets that are porous pebble-piles with particle sizes varying with depth. In the surface layers there is a mixture of primordial pebbles and pebble fragments. The interior, on the other hand, consists only of primordial pebbles with a narrower size distribution, yielding higher porosity there. Our results imply that the gas in the protoplanetary disc plays an important role in determining the radial distribution of pebble sizes and porosity inside planetesimals.

1.3-mm ALMA observations of the Fomalhaut debris system

1.3-mm ALMA observations of the Fomalhaut debris system

Authors:


White et al

Abstract:
We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations (1.3 mm/233 GHz) of Fomalhaut and its debris disc. The observations achieve a sensitivity of 17 μJy and a resolution of 0.28 arcsec (2.1 au at a distance of 7.66 pc), which are the highest resolution observations to date of the millimetre grains in Fomalhaut's main debris ring. The ring is tightly constrained to 139+2−3/139−3+2 au with a full width at half-maximum of 13 ± 3 au, following a Gaussian profile. The millimetre spectral index is constrained to αmm = −2.73 ± 0.13. We explore fitting debris disc models in the image plane, as well as fitting models using visibility data directly. The results are compared and the potential advantages/disadvantages of each approach are discussed. The detected central emission is indistinguishable from a point source, with a most probable flux of 0.90 ± 0.12 mJy (including calibration uncertainties). This implies that any inner debris structure, as was inferred from far-infrared observations, must contribute little to the total central emission. Moreover, the stellar flux is less than 70 per cent of that predicted by extrapolating a blackbody from the constrained stellar photosphere temperature. This result emphasizes that unresolved inner debris components cannot be fully characterized until the behaviour of the host star's intrinsic stellar emission at millimetre wavelengths is properly understood.

Saturday, September 16, 2017

The Planetary Accretion Shock: I. Framework for Radiation-hydrodynamical Simulations and First Results

The Planetary Accretion Shock: I. Framework for Radiation-hydrodynamical Simulations and First Results

Authors:


Marleau et al

Abstract:
The key aspect determining the post-formation luminosity of gas giants has long been considered to be the energetics of the accretion shock at the planetary surface. We use 1D radiation-hydrodynamical simulations to study the radiative loss efficiency and to obtain post-shock temperatures and pressures and thus entropies. The efficiency is defined as the fraction of the total incoming energy flux which escapes the system (roughly the Hill sphere), taking into account the energy recycling which occurs ahead of the shock in a radiative precursor. We focus here on a constant equation of state to isolate the shock physics but use constant and tabulated opacities. While robust quantitative results will require a self-consistent treatment including hydrogen dissocation and ionization, the results show the correct qualitative behavior and can be understood semi-analytically. The shock is found to be isothermal and supercritical for a range of conditions relevant to core accretion (CA), with Mach numbers greater than ca. 3. Across the shock, the entropy decreases significantly, by a few entropy units (k_B/baryon). While nearly 100 percent of the incoming kinetic energy is converted to radiation locally, the efficiencies are found to be as low as roughly 40 percent, implying that a meaningful fraction of the total accretion energy is brought into the planet. For realistic parameter combinations in the CA scenario, a non-zero fraction of the luminosity always escapes the system. This luminosity could explain, at least in part, recent observations in the LkCa 15 and HD 100546 systems.

Self-induced dust traps: overcoming planet formation barriers

Self-induced dust traps: overcoming planet formation barriers

Authors:


Gonzalez et al

Abstract:
Planet formation is thought to occur in discs around young stars by the aggregation of small dust grains into much larger objects. The growth from grains to pebbles and from planetesimals to planets is now fairly well understood. The intermediate stage has however been found to be hindered by the radial-drift and fragmentation barriers. We identify a powerful mechanism in which dust overcomes both barriers. Its key ingredients are (i) backreaction from the dust on to the gas, (ii) grain growth and fragmentation and (iii) large-scale gradients. The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped. We show that these self-induced dust traps are robust: they develop for a wide range of disc structures, fragmentation thresholds and initial dust-to-gas ratios. They are favoured locations for pebbles to grow into planetesimals, thus opening new paths towards the formation of planets.

GAPS IN PROTOPLANETARY DISKS AS SIGNATURES OF PLANETS. III. POLARIZATION

GAPS IN PROTOPLANETARY DISKS AS SIGNATURES OF PLANETS. III. POLARIZATION

Author:


Jang-Condell

Abstract:

Polarimetric observations of T Tauri and Herbig Ae/Be stars are a powerful way to image protoplanetary disks. However, interpretation of these images is difficult because the degree of polarization is highly sensitive to the angle of scattering of stellar light off the disk surface. We examine how disks with and without gaps created by planets appear in scattered polarized light as a function of inclination angle. Isophotes of inclined disks without gaps are distorted in polarized light, giving the appearance that the disks are more eccentric or more highly inclined than they truly are. Apparent gap locations are unaffected by polarization, but the gap contrast changes. In face-on disks with gaps, we find that the brightened far edge of the gap scatters less polarized light than the rest of the disk, resulting in slightly decreased contrast between the gap trough and the brightened far edge. In inclined disks, gaps can take on the appearance of being localized "holes" in brightness rather than full axisymmetric structures. Photocenter offsets along the minor axis of the disk in both total intensity and polarized intensity images can be readily explained by the finite thickness of the disk. Alone, polarized scattered light images of disks do not necessarily reveal intrinsic disk structure. However, when combined with total intensity images, the orientation of the disk can be deduced and much can be learned about disk structure and dust properties.

Friday, September 15, 2017

SCExAO AND GPI Y JH BAND PHOTOMETRY AND INTEGRAL FIELD SPECTROSCOPY OF THE YOUNG BROWN DWARF COMPANION TO HD 1160

SCExAO AND GPI Y JH BAND PHOTOMETRY AND INTEGRAL FIELD SPECTROSCOPY OF THE YOUNG BROWN DWARF COMPANION TO HD 1160

Authors:


Garcia et al

Abstract:
We present high signal-to-noise ratio, precise Y JH photometry and Y band (0.957–1.120 μm) spectroscopy of HD 1160 B, a young substellar companion discovered from the Gemini NICI Planet Finding Campaign using the Subaru Coronagraphic Extreme Adaptive Optics instrument and the Gemini Planet Imager. HD 1160 B has typical mid-M dwarf-like infrared colors and a spectral type of M5.5${}_{-0.5}^{+1.0}$, where the blue edge of our Y band spectrum rules out earlier spectral types. Atmospheric modeling suggests HD 1160 B has an effective temperature of 3000–3100 K, a surface gravity of log g = 4–4.5, a radius of 1.55 ± 0.10 R J, and a luminosity of log L/L ⊙ = −2.76 ± 0.05. Neither the primary's Hertzspring–Russell diagram position nor atmospheric modeling of HD 1160 B show evidence for a subsolar metallicity. Interpretation of the HD 1160 B spectroscopy depends on which stellar system components are used to estimate the age. Considering HD 1160 A, B and C jointly, we derive an age of 80–125 Myr, implying that HD 1160 B straddles the hydrogen-burning limit (70–90 M J). If we consider HD 1160 A alone, younger ages (20–125 Myr) and a brown dwarf-like mass (35–90 M J) are possible. Interferometric measurements of the primary, a precise Gaia parallax, and moderate-resolution spectroscopy can better constrain the system's age and how HD 1160 B fits within the context of (sub)stellar evolution.

2MASS 0213+3648 C: A wide T3 benchmark companion to an an active, old M dwarf binary

2MASS 0213+3648 C: A wide T3 benchmark companion to an an active, old M dwarf binary

Authors:


Deacon et al

Abstract:

We present the discovery of a 360 AU separation T3 companion to the tight (3.1 AU) M4.5+M6.5 binary 2MASS J02132062+3648506. This companion was identified using Pan-STARRS1 data and, despite its relative proximity to the Sun (22.2+6.4−4.0 pc; Pan-STARRS1 parallax) and brightness (J=15.3), appears to have been missed by previous studies due to its position near a diffraction spike in 2MASS. The close M~dwarf binary has active X-ray and Hα emission and shows evidence for UV flares. The binary's weak {\it GALEX} UV emission and strong Na I 8200\AA Na absorption leads us to an age range of ∼1-10Gyr. Applying this age range to evolutionary models implies the wide companion has a mass of 0.063±0.009\,M⊙. 2MASS J0213+3648 C provides a relatively old benchmark close to the L/T transition and acts as a key, older comparison to the much younger early-T companions HN~Peg~B and GU~Psc~b.

Physical properties and astrometry of radio-emitting brown dwarf TVLM513-46546 revisited

Physical properties and astrometry of radio-emitting brown dwarf TVLM513-46546 revisited


Authors:


Gawroński et al

Abstract:
We present multi-epoch astrometric observations of the M9 ultracool dwarf TVLM513-46546 that is placed at the brown dwarf boundary. The new observations have been performed with the European Very Large Baseline Interferometry Network at 6 cm band. The target has been detected at seven epochs spanning three years, with measured quiescent emission flux in the range 180–300 μJy. We identified four short-duration flaring events (0.5–2 mJy) with very high circular polarization (∼75 per cent–100 per cent). Properties of the observed radio flares support the physical model of the source that is characterized by the electron cyclotron maser instability responsible for outbursts of radio emission. Combined with Very Long Baseline Array earlier data, our detections make it possible to refine the absolute parallax π=93.27+0.18−0.17/π=93.27−0.17+0.18 mas. Our measurements rule out TVLM513-46546 companions more massive than Jupiter in orbits with periods longer than ∼1 yr.

Thursday, September 14, 2017

Possible formation pathways for the low density Neptune-mass planet HAT-P-26b

Possible formation pathways for the low density Neptune-mass planet HAT-P-26b

Authors:


Ali-Dib et al

Abstract:
We investigate possible pathways for the formation of the low density Neptune-mass planet HAT-P-26b. We use two formation different models based on pebbles and planetesimals accretion, and includes gas accretion, disk migration and simple photoevaporation. The models tracks the oxygen abundance and the properties of the forming planets, that we compare to HAT-P-26b. We find that pebbles accretion can explain this planet more naturally than planetesimals accretion that fails completely unless we artificially enhance the disk metallicity significantly. Pebble accretion models can reproduce HAT-P-26b with either a high initial core mass and low amount of core erosion, or the opposite, with both scenarios being equally plausible. Degeneracy between the formation time and location of the planet implies that more observables are needed to constraint its formation.

HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance

HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance

Authors:


Wakeford et al

Abstract:


A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System, and has served as a cornerstone of planet formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 microns, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands with a maximum base-to-peak amplitude of 525ppm in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b's atmospheric heavy element content [4.8 (-4.0 +21.5) times solar]. This likely indicates that HAT-P-26b's atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.

On Signatures of Clouds in Exoplanetary Transit Spectra

On Signatures of Clouds in Exoplanetary Transit Spectra

Authors:


Pinhas et al

Abstract:

Transmission spectra of exoplanetary atmospheres have been used to infer the presence of clouds/hazes. Such inferences are typically based on spectral slopes in the optical deviant from gaseous Rayleigh scattering or low-amplitude spectral features in the infrared. We investigate three observable metrics that could allow constraints on cloud properties from transmission spectra, namely, the optical slope, the uniformity of this slope, and condensate features in the infrared. We derive these metrics using model transmission spectra considering Mie extinction from a wide range of condensate species, particle sizes, and scale heights. Firstly, we investigate possible degeneracies among the cloud properties for an observed slope. We find, for example, that spectra with very steep optical slopes suggest sulphide clouds (e.g. MnS, ZnS, Na2S) in the atmospheres. Secondly, (non)uniformities in optical slopes provide additional constraints on cloud properties, e.g., MnS, ZnS, TiO2, and Fe2O3 have significantly non-uniform slopes. Thirdly, infrared spectra provide an additional powerful probe into cloud properties, with SiO2, Fe2O3, Mg2SiO4, and MgSiO3 bearing strong infrared features observable with JWST. We investigate observed spectra of eight hot Jupiters and discuss their implications. In particular, no single or composite condensate species considered here conforms to the steep and non-uniform optical slope observed for HD 189733b. Our work highlights the importance of the three above metrics to investigate cloud properties in exoplanetary atmospheres using high-precision transmission spectra and detailed cloud models. We make our Mie data publicly available to the community.

Wednesday, September 13, 2017

Quantifying the Impact of Spectral Coverage on the Retrieval of Molecular Abundances from Exoplanet Transmission Spectra

Quantifying the Impact of Spectral Coverage on the Retrieval of Molecular Abundances from Exoplanet Transmission Spectra

Authors:


Chapman et al

Abstract:

Using forward models for representative exoplanet atmospheres and a radiometric instrument model, we have generated synthetic observational data to explore how well the major C- and O-bearing chemical species (CO, CO2, CH4, and H2O), important for determining atmospheric opacity and radiation balance, can be constrained by transit measurements as a function of spectral wavelength coverage. This work features simulations for a notional transit spectroscopy mission and compares two cases for instrument spectral coverage (wavelength coverage from 0.5-2.5 {\mu}m and 0.5-5 {\mu}m). The simulation is conducted on a grid with a range of stellar magnitudes and incorporates a full retrieval of atmospheric model parameters. We consider a range of planets from sub-Neptunes to hot Jupiters and include both low and high mean molecular weight atmospheres. We find that including the 2.5-5 {\mu}m wavelength range provides a significant improvement, up to -3 orders of magnitude, in the degree of constraint on the retrieved molecular abundances, implying that broad spectral coverage between the visible and the mid-infrared is an important tool for understanding the chemistry and composition of exoplanet atmospheres. This analysis suggests that the JWST/NIRSpec 0.6-5 {\mu}m prism spectroscopy mode, or similar wavelength coverage with possible future missions, will be an important resource for exoplanet atmospheric characterization.

Star-planet interactions. IV. Possibility of detecting the orbit-shrinking of a planet around a red giant

Star-planet interactions. IV. Possibility of detecting the orbit-shrinking of a planet around a red giant

Authors:


Meynet et al

Abstract:
The surface rotations of some red giants are so fast that they must have been spun up by tidal interaction with a close companion, either another star, a brown dwarf, or a planet. We focus here on the case of red giants that are spun up by tidal interaction with a planet. When the distance between the planet and the star decreases, the spin period of the star decreases, the orbital period of the planet decreases, and the reflex motion of the star increases. We study the change rate of these three quantities when the circular orbit of a planet of 15 MJ that initially orbits a 2 M⊙ star at 1 au shrinks under the action of tidal forces during the red giant phase. We use stellar evolution models coupled with computations of the orbital evolution of the planet, which allows us to follow the exchanges of angular momentum between the star and the orbit in a consistent way. We obtain that the reflex motion of the red giant star increases by more than 1 m s−1 per year in the last ∼40 years before the planet engulfment. During this phase, the reflex motion of the star is between 660 and 710 m s−1. The spin period of the star increases by more than about 10 minutes per year in the last 3000 y before engulfment. During this period, the spin period of the star is shorter than 0.7 year. During this same period, the variation in orbital period, which is shorter than 0.18 year, is on the same order of magnitude. Changes in reflex-motion and spin velocities are very small and thus most likely out of reach of being observed. The most promising way of detecting this effect is through observations of transiting planets, that is, through{\it } changes of the beginning or end of the transit. A space mission like PLATO might be of great interest for detecting planets that are on the verge of being engulfed by red giants.

Effects of the Planetary Temperature on the Circumplanetary Disk and on the Gap

Effects of the Planetary Temperature on the Circumplanetary Disk and on the Gap

Author:


Szulágyi

Abstract:

Circumplanetary disks regulate the late accretion to the giant planet and serve as the birthplace for satellites. Understanding their characteristics via simulations also helps to prepare for their observations. Here we study disks around 1, 3, 5, 10 MJup planets with three dimensional, global radiative hydrodynamic simulations with sub-planet peak resolution, and various planetary temperatures. We found that as the 1 MJup planet radiates away its formation heat, the circumplanetary envelope transitions to a disk between Tp=6000 K and 4000 K. In the case of 3-10 MJup planets a disk always forms. The temperature profile of the circumplanetary disks is very steep, the inner 1/6th is over the silicate condensation temperature and the entire disk is above water freezing point, making satellite formation impossible in this early stage (less than 1 Myr). Satellites might form much later and first in the outer parts of the disk migrating inwards later on. Our disk masses are 1,7,20,40×10−3MJup for the 1, 3, 5, 10 MJup gas giants respectively, and we provide an empirical formula to estimate the subdisk masses based on the planet- and circumstellar disk mass. Our finding is that the cooler the planet, the lower the temperature of the subdisk, the higher the vertical influx velocities, and the planetary gap is both deeper and wider. We also show that the gaps in 2D and 3D are different. The subdisk eccentricity increases with planetary mass and violently interacts with the circumstellar disk, making satellite-formation less likely, if Mp≳5MJup.

Tuesday, September 12, 2017

The Exoplanetary Phylogeny



Link.

Collisional Fragmentation is not a Barrier to Close-in Planet Formation

Collisional Fragmentation is not a Barrier to Close-in Planet Formation 
Authors:

Wallace et al

Abstract:
Collisional fragmentation is shown to not be a barrier to rocky planet formation at small distances from the host star. Simple analytic arguments demonstrate that rocky planet formation via collisions of homogeneous gravity-dominated bodies is possible down to distances of order the Roche radius (rRoche). Extensive N-body simulations that include plausible models for fragmentation and merging of gravity-dominated bodies confirm this conclusion and demonstrate that rocky planet formation is possible down to ∼1.1 rRoche. At smaller distances, tidal effects cause collisions to be too fragmenting to allow mass build-up to a final, dynamically stable planetary system. We argue that even differentiated bodies can accumulate to form planets at distances that are not much larger than rRoche.

Spin dynamics of close-in planets exhibiting large TTVs

Spin dynamics of close-in planets exhibiting large TTVs

Authors:


Delisle et al

Abstract:
We study the spin evolution of close-in planets in compact multi-planetary systems. The rotation period of these planets is often assumed to be synchronous with the orbital period due to tidal dissipation. Here we show that planet-planet perturbations can drive the spin of these planets into non-synchronous or even chaotic states. In particular, we show that the transit timing variation (TTV) is a very good probe to study the spin dynamics, since both are dominated by the perturbations of the mean longitude of the planet. We apply our model to KOI-227b and Kepler-88b, which are both observed undergoing strong TTVs. We also perform numerical simulations of the spin evolution of these two planets. We show that for KOI-227b non-synchronous rotation is possible, while for Kepler-88b the rotation can be chaotic.

Rocky Terrestrial Exoplanets may only go up to 2 Earth Radius

Hydrodynamic Escape of Planetary Atmospheres during a Star's X-ray and Extreme Ultraviolet Saturation May Impose a Size Limit of ~2 Earth Radii on Rocky Exoplanets

Authors:


Lehmer et al

Abstract:
Recent observations and analysis of small exoplanets have found that rocky planets like the Earth only have radii up to 1.5-2R⊕. Two general hypotheses exist for the cause of the dichotomy between rocky and gas-enveloped planets (or possible water worlds): either small planets do not easily form thick atmospheres, or the thick atmospheres on small planets easily escape driven by x-ray and extreme ultraviolet (XUV) emissions from young parent stars. Here we show that a cutoff between rocky and gas-enveloped planets due to hydrodynamic escape is most likely to occur at a mean radius of 1.76±0.38R⊕ (2σ) Earth radii around Sun-like stars. We examine the limit in rocky planet radii predicted by hydrodynamic escape across a wide range of possible model inputs using 10,000 parameter combinations drawn randomly from plausible parameter ranges. We find a cutoff between rocky and gas-enveloped planets that agrees with the observed cutoff. The large cross-section available for XUV absorption in the extremely distended primitive atmospheres of low mass planets results in complete loss of atmospheres during the ~100 Myr phase of stellar XUV saturation. In contrast, more massive planets have less distended atmospheres and less escape, and so retain thick atmospheres through XUV saturation and then indefinitely as the XUV and escape fluxes drop over time. The agreement between our model and exoplanet data leads us to conclude that hydrodynamic escape plausibly explains the observed upper limit on rocky planet size.

Monday, September 11, 2017

Quasars Could Strip Exoplanetary Atmospheres

Evaporation of planetary atmospheres due to XUV illumination by quasars

Authors:


Forbes et al

Abstract:
Planetary atmospheres are subject to mass loss through a variety of mechanisms including irradiation by XUV photons from their host star. Here we explore the consequences of XUV irradiation by supermassive black holes as they grow by the accretion of gas in galactic nuclei. Based on the mass distribution of stars in galactic bulges and disks and the luminosity history of individual black holes, we estimate the probability distribution function of XUV fluences as a function of galaxy halo mass, redshift, and stellar component. We find that about 50% of all planets in the universe may lose the equivalent of a Martian atmosphere, 10% may lose an Earth's atmosphere, and 0.2% may lose the mass of Earth's oceans. The fractions are appreciably higher in the spheroidal components of galaxies, and depend strongly on galaxy mass, but only weakly on redshift.

A Statistical Comparative Planetology Approach to the Hunt for Habitable Exoplanets and Life Beyond the Solar System

A Statistical Comparative Planetology Approach to the Hunt for Habitable Exoplanets and Life Beyond the Solar System

Authors:


Bean et al

Abstract:

The search for habitable exoplanets and life beyond the Solar System is one of the most compelling scientific opportunities of our time. Nevertheless, the high cost of building facilities that can address this topic and the keen public interest in the results of such research requires the rigorous development of experiments that can deliver a definitive advance in our understanding. Most work to date in this area has focused on a "systems science" approach of obtaining and interpreting comprehensive data for individual planets to make statements about their habitability and the possibility that they harbor life. This strategy is challenging because of the diversity of exoplanets, both observed and expected, and the limited information that can be obtained with astronomical instruments. Here we propose a complementary approach that is based on performing surveys of key planetary characteristics and using statistical marginalization to answer broader questions than can be addressed with a small sample of objects. The fundamental principle of this comparative planetology approach is maximizing what can be learned from each type of measurement by applying it widely rather than requiring that multiple kinds of observations be brought to bear on a single object. As a proof of concept, we outline a survey of terrestrial exoplanet atmospheric water and carbon dioxide abundances that would test the habitable zone hypothesis and lead to a deeper understanding of the frequency of habitable planets. We also discuss ideas for additional surveys that could be developed to test other foundational hypotheses is this area.