Fitting Formulae and Constraints for the Existence of S-type and P-type Habitable Zones in Binary Systems

Fitting Formulae and Constraints for the Existence of S-type and P-type Habitable Zones in Binary Systems

Authors:

Wang et al

Abstract: 

We derive fitting formulae for the quick determination of the existence of S-type and P-type habitable zones (HZs) in binary systems. Based on previous work, we consider the limits of the climatological HZ in binary systems (which sensitively depend on the system parameters) based on a joint constraint encompassing planetary orbital stability and a habitable region for a possible system planet. Additionally, we employ updated results on planetary climate models obtained by Kopparapu and collaborators. Our results are applied to four P-type systems (Kepler-34, Kepler-35, Kepler-413, and Kepler-1647) and two S-type systems (TrES-2 and KOI-1257). Our method allows us to gauge the existence of climatological HZs for these systems in a straightforward manner with detailed consideration of the observational uncertainties. Further applications may include studies of other existing systems as well as systems to be identified through future observational campaigns.

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 $).

The Fate of Tatooine-like Circumbinary Exoplanets

TATOOINE'S FUTURE: THE ECCENTRIC RESPONSE OF KEPLER'S CIRCUMBINARY PLANETS TO COMMON-ENVELOPE EVOLUTION OF THEIR HOST STARS

Authors:

Kostov et al

Abstract:

Inspired by the recent Kepler discoveries of circumbinary planets orbiting nine close binary stars, we explore the fate of the former as the latter evolve off the main sequence. We combine binary star evolution models with dynamical simulations to study the orbital evolution of these planets as their hosts undergo common-envelope (CE) stages, losing in the process a tremendous amount of mass on dynamical timescales. Five of the systems experience at least one Roche-lobe overflow and CE stage (Kepler-1647 experiences three), and the binary stars either shrink to very short orbits or coalesce; two systems trigger a double-degenerate supernova explosion. Kepler's circumbinary planets predominantly remain gravitationally bound at the end of the CE phase, migrate to larger orbits, and may gain significant eccentricity; their orbital expansion can be more than an order of magnitude and can occur over the course of a single planetary orbit. The orbits these planets can reach are qualitatively consistent with those of the currently known post-CE, eclipse-time variations circumbinary candidates. Our results also show that circumbinary planets can experience both modes of orbital expansion (adiabatic and nonadiabatic) if their host binaries undergo more than one CE stage; multiplanet circumbinary systems like Kepler-47 can experience both modes during the same CE stage. Additionally, unlike Mercury orbiting the Sun, a circumbinary planet with the same semimajor axis can survive the CE evolution of a close binary star with a total mass of 1 ${M}_{\odot }$.

Hot Jupiter WASP-20b is Really in a Binary Star System

WASP-20 is a close visual binary with a transiting hot Jupiter

Authors:

Evans et al

Abstract:

We announce the discovery that WASP-20 is a binary stellar system, consisting of two components separated by 0.2578±0.0007′′ on the sky, with a flux ratio of 0.4639±0.0015 in the K-band. It has previously been assumed that the system consists of a single F9 V star, with photometric and radial velocity signals consistent with a low-density transiting giant planet. With a projected separation of approximately 60 au between the two components, the detected planetary signals almost certainly originate from the brighter of the two stars. We reanalyse previous observations allowing for two scenarios, `planet transits A' and `planet transits B', finding that both cases remain consistent with a transiting gas giant. However, we rule out the `planet transits B' scenario because the observed transit duration requires star B to be significantly evolved, and therefore have an age much greater than star A. We outline further observations which can be used to confirm this finding. Our preferred `planet transits A' scenario results in the measured mass and radius of the planet increasing by 4σ and 1σ, respectively.

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.

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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How Circumbinary Gas Giants Kepler-16 & Kepler-34 Could Have Formed

Modelling circumbinary protoplanetary disks II. Gas disk feedback on planetesimal dynamical and collisional evolution in the circumbinary systems Kepler-16 and 34

Authors:

Lines et al

Abstract:

Aims.

We investigate the feasibility of planetesimal growth in circumbinary protoplanetary disks around the observed systems Kepler- 16 and Kepler-34 under the gravitational influence of a precessing eccentric gas disk.

Methods.

We embed the results of our previous hydrodynamical simulations of protoplanetary disks around binaries into an N-body code to perform 3D, high-resolution, inter-particle gravity-enabled simulations of planetesimal growth and dynamics that include the gravitational force imparted by the gas.

Results.

Including the full, precessing asymmetric gas disk generates high eccentricity orbits for planetesimals orbiting at the edge of the circumbinary cavity, where the gas surface density and eccentricity have their largest values. The gas disk is able to efficiently align planetesimal pericenters in some regions leading to phased, non-interacting orbits. Outside of these areas eccentric planetesimal orbits become misaligned and overlap leading to crossing orbits and high relative velocities during planetesimal collisions. This can lead to an increase in the number of erosive collisions that far outweighs the number of collisions that result in growth. Gravitational focusing from the static axisymmetric gas disk is weak and does not significantly alter collision outcomes from the gas free case.

Conclusions.

Due to asymmetries in the gas disk, planetesimals are strongly perturbed onto highly eccentric orbits. Where planetesimals orbits are not well aligned, orbit crossings lead to an increase in the number of erosive collisions. This makes it difficult for sustained planetesimal accretion to occur at the location of Kepler-16b and Kepler-34b and we therefore rule out in-situ growth. This adds further support to our initial suggestions that most circumbinary planets should form further out in the disk and migrate inwards.

EPIC 211089792b: an Inflated hot Jupiter in a Binary Star System

EPIC211089792 b: an aligned and inflated hot jupiter in a young visual binary

Authors:

Santerne et al

Abstract:

In the present paper we report the discovery of a new hot Jupiter, EPIC211089792 b, first detected by the Super-WASP observatory and then by the K2 space mission during its campaign 4. The planet has a period of 3.25d, a mass of 0.73 +/- 0.04 Mjup, and a radius of 1.19 +/- 0.02 Rjup. The host star is a relatively bright (V=12.5) G7 dwarf with a nearby K5V companion. Based on stellar rotation and the abundance of Lithium, we find that the system might be as young as about 450 Myr. The observation of the Rossiter-McLaughlin effect shows the planet is aligned with respect to the stellar spin. Given the deep transit (20mmag), the magnitude of the star and the presence of a nearby stellar companion, the planet is a good target for both space- and ground-based transmission spectroscopy, in particular in the near-infrared where the both stars are relatively bright.

Binary Star System AK Sco Appears to Have Circumbinary Planets

Detection of Sharp Symmetric Features in the Circumbinary Disk Around AK Sco

Authors:

Jansen et al

Abstract:

The Search for Planets Orbiting Two Stars (SPOTS) survey aims to study the formation and distribution of planets in binary systems by detecting and characterizing circumbinary planets and their formation environments through direct imaging. With the SPHERE Extreme Adaptive Optics instrument, a good contrast can be achieved even at small (less than 300 mas) separations from bright stars, which enables studies of planets and disks in a separation range that was previously inaccessible. Here, we report the discovery of resolved scattered light emission from the circumbinary disk around the well-studied young double star AK Sco, at projected separations in the ~13--40 AU range. The sharp morphology of the imaged feature is surprising, given the smooth appearance of the disk in its spectral energy distribution. We show that the observed morphology can be represented either as a highly eccentric ring around AK Sco, or as two separate spiral arms in the disk, wound in opposite directions. The relative merits of these interpretations are discussed, as well as whether these features may have been caused by one or several circumbinary planets interacting with the disk.

The Fate of Circumbinary ExoPlanets - Tatooines - in Unstable Orbits

On the Fate of Unstable Circumbinary Planets: Tatooine's Close Encounters with a Death Star

Authors:

Sutherland et al

Abstract:

Circumbinary planets whose orbits become unstable may be ejected, accreted, or even captured by one of the stars. We quantify the relative rates of these channels, for a binary of secondary star's mass fraction 0.1 with an orbit of 1AU. The most common outcome is ejection, which happens ~80% of the time. If binary systems form circumbinary planets readily and sloppily, this process may fill the Milky Way with free-floating planets. A significant fraction of the time, ~20%, the unstable planet strikes the primary or secondary. We tracked whether a Jupiter-like planet would undergo tidal stripping events during close passages, and find that these events are not strong enough to change the trajectory of the planet, though this may be observable from a changed structured for free-floating planets that are produced by this process.

Detectability of Circumbinary ExoPlanets With Microlensing

Caustic Structures and Detectability of Circumbinary Planets in Microlensing

Authors:

Luhn et al

Abstract:

Recent discoveries of circumbinary planets in Kepler data show that there is a viable channel of planet formation around binary main sequence stars. Motivated by these discoveries, we have investigated the caustic structures and detectability of circumbinary planets in microlensing events. We have produced a suite of animations of caustics as a function of the projected separation and angle of the binary host to efficiently explore caustic structures over the entire circumbinary parameter space. Aided by these animations, we have derived a semi-empirical analytic expression for the location of planetary caustics, which are displaced in circumbinary lenses relative to those of planets with a single host. We have used this expression to show that the dominant source of caustic motion will be due to the planet's orbital motion and not that of the binary star. Finally, we estimate the fraction of circumbinary microlensing events that are recognizable as such to be significant (5-50 percent) for binary projected separations in the range 0.1-0.5 in units of Einstein radii.

Eclipsing Binary FL Lyr has a Circumbinary Jupiter Analog Gas Giant

An Exo-Jupiter Candidate in the Eclipsing Binary FL Lyr

Authors:

Kozyreva et al

Abstract:

Light curves of the eclipsing binary FL Lyr acquired by the Kepler space telescope are analyzed. Eclipse timing measurements for FL Lyr testify to the presence of a third body in the system. Preliminary estimates of its mass and orbital period are greater than 2 MJupiter and greater than 7 yrs. The times of primary minimum in the light curve of FL Lyr during the operation of the Kepler mission are presented.

Eclipsing Binary CoRoT 223992193 Appears to Have a Circumpibary Protoplanetary Disk

A circumbinary disc model for the variability of the eclipsing binary CoRoT 223992193

Authors:

Terquem et al

Abstract:

We calculate the flux received from a binary system obscured by a circumbinary disc. The disc is modelled using two dimensional hydrodynamical simulations, and the vertical structure is derived by assuming it is isothermal. The gravitational torque from the binary creates a cavity in the disc's inner parts. If the line of sight along which the system is observed has a high inclination I, it intersects the disc and some absorption is produced. As the system is not axisymmetric, the resulting light curve displays variability. We calculate the absorption and produce light curves for different values of the dust disc aspect ratio H/r and mass of dust in the cavity Mdust. This model is applied to the high inclination (I=85∘) eclipsing binary CoRoT 223992193, which shows 5-10% residual photometric variability after the eclipses and a spot model are subtracted. We find that such variations for I∼85∘ can be obtained for H/r=10−3 and Mdust≥10−12 M⊙. For higher H/r, Mdust would have to be close to this lower value and I somewhat less than 85∘. Our results show that such variability in a system where the stars are at least 90% visible at all phases can be obtained only if absorption is produced by dust located inside the cavity. If absorption is dominated by the parts of the disc located close to or beyond the edge of the cavity, the stars are significantly obscured.

Tatooine Nurseries

Tatooine Nurseries: Structure and Evolution of Circumbinary Protoplanetary Disks

Authors:

Vartanyan et al

Abstract:

Recent discoveries of circumbinary planets by Kepler mission provide motivation for understanding their birthplaces - protoplanetary disks around stellar binaries with separations less than 1 AU. We explore properties and evolution of such circumbinary disks focusing on modification of their structure caused by tidal coupling to the binary. We develop a set of analytical scaling relations describing viscous evolution of the disk properties, which are verified and calibrated using 1D numerical calculations with realistic inputs. Injection of angular momentum by the central binary suppresses mass accretion onto the binary and causes radial distribution of the viscous angular momentum flux F_J to be different from that in a standard accretion disk around a single star with no torque at the center. Disks with no mass accretion at the center develop F_J profile which is flat in radius. Radial profiles of temperature and surface density are also quite different from those in disks around single stars. Damping of the density waves driven by the binary and viscous dissipation dominate heating of the inner disk (within 1-2 AU), pushing the iceline beyond 3-5 AU, depending on disk mass and age. Irradiation by the binary governs disk thermodynamics beyond ~10 AU. However, self-shadowing by the hot inner disk may render central illumination irrelevant out to ~20 AU. Spectral energy distribution of a circumbinary disk exhibits a distinctive bump around 10 micron, which may facilitate identification of such disks around unresolved binaries. Efficient tidal coupling to the disk drives orbital inspiral of the binary and may cause low-mass and compact binaries to merge into a single star within the disk lifetime. We generally find that circumbinary disks present favorable sites for planet formation (despite wider zone of volatile depletion), in agreement with the statistics of Kepler circumbinary planets.

The Lack of Kozai-Lidov Cycles for Most Circumbinary Exoplanets

Kozai-Lidov cycles towards the limit of circumbinary planets

Authors:

Martin et al

Abstract:

In this paper we answer a simple question: can a misaligned circumbinary planet induce Kozai-Lidov cycles on an inner stellar binary? We use known analytic equations to analyse the behaviour of the Kozai-Lidov effect as the outer mass is made small. We demonstrate a significant departure from the traditional symmetry, critical angles and amplitude of the effect. Aside from massive planets on near-polar orbits, circumbinary planetary systems are devoid of Kozai-Lidov cycles. This has positive implications for the existence of highly misaligned circumbinary planets: an observationally unexplored and theoretically important parameter space.

Near-Infrared Polarimetry of the GG Tauri A Binary System

Near-Infrared Polarimetry of the GG Tauri A Binary System

Authors:

Itoh et al

Abstract:

A high angular resolution near-infrared polarized-intensity image of the GG Tau A binary system was obtained with the Subaru Telescope. The image shows the circumbinary disk scattering the light from the central binary. The azimuthal profile of the polarized intensity of the circumbinary disk is roughly reproduced by a simple disk model with the Henyey-Greenstein function and the Rayleigh function, indicating small dust grains at the surface of the disk. Combined with a previous observation of the circumbinary disk, our image indicates that the gap structure in the circumbinary disk orbits anti-clockwise, while material in the disk orbit clockwise. We propose a shadow of material located between the central binary and the circumbinary disk. The separations and position angles of the stellar components of the binary in the past 20 years are consistent with the binary orbit with a = 33.4 AU and e = 0.34.

Modeling the Formation of Kepler-16 and Kepler-34 Circumbinary Exoplanetary Systems

Modelling circumbinary protoplanetary disks: I. Fluid simulations of the Kepler-16 and 34 systems

Authors:

Lines et al

Abstract:

The Kepler mission's discovery of a number of circumbinary planets orbiting close (a_p less than 1.1 au) to the stellar binary raises questions as to how these planets could have formed given the intense gravitational perturbations the dual stars impart on the disk. The gas component of circumbinary protoplanetary disks is perturbed in a similar manner to the solid, planetesimal dominated counterpart, although the mechanism by which disk eccentricity originates differs. This is the first work of a series that aims to investigate the conditions for planet formation in circumbinary protoplanetary disks. We present a number of hydrodynamical simulations that explore the response of gas disks around two observed binary systems: Kepler-16 and Kepler-34. We probe the importance of disk viscosity, aspect-ratio, inner boundary condition, initial surface density gradient, and self-gravity on the dynamical evolution of the disk, as well as its quasi steady-state profile. We find there is a strong influence of binary type on the mean disk eccentricity, e_d, leading to e_d = 0.02 - 0.08 for Kepler-16 and e_d = 0.10 - 0.15 in Kepler-34. The value of alpha-viscosity has little influence on the disk, but we find a strong increase in mean disk eccentricity with increasing aspect-ratio due to wave propagation effects. The choice of inner boundary condition only has a small effect on the surface density and eccentricity of the disk. Our primary finding is that including disk self-gravity has little impact on the evolution or final state of the disk for disks with masses less than 12.5 times that of the minimum-mass solar nebula. This finding contrasts with the results of self-gravity relevance in circumprimary disks, where its inclusion is found to be an important factor in describing the disk evolution.

How a Circumbinary Gas Giant can end up Misaligned

Decoupling of a giant planet from its disk in an inclined binary system

Authors:

Picogna et al

Abstract:

We explore the dynamical evolution of a planet embedded in a disk surrounding a star part of a binary system where the orbital plane of the binary is significantly tilted respect to the initial disk plane. Our aim is to test whether the planet remains within the disk and continues to migrate towards the star in a Type I/II mode in spite of the secular perturbations of the companion star. This would explain observed exoplanets with significant inclination respect to the equatorial plane of their host star. We have used two different SPH codes, vine and phantom, to model the evolution of a system star+disk+planet and companion star with time. After an initial coupled evolution, the inclination of the disk and that of the planet begin to differ significantly. The period of oscillation of the disk inclination, respect to the initial plane, is shorter than that of the planet which evolves independently after about 10^4 yr following a perturbed N-body behavior. However, the planet keeps migrating towards the star because during its orbital motion it crosses the disk plane and the friction with the gas causes angular momentum loss. Disk and planet in a significantly inclined binary system are not dynamically coupled for small binary separations but evolve almost independently. The planet abandons the disk and, due to the onset of a significant mutual inclination, it interacts with the gas only when its orbit intersects the disk plane. The drift of the planet towards the star is not due to type I/II with the planet embedded in the disk but to the friction with the gas during the disk crossing.

Kepler-453b Artwork: the 10th Transiting Tatooine

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Kepler Compact Binary Stellar Systems With Exoplanets may be False Positives, Really Trinary Systems

No circumbinary planets transiting the tightest Kepler binaries - a fingerprint of a third star

Authors:

Martin et al

Abstract:

The Kepler mission has yielded the discovery of eight eclipsing binaries, within period range of 7 - 40 d, hosting circumbinary planets. This is longer than the typical eclipsing binary period found by Kepler, and hence there is a dearth of planets around the closest binaries. In this paper we demonstrate how this dearth may be explained by the presence of a distant stellar tertiary companion, which shrunk the inner binary orbit by the process of Kozai cycles and tidal friction, a mechanism that has been implicated for producing most binaries with periods below 7 d. We show that the geometry and orbital dynamics of these evolving triple-star systems are highly restrictive for a circumbinary planet, which is subject itself to Kozai modulation, on one hand, and can shield the two inner stars from their Kozai cycle and subsequent shrinking, on the other hand. Only small planets on wide and inclined orbits may form, survive and allow for the inner binary shrinkage. Those are difficult to detect.