First Detection of Hydrogen in the β Pictoris Gas Disk

First Detection of Hydrogen in the β Pictoris Gas Disk

Authors:

Wilson et al

Abstract:

The young and nearby star \beta\ Pictoris (\beta\ Pic) is surrounded by a debris disk composed of dust and gas known to host a myriad evaporating exocomets, planetesimals and at least one planet. At an edge-on inclination, as seen from Earth, this system is ideal for debris disk studies providing an excellent opportunity to use absorption spectroscopy to study the planet forming environment. Using the Cosmic Origins Spectrograph (COS) instrument on the Hubble Space Telescope (HST) we observe the most abundant element in the disk, hydrogen, through the HI Lyman \alpha\ (Ly-\alpha\) line. We present a new technique to decrease the contamination of the Ly-\alpha\ line by geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique allows us to shift the Ly-\alpha\ line of the astrophysical target away from the contaminating airglow emission revealing more of the astrophysical line profile. The column density of hydrogen in the \beta\ Pic stable gas disk at the stellar radial velocity is measured to be log(NH/1cm2)≪18.5. The Ly-\alpha\ emission line profile is found to be asymmetric and we propose that this is caused by HI falling in towards the star with a bulk radial velocity of 41±6 km/s relative to \beta\ Pic and a column density of log(NH/1cm2)=18.6±0.1. The high column density of hydrogen relative to the hydrogen content of CI chondrite meteorites indicates that the bulk of the hydrogen gas does not come from the dust in the disk. This column density reveals a hydrogen abundance much lower than solar, which excludes the possibility that the detected hydrogen could be a remnant of the protoplanetary disk or gas expelled by the star. We hypothesise that the hydrogen gas observed falling towards the star arises from the dissociation of water originating from evaporating exocomets.

A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

Authors:

Kral et al

Abstract:

This paper presents a self-consistent model for the evolution of gas produced in the debris disc of β Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous α model. We present new gas observations of β Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity α > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in β Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.

A Comprehensive Dust Model Applied to the Resolved Beta Pictoris Debris Disk from Optical to Radio Wavelengths

A Comprehensive Dust Model Applied to the Resolved Beta Pictoris Debris Disk from Optical to Radio Wavelengths

Authors:

Ballering et al

Abstract:

We investigate whether varying the dust composition (described by the optical constants) can solve a persistent problem in debris disk modeling--the inability to fit the thermal emission without over-predicting the scattered light. We model five images of the beta Pictoris disk: two in scattered light from HST/STIS at 0.58 microns and HST/WFC3 at 1.16 microns, and three in thermal emission from Spitzer/MIPS at 24 microns, Herschel/PACS at 70 microns, and ALMA at 870 microns. The WFC3 and MIPS data are published here for the first time. We focus our modeling on the outer part of this disk, consisting of a parent body ring and a halo of small grains. First, we confirm that a model using astronomical silicates cannot simultaneously fit the thermal and scattered light data. Next, we use a simple, generic function for the optical constants to show that varying the dust composition can improve the fit substantially. Finally, we model the dust as a mixture of the most plausible debris constituents: astronomical silicates, water ice, organic refractory material, and vacuum. We achieve a good fit to all datasets with grains composed predominantly of silicates and organics, while ice and vacuum are, at most, present in small amounts. This composition is similar to one derived from previous work on the HR 4796A disk. Our model also fits the thermal SED, scattered light colors, and high-resolution mid-IR data from T-ReCS for this disk. Additionally, we show that sub-blowout grains are a necessary component of the halo.

Oxygen Detected in beta Pictoris' Protoplanetary Disk

Herschel detects oxygen in the beta Pictoris debris disk

Authors:

Brandeker et al

Abstract:

The young star beta Pictoris is well known for its dusty debris disk, produced through the grinding down by collisions of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star, likely the result from vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio 20x higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported being observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher-density region in the disk, perhaps in the shape of a clump or a dense torus, required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by ALMA in the disk, and that the re-distribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution.

New Orbital Parameters for β Pictoris b

β Pictoris' inner disk in polarized light and new orbital parameters for β Pictoris b

Authors:

Millar-Blanchaer et al

Abstract:

We present H-band observations of β Pic with the Gemini Planet Imager's (GPI's) polarimetry mode that reveal the debris disk between ~0.3" (~6 AU) and ~1.7" (~33 AU), while simultaneously detecting β Pic b. The polarized disk image was fit with a dust density model combined with a Henyey-Greenstein scattering phase function. The best fit model indicates a disk inclined to the line of sight (ϕ=85.27deg+0.26−0.19) with a position angle θPA=30.35deg+0.29−0.28 (slightly offset from the main outer disk, θPA≈29deg), that extends from an inner disk radius of 23.6+0.9−0.6 AU to well outside GPI's field of view. In addition, we present an updated orbit for β Pic b based on new astrometric measurements taken in GPI's spectroscopic mode spanning 14 months. The planet has a semi-major axis of a=9.2+1.5−0.4AU, with an eccentricity e≤0.26. The position angle of the ascending node is Ω=31.75deg±0.15, offset from both the outer main disk and the inner disk seen in the GPI image. The orbital fit constrains the stellar mass of β Pic to 1.60±0.05M⊙. Dynamical sculpting by β Pic b cannot easily account for the following three aspects of the inferred disk properties: 1) the modeled inner radius of the disk is farther out than expected if caused by β Pic b; 2) the mutual inclination of the inner disk and β Pic b is 4deg, when it is expected to be closer to zero; and 3) the aspect ratio of the disk (h0=0.137+0.005−0.006) is larger than expected from interactions with β Pic b or self-stirring by the disk's parent bodies.

Modeling Collisions of Planetesimals and Dust in the β Pictoris Debris Disk

A SMACK Model of Colliding Planetesimals and Dust in the β Pictoris Debris Disk: Thermal Radiation and Scattered Light

Authors:

Nesvold et al

Abstract:

We present a new model of the β Pictoris disk-and-planet system that simulates both the planetesimal collisions and the dynamics of the resulting dust grains, allowing us to model features and asymmetries in both thermal and scattered light images of the disk. Given the observed inclination and eccentricity of the β Pictoris b planet, the model neatly ties together several features of the disk: the central hole in the submillimeter images, the two-disk "x"-pattern seen in scattered light, the "wing-tilt" asymmetry, and possibly even the clumpy gas seen by ALMA. We also find that most of the dust in the β Pictoris system is likely produced outside the ring at 60-100 AU. Instead of a birth ring, this disk has a "stirring ring" at 60-100 AU where the high-velocity collisions produced by the secular wave launched by the planet are concentrated. The two-disk x-pattern arises because collisions occur more frequently at the peaks and troughs of the secular wave. The perturbations of the disk in this region create an azimuthally and vertically asymmetric spatial distribution of collisions, which could yield an azimuthal clump of gas without invoking resonances or an additional planet.

SMACK: Modeling Beta Pictoris' Disk

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A Model for Young Giant Exoplanets Applied beta Pictoris b

A radiative-convective equilibrium model to study young giant exoplanets by direct imaging

Authors:

Baudino et al

Abstract:

We developed a model for young giant exoplanets (Exoplanet Radiative-convective Equilibrium Model or Exo-REM). Input parameters are planet's surface gravity (g), effective temperature (Teff ) and elemental composition. Under the additional assumption of thermochemical equilibrium, the model predicts the equilibrium temperature profile and mixing ratio profiles of the most important gases. Opacity sources include the H2-He collision-induced absorption and molecular lines from H2O, CO, CH4 (updated with the Exomol linelist), NH3, VO, TiO, Na and K. Absorption by iron and silicate cloud particles is added above the expected condensation levels with a fixed scale height and a given optical depth at some reference wavelength. Scattering was not included at this stage.

We applied Exo-REM to photometric and spectral observations of the planet beta Pictoris b obtained in a series of near IR filters. We derived Teff = 1550 ± 150 K, log(g) = 3.5 ± 1, and a radius R = 1.76 ± 0.24 R Jup (2-σ error bars). These values are comparable to those found in the literature, although with more conservative error bars, but consistent with the model accuracy. We finally investigated the precision to which the above parameters can be constrained from SPHERE measurements using different sets of near IR filters as well as near low resolution spectroscopy.

Best Picture Yet by Hubble Space Telescope of beta Pictoris Circumstellar Disk

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Evolution of the Beta Pictoris System

The Inner Disk Structure, Disk-Planet Interactions, and Temporal Evolution in the Beta Pictoris System: A Two-Epoch HST/STIS Coronagraphic Study

Authors:

Apai et al

Abstract:

We present deep HST/STIS coronagraphic images of the Beta Pic debris disk obtained at two epochs separated by 15 years. The new images and the re-reduction of the 1997 data provide the most sensitive and detailed views of the disk at optical wavelengths as well as the yet smallest inner working angle optical coronagraphic image of the disk. Our observations characterize the large-scale and inner-disk asymmetries and we identify multiple breaks in the disk radial surface brightness profile. We study in detail the radial and vertical disk structure and show that the disk is warped. We explore the disk at the location of the Beta Pic b super-jupiter and find that the disk surface brightness slope is continuous between 0.5 and 2.0 arcsec, arguing for no change at the separations where Beta Pic b orbits. The two epoch images constrain the disk surface brightness evolution on orbital and radiation pressure blow-out timescales. We place an upper limit of 3% on the disk surface brightness change between 3-5 arcsec, including the locations of the disk warp, and the CO and dust clumps. We discuss the new observations in the context of high-resolution multi-wavelength images and divide the disk asymmetries in two groups: axisymmetric and non-axisymmetric. The axisymmetric structures (warp, large-scale butterfly, etc.) are consistent with disk structure models that include interactions of a planetesimal belt and a non-coplanar giant planet. The non-axisymmetric features, however, require a different explanation.

Simulating Gap Clearing by ExoPlanets in Protoplanetary Disks

Gap Clearing by Planets in a Collisional Debris Disk

Authors:

Nesvold et al

Abstract:

We apply our 3D debris disk model, SMACK, to simulate a planet on a circular orbit near a ring of planetesimals that are experiencing destructive collisions. Previous simulations of a planet opening a gap in a collisionless debris disk have found that the width of the gap scales as the planet mass to the 2/7th power (α=2/7). We find that the gap sizes in a collisional disk still obey a power law scaling with planet mass, but that the index alpha of the power law depends on the age of the system t relative to the collisional timescale tcoll of the disk by α=0.32(t/tcoll)−0.04, with inferred planet masses up to five times smaller than those predicted by the classical gap law. The increased gap sizes likely stem from the interaction between the collisions and the mean motion resonances near the chaotic zone. We investigate the effects of the initial eccentricity distribution of the disk particles and find a negligible effect on the gap size at Jovian planet masses, since collisions tend to erase memory of the initial particle eccentricity distributions. Finally, we find that the presence of Trojan analogs is a potentially powerful diagnostic of planets in the mass range ~1-10 Mjup. We apply our model to place new upper limits on planets around Fomalhaut, HR 4796 A, HD 202628, HD 181327, and β Pictoris.

Two Families of Exocomets in the β Pictoris System

Two families of exocomets in the β Pictoris system

Authors:

Kiefer et al

Abstract:

The young planetary system surrounding the star β Pictoris harbours active minor bodies . These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of our Solar System. Spectroscopic observations of β Pictoris reveal a high rate of transits of small evaporating bodies, that is, exocomets. Here we report an analysis of more than 1,000 archival spectra gathered between 2003 and 2011, which provides a sample of about 6,000 variable absorption signatures arising from exocomets transiting the disk of the parent star. Statistical analysis of the observed properties of these exocomets allows us to identify two populations with different physical properties. One family consists of exocomets producing shallow absorption lines, which can be attributed to old exhausted (that is, strongly depleted in volatiles) comets trapped in a mean motion resonance with a massive planet. Another family consists of exocomets producing deep absorption lines, which may be related to the recent fragmentation of one or a few parent bodies. Our results show that the evaporating bodies observed for decades in the β Pictoris system are analogous to the comets in our own Solar System.

Observations of BD-21 1074 Suggest Close-in Planets do NOT Form Through Core Accretion

Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the β

Authors:

Messina et al

Abstract:

The early stage of stellar evolution is characterized by a star-disc locking mechanism. The disc-locking prevents the star to spin its rotation up, and its timescale depends on the disc lifetime. Some mechanisms can significantly shorten this lifetime, allowing a few stars to start spinning up much earlier than other stars. In the present study, we aim to investigate how the properties of the circumstellar environment can shorten the disc lifetime. We have identified a few multiple stellar systems, composed of stars with similar masses, which belong to associations with a known age. Since all parameters that are responsible for the rotational evolution, with the exception of environment properties and initial stellar rotation, are similar for all components, we expect that significant differences among the rotation periods can only arise from differences in the disc lifetimes. A photometric timeseries allowed us to measure the rotation periods of each component, while high-resolution spectra provided us with the fundamental parameters, vsini and chromospheric line fluxes. The rotation periods of the components differ significantly, and the component B, which has a closer companion C, rotates faster than the more distant and isolated component A. We can ascribe the rotation period difference to either different initial rotation periods or different disc-locking phases arising from the presence of the close companion C. In the specific case of BD−21 1074, the second scenario seems to be more favored. In our hypothesis of different disc-locking phase, any planet orbiting this star is likely formed very rapidly owing to a gravitational instability mechanism, rather than core accretion. Only a large difference of initial rotation periods alone could account for the observed period difference, leaving comparable disc lifetimes.

Gemini Planet Imager Examines Massive Gas Giant Planet beta Pictoris b

The First H-band Spectrum of the Massive Gas Giant Planet beta Pictoris b with the Gemini Planet Imager

Authors:

Chilcote et al

Abstract:

Using the recently installed Gemini Planet Imager (GPI), we have taken the first H-band spectrum of the planetary companion to the nearby young star beta Pictoris. GPI is designed to image and provide low-resolution spectra of Jupiter sized, self-luminous planetary companions around young nearby stars. These observations were taken covering the H-band (1.65 microns). The spectrum has a resolving power of ∼ 45 and demonstrates the distinctive triangular shape of a cool substellar object with low surface gravity. Using atmospheric models, we find an effective temperature of 1650±50 K and a surface gravity of log(g)=4.0±0.25 (cgs units). These values agree well with predictions from planetary evolution models for a gas giant with mass between 10 and 12 MJup and age between 10 and 20 Myrs.

Refining the Characteristics of Beta Pictoris b

Physical and orbital properties of Beta Pictoris b

Authors:

Bonnefoy et al

Abstract:

The intermediate-mass star Beta Pictoris is known to be surrounded by a structured edge-on debris disk within which a gas giant planet was discovered orbiting at 8-10 AU. The physical properties of Beta Pic b were previously inferred from broad and narrow-band 0.9-4.8 microns photometry. We used commissioning data of the Gemini Planet Imager (GPI) to obtain new astrometry and a low-resolution (R=35-39) J-band (1.12-1.35 microns) spectrum of the planet. We find that the planet has passed the quadrature. We constrain its semi-major axis to ≤ 10 AU (90 % prob.) with a peak at 8.9+0.4-0.6 AU. The joint fit of the planet astrometry and the most recent radial velocity measurements of the star yields a planet's dynamical mass ≤ 20 MJup (greater than 96 % prob.). The extracted spectrum of Beta Pic b is similar to those of young L1-1.5+1 dwarfs. We use the spectral type estimate to revise the planet luminosity to log(L/Lsun)=-3.90+-0.07. The 0.9-4.8 microns photometry and spectrum are reproduced for Teff=1650+-150 K and a log g lower than 4.7 dex by 12 grids of PHOENIX-based and LESIA atmospheric models. If we adopt the most recent system age estimate (21+-4 Myr), the bolometric luminosity and the constraints on the dynamical mass of Beta Pic b are only reproduced by warm- and hot-start tracks with initial entropies Si greater than 10.5 kB/baryon. Such initial conditions may result from an inefficient accretion shock and/or a planetesimal density at formation higher than in the classical core accretion model. Considering a younger age for the system or a conservative formation time for Beta Pic b does not change these conclusions.

Imaging the Innermost Regions of beta Pictoris' Disk

Very deep images of the innermost regions of the beta Pictoris debris disc at Lp

Authors:

Milli et al

Abstract:

Very few debris discs have been imaged in scattered light at wavelengths beyond 3 microns because the thermal emission from both the sky and the telescope is generally too strong with respect to the faint emission of a debris disc. We present here the first analysis of a high angular resolution image of the disc of beta Pictoris at 3.8 microns. Our primary objective is to probe the innermost parts of the beta Pictoris debris disc and describe its morphology. We performed extensive forward modelling to correct for the biases induced by angular differential imaging on extended objects and derive the physical parameters of the disc. This work relies on a new analysis of seven archival datasets of beta Pictoris observed with VLT/NaCo in the Lp band, associated with disc forward modelling to correct for the biases induced by that technique. The disc is detected above a 5 sigma level between 0.4" and 3.8". The two extensions have a similar brightness within error bars. We confirm an asymmetry previously observed at larger distances from the star and at shorter wavelengths: the isophotes are more widely spaced on the north-west side (above the disc apparent midplane) than on the south-east side. This is interpreted as a small inclination of the disc combined with anisotropic scattering. Our best-fit model has an inclination of 86 degrees with an anisotropic Henyey- Greenstein coefficient of 0.36. This interpretation is supported by a new asymmetry detected in the disc: the disc is significantly bowed towards the north-west within 3" (above the apparent midplane). We also detect a possible new asymmetry within 1", but at this stage we cannot discern between a real feature and an underlying speckle.

beta Pictoris b's Fast Day

The fast spin-rotation of a young extrasolar planet

Authors:

Snellen et al

Abstract:

The spin-rotation of a planet arises from the accretion of angular momentum during its formation, but the details of this process are still unclear. In the solar system, the equatorial rotation velocities and spin angular momentum of the planets show a clear trend with mass, except for Mercury and Venus which have significantly spun down since their formation due to tidal interactions. Here we report on near-infrared spectroscopic observations at R=100,000 of the young extra-solar gas giant beta Pictoris b. The absorption signal from carbon monoxide in the planet's thermal spectrum is found to be blueshifted with respect to the velocity of the parent star by (-15+-1.7) km/sec, consistent with a circular orbit. The combined line profile exhibits a rotational broadening of 25+-3 km/sec, meaning that Beta Pictoris b spins significantly faster than any planet in the solar system, in line with the extrapolation of the known trend in spin velocity with planet mass.

β Pictoris Gas Giant or Shattered Martian Terrestrial Planets Detected in Protoplanetary Disk?

Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk

Authors:

Dent et al

Abstract:

Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets and dwarf planets. But is gas also released in such events? Observations at submm wavelengths of the archetypal debris disk around β Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85AU from the star, in a plane closely aligned with the orbit of the inner planet, β Pic b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet, or from the remnants of a collision of Mars-mass planets.

The Orbit of the Young Exoplanet beta Pictoris b

The Gemini NICI Planet-Finding Campaign: The Orbit of the Young Exoplanet beta Pictoris b

Authors:

Nielsen et al

Abstract:

We present new astrometry for the young (12-21 Myr) exoplanet beta Pictoris b taken with the Gemini/NICI, Magellan/MagAO+Clio2, and Magellan/MagAO+VisAO instruments between 2009 and 2012. The high dynamic range of our observations allows us to measure the relative position of beta Pic b with respect to its primary star with greater accuracy than previous observations. Based on a Markov Chain Monte Carlo analysis, we find the planet has an orbital semi-major axis of 9.2 [+8.3, -0.5] AU and orbital eccentricity less than 0.15 at 68% confidence (with 95% confidence intervals of 8.2-72.3 AU and 0.00-0.88 for semi-major axis and eccentricity, respectively). We find that the planet has reached its maximum projected elongation, enabling higher precision determination of the orbital parameters than previously possible, and that the planet's projected separation is currently decreasing. With unsaturated data of the entire beta Pic system (primary star, planet, and disk) obtained thanks to NICI's semi-transparent focal plane mask, we are able to tightly constrain the relative orientation of the circumstellar components. We find the orbital plane of the planet lies between the inner and outer disks but closer to the inner (warped) disk: the position angle (PA) of nodes (211.8 +/- 0.3 degrees) is 6-sigma greater than the PA of the spine of the outer disk and 1.8-sigma less than the warped inner disk PA. Finally, for the first time we are able to dynamically constrain the mass of the primary star beta Pic to 1.7 +/- 0.3 M_sun.

Beta Pictoris Exhibits Large Planetary Embryos Colliding

Debris from giant impacts between planetary embryos at large orbital radii

Authors:

Jackson et al

Abstract:

We consider the observational signatures of giant impacts between planetary embryos. While the debris released in the impact remains in a clump for only a single orbit, there is a much longer lasting asymmetry caused by the fact that all debris must pass through the collision-point. The resulting asymmetry is stationary, it does not orbit the star. The debris is concentrated in a clump at the collision-point, with a more diffuse structure on the opposite side. The asymmetry lasts for typically around 1000 orbital periods of the progenitor, which can be several Myr at distances of ~50 AU. We describe how the appearance of the asymmetric disc depends on the mass and eccentricity of the progenitor, as well as viewing orientation. The wavelength of observation, which determines the grain sizes probed, is also important. Notably, the increased collision rate of the debris at the collision-point makes this the dominant production site for any secondary dust and gas created. For dust small enough to be removed by radiation pressure, and gas with a short lifetime, this causes their distribution to resemble a jet emanating from the (stationary) collision-point. We suggest that the asymmetries seen at large separations in some debris discs, like Beta Pictoris, could be the result of giant impacts. If so this would indicate that planetary embryos are present and continuing to grow at several tens of AU at ages of up to tens of Myr.