Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars

Kronos & Krios: Evidence for accretion of a massive, rocky planetary system in a comoving pair of solar-type stars 

Authors:

Oh et al

Abstract: 

We report and discuss the discovery of a comoving pair of bright solar-type stars, HD 240430 and HD 240429, with a significant difference in their chemical abundances. The two stars have an estimated 3D separation of ≈0.6 pc (≈0.01 pc projected) at a distance of r≈100 pc with nearly identical three-dimensional velocities, as inferred from Gaia TGAS parallaxes and proper motions, and high-precision radial velocity measurements. Stellar parameters determined from high-resolution Keck HIRES spectra indicate that both stars are ∼4 Gyr old. The more metal-rich of the two, HD 240430, shows an enhancement of refractory (TC greater than 1200 K) elements by ≈0.2 dex and a marginal enhancement of (moderately) volatile elements (TC less than 1200 K, C, N, O, Na, and Mn). This is the largest metallicity difference found in a wide binary pair yet. Additionally, HD 240430 shows an anomalously high surface lithium abundance (A(Li)=2.75), higher than its companion by 0.5 dex. The proximity in phase-space and ages between the two stars suggests that they formed together with the same composition, at odds with the observed differences in metallicity and abundance patterns. We therefore suggest that the star HD~240430, "Kronos", accreted 15 M⊕ of rocky material after birth, selectively enhancing the refractory elements as well as lithium in its surface and convective envelope.

The scattering outcomes of Kepler circumbinary planets: planet mass ratio

The scattering outcomes of Kepler circumbinary planets: planet mass ratio

Authors:

Gong et al

Abstract:
Recent studies reveal that the free eccentricities of Kepler-34b and Kepler-413b are much larger than their forced eccentricities, implying that the scattering events may take place in their formation. The observed orbital configuration of Kepler-34b cannot be well reproduced in disk-driven migration models, whereas a two-planet scattering scenario can play a significant role of shaping the planetary configuration. These studies indicate that circumbinary planets discovered by Kepler may have experienced scattering process. In this work, we extensively investigate the scattering outcomes of circumbinary planets focusing on the effects of planet mass ratio. We find that the planetary mass ratio and the the initial relative locations of planets act as two important parameters which affect the eccentricity distribution of the surviving planets. As an application of our model, we discuss the observed orbital configurations of Kepler-34b and Kepler-413b. We first adopt the results from the disk-driven models as the initial conditions, then simulate the scattering process occurred in the late evolution stage of circumbinary planets. We show that the present orbital configurations of Kepler-34b and Kepler-413b can be well reproduced when considering two unequal-mass planet ejection model. Our work further suggests that some of the currently discovered circumbinary single-planet systems may be the survivals of original multiple-planet systems. The disk-driven migration and the scattering events occurring in the late stage both play an irreplaceable role in sculpting the final systems.

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.

The Influence of Orbital Resonances on the Water Transport to Objects in the Circumprimary Habitable Zone of Binary Star Systems

The Influence of Orbital Resonances on the Water Transport to Objects in the Circumprimary Habitable Zone of Binary Star Systems

Authors:

Bancelin et al

Abstract:
We investigate the role of secular and mean motion resonances on the water transport from a belt of icy asteroids onto planets or embryos orbiting inside the circumprimary habitable zone (HZ) of a binary star system. In addition, the host-star has an accompanying gas giant planet. For a comparison, we perform two case studies where a secular resonance (SR) is located either inside the HZ close to 1.0 au (causing eccentric motion of a planet or embryos therein) or in the asteroid belt, beyond the snow line. In the latter case, a higher flux of icy objects moving toward the HZ is expected. Collisions between asteroids and objects in the HZ are treated analytically. Our purely dynamical study shows that the SR in the HZ boosts the water transport; however, collisions can occur at very high impact speeds. In this paper, we treat for the first time, realistic collisions using a GPU 3D-SPH code to assess the water loss in the projectile. Including the water loss into the dynamical results, we get more realistic values for the water mass fraction of the asteroid during an impact. We highlight that collisions occurring at high velocities greatly reduce the water content of the projectile and thus the amount of water transported to planets or embryos orbiting inside the HZ. Moreover, we discuss other effects that could modify our results, namely the asteroid's surface rate recession due to ice sublimation and the atmospheric drag contribution on the asteroids' mass loss.

HD 202206: A Circumbinary Brown Dwarf System

HD 202206: A Circumbinary Brown Dwarf System 

Authors:

Benedict et al

Abstract:
Using Hubble Space Telescope Fine Guidance Sensor astrometry and previously published radial velocity measures, we explore the exoplanetary system HD 202206. Our modeling results in a parallax, ${\pi }_{\mathrm{abs}}=21.96\pm 0.12$ milliseconds of arc, a mass for HD 202206 B of ${{ \mathcal M }}_{B}={0.089}_{-0.006}^{+0.007}\,{{ \mathcal M }}_{\odot }$, and a mass for HD 202206 c of ${{ \mathcal M }}_{c}={17.9}_{-1.8}^{+2.9}\,{{ \mathcal M }}_{\mathrm{Jup}}$. HD 202206 is a nearly face-on G + M binary orbited by a brown dwarf. The system architecture that we determine supports past assertions that stability requires a 5:1 mean motion resonance (we find a period ratio, ${P}_{c}/{P}_{B}=4.92\pm 0.04$) and coplanarity (we find a mutual inclination, ${\rm{\Phi }}=6^\circ \pm 2^\circ $).

Habitability Properties of Circumbinary Planets

Habitability Properties of Circumbinary Planets

Author:

Shevchenko

Abstract:
It is shown that several habitability conditions (in fact, at least seven such conditions) appear to be fulfilled automatically by circumbinary planets of main-sequence stars (CBP-MS), whereas on Earth, these conditions are fulfilled only by chance. Therefore, it looks natural that most of the production of replicating biopolymers in the Galaxy is concentrated on particular classes of CBP-MS, and life on Earth is an outlier, in this sense. In this scenario, Lathe's mechanism for the tidal "chain reaction" abiogenesis on Earth is favored as generic for CBP-MS, due to photo-tidal synchronization inherent to them. Problems with this scenario are discussed in detail.

Dynamics and habitability in circumstellar planetary systems of known binary stars

Dynamics and habitability in circumstellar planetary systems of known binary stars

Authors:

Bazsó et al

Abstract:

We present a survey on exoplanetary systems of binary stars with stellar separations less than 100 au. For a sample of 11 binaries that harbour detected circumstellar giant planets we investigate the frequency of systems with secular resonances (SR) affecting the habitable zone (HZ). Secular resonances are connected to dynamically unstable or chaotic regions by enforcing highly eccentric motion. We apply a semi-analytical method to determine the locations of linear SR, which is based on finding the apsidal precession frequencies of the massive bodies. For configurations where the giant planet is located exterior to the HZ we find that there is always a SR interior to its orbit, the exact location of the SR strongly depends on the system's architecture. In systems with the giant planet interior to the HZ no SR can occur in the Newtonian framework. Taking into account the general relativistic precession of the perihelion, which increases the precession frequencies, planets with a < 0.1 au can cause SR in the HZ. We find two cases where the SR is located inside the HZ, and some more where it is close to the HZ. Generally, giant planets interior to the HZ are more favourable than exterior planets to avoid SR in the HZ. Around the location of the SR weaker mean-motion resonances are excited, and resonance overlap is possible. Existing analytical models are not as accurate as the semi-analytical method in locating the SR and deviate by ∼0.1 au or more.

OGLE-2007-BLG-349L(AB)c: The First Circumbinary Planet Found by Microlensing

The First Circumbinary Planet Found by Microlensing: OGLE-2007-BLG-349L(AB)c

Authors:

Bennett et al

Abstract:

We present the analysis of the first circumbinary planet microlensing event, OGLE-2007-BLG-349. This event has a strong planetary signal that is best fit with a mass ratio of q≈3.4×10−4, but there is an additional signal due to an additional lens mass, either another planet or another star. We find acceptable light curve fits with two classes of models: 2-planet models (with a single host star) and circumbinary planet models. The light curve also reveals a significant microlensing parallax effect, which constraints the mass of the lens system to be ML≈0.7M⊙. Hubble Space Telescope images resolve the lens and source stars from their neighbors, and indicate excess flux due to the star(s) in the lens system. This is consistent with the predicted flux from the circumbinary models, where the lens mass is shared between two stars, but there is not enough flux to be consistent with the 2-planet, 1-star models. So, only the circumbinary models are consistent with the HST data. They indicate a planet of mass mc=80±13M⊕, orbiting a pair of M-dwarfs with masses of MA=0.41±0.07M⊙ and MB=0.30±0.07M⊙, which makes this the lowest mass circumbinary planet system known. The ratio of the planet:center-of-mass separation to the separations of the two stars is ~40, so unlike most of the circumbinary planets found by Kepler, the planet does not orbit near the stability limit.

Secular and tidal evolution of circumbinary systems

Secular and tidal evolution of circumbinary systems

Authors:

Corriea et al

Abstract:

We investigate the secular dynamics of three-body circumbinary systems under the effect of tides. We use the octupolar non-restricted approximation for the orbital interactions, general relativity corrections, the quadrupolar approximation for the spins, and the viscous linear model for tides. We derive the averaged equations of motion in a simplified vectorial formalism, which is suitable to model the long-term evolution of a wide variety of circumbinary systems in very eccentric and inclined orbits. In particular, this vectorial approach can be used to derive constraints for tidal migration, capture in Cassini states, and stellar spin-orbit misalignment. We show that circumbinary planets with initial arbitrary orbital inclination can become coplanar through a secular resonance between the precession of the orbit and the precession of the spin of one of the stars. We also show that circumbinary systems for which the pericenter of the inner orbit is initially in libration present chaotic motion for the spins and for the eccentricity of the outer orbit. Because our model is valid for the non-restricted problem, it can also be applied to any three-body hierarchical system such as star-planet-satellite systems and triple stellar systems.

What is the Frequency of Exoplanets in Wide Circumbinary Orbits

SPOTS: The Search for Planets Orbiting Two Stars: II. First constraints on the frequency of sub-stellar companions on wide circumbinary orbits

Authors:

Bonavita et al

Abstract:

A large number of direct imaging surveys for exoplanets have been performed in recent years, yielding the first directly imaged planets and providing constraints on the prevalence and distribution of wide planetary systems. However, like most of the radial velocity ones, these surveys generally focus on single stars, hence binaries and higher-order multiples have not been studied to the same level of scrutiny. This motivated the SPOTS (Search for Planets Orbiting Two Stars) survey, which is an ongoing direct imaging study of a large sample of close binaries, started with VLT/NACO and now continuing with VLT/SPHERE. To complement this survey, we have identified the close binary targets in 24 published direct imaging surveys. Here we present our statistical analysis of this combined body of data. We analysed a sample of 117 tight binary systems, using a combined Monte Carlo and Bayesian approach to derive the expected values of the frequency of companions, for different values of the companion's semi-major axis. Our analysis suggest that the frequency of sub-stellar companions in wide orbit is moderately low (≲13% with a best value of 6% at 95% confidence level) and not significantly different between single stars and tight binaries. One implication of this result is that the very high frequency of circumbinary planets in wide orbits around post-common envelope binaries, implied by eclipse timing (up to 90% according to Zorotovic & Schreiber 2013), can not be uniquely due to planets formed before the common-envelope phase (first generation planets), supporting instead the second generation planet formation or a non-Keplerian origin of the timing variations.

Milankovitch Cycles of Terrestrial Planets in Binary Star Systems

Milankovitch Cycles of Terrestrial Planets in Binary Star Systems

Author:

Forgan

Abstract:

The habitability of planets in binary star systems depends not only on the radiation environment created by the two stars, but also on the perturbations to planetary orbits and rotation produced by the gravitational field of the binary and neighbouring planets. Habitable planets in binaries may therefore experience significant perturbations in orbit and spin. The direct effects of orbital resonances and secular evolution on the climate of binary planets remain largely unconsidered. We present latitudinal energy balance modelling of exoplanet climates with direct coupling to an N Body integrator and an obliquity evolution model. This allows us to simultaneously investigate the thermal and dynamical evolution of planets orbiting binary stars, and discover gravito-climatic oscillations on dynamical and secular timescales. We investigate the Kepler-47 and Alpha Centauri systems as archetypes of P and S type binary systems respectively. In the first case, Earthlike planets would experience rapid Milankovitch cycles (of order 1000 years) in eccentricity, obliquity and precession, inducing temperature oscillations of similar periods (modulated by other planets in the system). These secular temperature variations have amplitudes similar to those induced on the much shorter timescale of the binary period. In the Alpha Centauri system, the influence of the secondary produces eccentricity variations on 15,000 year timescales. This produces climate oscillations of similar strength to the variation on the orbital timescale of the binary. Phase drifts between eccentricity and obliquity oscillations creates further cycles that are of order 100,000 years in duration, which are further modulated by neighbouring planets.

Close Binary System HD 87646 Appears to Have a Brown Dwarf and a hot Jupiter

Very Low-Mass Stellar and Substellar Companions to Solar-like Stars From MARVELS VI: A Giant Planet and a Brown Dwarf Candidate in a Close Binary System HD 87646

Authors:

Ma et al

Abstract:

We report the detections of a giant planet (MARVELS-7b) and a brown dwarf candidate (MARVELS-7c) around the primary star in the close binary system, HD 87646. It is the first close binary system with more than one substellar circum-primary companion discovered to the best of our knowledge. The detection of this giant planet was accomplished using the first multi-object Doppler instrument (KeckET) at the Sloan Digital Sky Survey (SDSS) telescope. Subsequent radial velocity observations using ET at Kitt Peak National Observatory, HRS at HET, the "Classic" spectrograph at the Automatic Spectroscopic Telescope at Fairborn Observatory, and MARVELS from SDSS-III confirmed this giant planet discovery and revealed the existence of a long-period brown dwarf in this binary. HD 87646 is a close binary with a separation of ∼22 AU between the two stars, estimated using the Hipparcos catalogue and our newly acquired AO image from PALAO on the 200-inch Hale Telescope at Palomar. The primary star in the binary, HD 87646A, has Teff = 5770±80K, log(g)=4.1±0.1 and [Fe/H] = −0.17±0.08. The derived minimum masses of the two substellar companions of HD 87646A are 12.4±0.7MJup and 57.0±3.7MJup. The periods are 13.481±0.001 days and 674±4 days and the measured eccentricities are 0.05±0.02 and 0.50±0.02 respectively. Our dynamical simulations show the system is stable if the binary orbit has a large semi-major axis and a low eccentricity, which can be verified with future astrometry observations.

Are the NY Vir Circumbinary Exoplanets False Positives?

Observing NY Vir and the quest for circumbinary planets

Authors:

Pulley et al

Abstract:

We add 9 new observations of NY Vir and identify four others from AASVO database. Our results indicste that the one and two exo-planet predictions made by earlier authors do match these new results.

Relating binary-star planetary systems to central configurations

Relating binary-star planetary systems to central configurations

Author:

Veras

Abstract:

Binary-star exoplanetary systems are now known to be common, for both wide and close binaries. However, their orbital evolution is generally unsolvable. Special cases of the N-body problem which are in fact completely solvable include dynamical architectures known as central configurations. Here, I utilize recent advances in our knowledge of central configurations to assess the plausibility of linking them to coplanar exoplanetary binary systems. By simply restricting constituent masses to be within stellar or substellar ranges characteristic of planetary systems, I find that (i) this constraint reduces by over 90 per cent the phase space in which central configurations may occur, (ii) both equal-mass and unequal-mass binary stars admit central configurations, (iii) these configurations effectively represent different geometrical extensions of the Sun-Jupiter-Trojan-like architecture, (iv) deviations from these geometries are no greater than ten degrees, and (v) the deviation increases as the substellar masses increase. This study may help restrict future stability analyses to architectures which resemble exoplanetary systems, and might hint at where observers may discover dust, asteroids and/or planets in binary star systems.

Circumbinary Exoplanets in P Type Systems Survive Host Star Stellar Deaths

Planets that revolve around two suns may surprisingly survive the violent late stages of the stars' lives, according to new research out of the NASA Goddard Space Flight Centre and York University. The finding is surprising because planets orbiting close to a single sun, like Mercury and Venus in our solar system, would be destroyed when the aging star swells into a red giant.

Led by Veselin Kostov at the NASA Goddard Space Flight Centre, in collaboration with York University master's student Keavin Moore and Professor Ray Jayawardhana, the study found that planets orbiting two (binary) stars - also referred to as circumbinary planets or "Tatooine worlds" after the iconic planetary home of Luke Skywalker in Star Wars - often escape death and destruction by moving out to wider orbits.

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Exoplanets Around Binary Stars are Ejected, Don't Collide

Planet Scattering Around Binaries: Ejections, Not Collisions

Authors:

Smullen et al

Abstract:

Transiting circumbinary planets discovered by Kepler provide unique insight into binary star and planet formation. Several features of this new found population, for example the apparent pile-up of planets near the innermost stable orbit, may distinguish between formation theories. In this work, we determine how planet-planet scattering shapes planetary systems around binaries as compared to single stars. In particular, we look for signatures that arise due to differences in dynamical evolution in binary systems. We carry out a parameter study of N-body scattering simulations for four distinct planet populations around both binary and single stars. While binarity has little influence on the final system multiplicity or orbital distribution, the presence of a binary dramatically effects the means by which planets are lost from the system. Most circumbinary planets are lost due to ejections rather than planet-planet or planet-star collisions. The most massive planet in the system tends to control the evolution. Systems similar to the only observed multi-planet circumbinary system, Kepler-47, can arise from much more tightly packed, unstable systems. Only extreme initial conditions introduce differences in the final planet populations. Thus, we suggest that any intrinsic differences in the populations are imprinted by formation.

The Impact of Secular Resonances on Habitable Zones in Circumbinary Exoplanetary Systems

The impact of secular resonances on habitable zones in circumstellar planetary systems of known binary stars

Authors:

Baszó et al

Abstract:

We present a survey on binary star systems with stellar separations less than 100 astronomical units. For a selection of 11 binaries with a detected (giant) planet in circumstellar motion we determine the conditions that would allow additional planets to be present inside or nearby the habitable zone (HZ) of the host star.

First we calculate the three-body HZ for these systems, in order to investigate the dynamics of bodies in those regions. After adding the giant planet's influence the final HZ is considerably modified in particular by mean motion and secular resonances. We apply a semi-analytical method to determine the locations of linear secular resonances, which is based on finding the apsidal precession frequencies of the massive bodies.

For very close-in giant planets we also take the general relativistic precession of the pericenter into account.

Our results demonstrate that there is a qualitative difference in the dynamics whether the giant planet is located exterior or interior to the HZ. An exterior giant planet is more likely to cause a secular resonance than an interior planet. Exterior planets play an important role for the HZ by enforcing highly eccentric orbits on potentially habitable planets. For interior planets generally we do not find a secular resonance in the Newtonian framework, but the general relativistic precession introduces another source for a secular resonance.

In 7 out of 11 systems secular resonances play a role. We compare our semi-analytical method to various purely analytical models, and show that it is more accurate in determining the location of secular resonances.

Line-depth-ratio temperatures for the close binary nu Octantis: new evidence supporting the conjectured retrograde planet

Line-depth-ratio temperatures for the close binary nu Octantis: new evidence supporting the conjectured retrograde planet

Author:

Ramm et al

Abstract:

We explore the possibly that either star-spots or pulsations are the cause of a periodic radial-velocity signal (P~400 days) from the K-giant binary nu Octantis (P~1050 days, e~0.25), alternatively conjectured to have a retrograde planet. Our study is based on temperatures derived from 22 line-depth ratios (LDRs) for nu Oct and twenty calibration stars. Empirical evidence and stability modelling provide unexpected support for the planet since other standard explanations (star-spots, pulsations and additional stellar masses) each have credibility problems. However, the proposed system presents formidable challenges to planet-formation and stability theories: it has by far the smallest stellar separation of any claimed planet-harbouring binary (a_bin~2.6 au) and an equally unbelievable separation ratio (a_pl/a_bin~0.5), hence the necessity that the circumstellar orbit be retrograde.

The LDR analysis of 215 nu Oct spectra acquired between 2001--2007, from which the RV perturbation was first revealed, have no significant periodicity at any frequency. The LDRs recover the original 21 stellar temperatures with an average accuracy of 45+/-25 K. The 215 nu Oct temperatures have a standard deviation of only 4.2 K. Assuming the host primary is not pulsating, the temperatures converted to magnitude differences strikingly mimic the very stable photometric Hipparcos observations 15 years previously, implying the long-term stability of the star and demonstrating a novel use of LDRs as a photometric gauge. Our results provide substantial new evidence that conventional star-spots and pulsations are unlikely causes of the RV perturbation. The controversial system deserves continued attention, including with higher resolving-power spectra for bisector and LDR analyses.

Studying the HD 41004AB Circumbinary System

Circumstellar multi-planetary systems in binary stars: secular resonances and a semi-analytical approach to determine the location

Authors:

Pilat-Lohinger et al

Abstract:

Binary stars are of special interest for studies of planetary motion and habitability as most of the stars in the solar neighborhood are part of such stellar systems. Since a secondary star causes gravitational perturbations the planetary motion is restricted to certain regions of the phase space depending on the binary configuration. In case a binary system hosts a giant planet it is obvious that additional perturbations will occur. These perturbations will be studied in detail in this investigation where we take into account various binary-planet configurations. We show how the dynamics of another test-planet is influenced by mean motion and secular resonances. Therefore, it is important to know the locations of these resonances. First, we study the binary system HD41004AB to visualize the perturbations on the dynamics of test-planets caused by the secondary star and the detected giant planet. Then we perform a frequency analysis of the orbits to identify of the secular resonance. And finally, we develop a new semi-analytical approach to determine the location of this resonance.

Circumbinary Exoplanets get Scattered, not Collide

Planet Scattering Around Binaries: Ejections, Not Collisions

Authors:

Smullen et al

Abstract:

Transiting circumbinary planets discovered by Kepler provide unique insight into binary and planet formation. Several features of this new found population, for example the apparent pile-up of planets near the innermost stable orbit, may distinguish between formation theories. In this work, we determine how planet-planet scattering shapes planetary systems around binaries as compared to single stars. In particular, we look for signatures that arise due to differences in dynamical evolution in binary systems. We carry out a parameter study of N-body scattering simulations for four distinct planet populations around both binary and single stars. While binarity has little influence on the final system multiplicity or orbital distribution, the presence of a binary dramatically effects the means by which planets are lost from the system. Most circumbinary planets are lost due to ejections rather than planet-planet or planet-star collisions. The most massive planet in the system tends to control the evolution. Aside from the absence of very close-in planets, which are unstable around a binary, dynamical evolution alone does not introduce systematic differences in the final planet populations. Thus, we suggest that any intrinsic differences in the populations are imprinted by formation.