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Circumbinary planets
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Title: On the Fate of Unstable Circumbinary Planets: Tatooine's Close Encounters with a Death Star
Author: Adam P. Sutherland, Daniel C. Fabrycky

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.

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Posts: 131433
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Tatooine planets
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Astronomers bring a new hope to find 'Tatooine' planets

Astronomers could discover a plethora of planets around binary star systems (stars that rotate around each other) by measuring with high precision how stars move around each other, looking for disturbances exerted by possible exoplanets.
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Earthlike 'Star Wars' Tatooines may be common

Luke Skywalker's home in "Star Wars" is the desert planet Tatooine, with twin sunsets because it orbits two stars. So far, only uninhabitable gas-giant planets have been identified circling such binary stars, and many researchers believe rocky planets cannot form there. Now, mathematical simulations show that Earthlike, solid planets such as Tatooine likely exist and may be widespread.
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Posts: 131433
Date:
Binary exoplanet host stars
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Title: Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound
Author: Elliott P. Horch, Steve B. Howell, Mark E. Everett, David R. Ciardi

Using the known detection limits for high-resolution imaging observations and the statistical properties of true binary and line-of-sight companions, we estimate the binary fraction of Kepler exoplanet host stars. Our speckle imaging programs at the WIYN 3.5-m and Gemini North 8.1-m telescopes have observed over 600 Kepler objects of interest (KOIs) and detected 49 stellar companions within ~1 arcsecond. Assuming binary stars follow a log-normal period distribution for an effective temperature range of 3,000 to 10,000 K, then the model predicts that the vast majority of detected sub-arcsecond companions are long period (P>50 years), gravitationally bound companions. In comparing the model predictions to the number of real detections in both observational programs, we conclude that the overall binary fraction of host stars is similar to the 40-50\% rate observed for field stars.

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RE: Double sun sunset
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Title: Circumbinary Habitability Niches
Author: Paul A. Mason, Jorge I. Zuluaga, Pablo A. Cuartas-Restrepo, Joni M. Clark

Binaries could provide the best niches for life in the galaxy. Though counterintuitive, this assertion follows directly from stellar tidal interaction theory and the evolution of lower mass stars. There is strong evidence that chromospheric activity of rapidly rotating young stars may be high enough to cause mass loss from atmospheres of potentially habitable planets. The removal of atmospheric water is most critical. Tidal breaking in binaries could help reduce magnetic dynamo action and thereby chromospheric activity in favor of life. We call this the Binary Habitability Mechanism (BHM), that we suggest allows for water retention at levels comparable to or better than Earth. We discuss novel advantages that life may exploit, in these cases, and suggest that life may even thrive on some circumbinary planets. We find that while many binaries do not benefit from BHM, high quality niches do exist for various combinations of stars between 0.55 and 1.0 solar masses. For a given pair of stellar masses, BHM operates only for certain combinations of period and eccentricity. Binaries having a solar-type primary seem to be quite well suited niches having wide and distant habitable zones with plentiful water and sufficient light for photosynthetic life. We speculate that, as a direct result of BHM, conditions may be suitable for life on several planets and possibly even moons of giant planets orbiting some binaries. Lower mass combinations, while more restrictive in parameter space, provide niches lasting many billions of years and are rich suppliers of photosynthetic photons. We provide a publicly available web-site (\href{http://bit.ly/BHM-calculator}{this http URL}, \href{http://bit.ly/BHM-calculator-mirror}{this http URL}), which calculates the BHM effects presented in this paper.

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Posts: 131433
Date:
Binary Star Exoplanets
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Title: Exoplanets Bouncing Between Binary Stars
Authors: Nickolas Moeckel, Dimitri Veras

Exoplanetary systems are found not only among single stars, but also binaries of widely varying parameters. Binaries with separations of 100--1000 au are prevalent in the Solar neighbourhood; at these separations planet formation around a binary member may largely proceed as if around a single star. During the early dynamical evolution of a planetary system, planet--planet scattering can eject planets from a star's grasp. In a binary, the motion of a planet ejected from one star has effectively entered a restricted three-body system consisting of itself and the two stars, and the equations of motion of the three body problem will apply as long as the ejected planet remains far from the remaining planets. Depending on its energy, escape from the binary as a whole may be impossible or delayed until the three-body approximation breaks down, and further close interactions with its planetary siblings boost its energy when it passes close to its parent star. Until then this planet may be able to transition from the space around one star to the other, and chaotically `bounce' back and forth. In this paper we directly simulate scattering planetary systems that are around one member of a circular binary, and quantify the frequency of bouncing in scattered planets. We find that a great majority (70 to 85 per cent) of ejected planets will pass at least once through the space of it's host's binary companion, and depending on the binary parameters about 45 to 75 per cent will begin bouncing. The time spent bouncing is roughly log-normally distributed with a peak at about 10^4 years, with only a small percentage bouncing for more than a Myr. This process may perturb and possibly incite instability among existing planets around the companion star. In rare cases, the presence of multiple planets orbiting both stars may cause post-bouncing capture or planetary swapping.

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Posts: 131433
Date:
T Tauri Binary Systems
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Title: Protoplanetary Disks of T T Binary Systems in the Orion Nebula Cluster
Authors: Sebastian Daemgen, Serge Correia, Monika G. Petr-Gotzens

We present a study of protoplanetary disks in spatially resolved low-mass binary stars in the well-known Orion Nebula Cluster (ONC) in order to assess the impact of binarity on the properties of circumstellar disks and its relation to the cluster environment. This is the currently largest such study in a clustered high stellar density star forming environment. We particularly aim at determining the presence of magnetospheric accretion and dust disks for each binary component, and at measuring the overall disk frequency. We carried out spatially resolved Adaptive Optics assisted near-IR photometry and spectroscopy of 26 binaries in the ONC, and determine stellar parameters such as effective temperatures and spectral types, luminosities, masses, as well as accretion properties and near-infrared excess for individual binary components. A fraction of 40(+10/-9)% of the binary components in the sample can be inferred to be T Tauri stars possessing an accretion disk. This is marginally lower than the disk fraction of single stars of ~50% in the ONC. We find that disks in wide binaries of >200AU separation are consistent with random pairing, while the evolution of circumprimary and circumsecondary disks is observed to be synchroniSed in closer binaries. Circumbinary disks appear to be not suited to explain this difference. Further, we identify several mixed pairs of accreting and non-accreting components, suggesting that these systems are common, and without preference for the more or less massive component to evolve faster. The derived mass accretion rates of the ONC binary components are of similar magnitude as those for ONC single stars and for binaries in the Taurus star forming region. The paper concludes with a discussion of the (presumably weak) connection between the presence of inner accretion disks in young binary systems and the existence of planets in stellar multiples.

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Posts: 131433
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RE: Double sun sunset
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Title: Eccentricity of radiative discs in close binary-star systems
Authors: F. Marzari, C. Baruteau, H. Scholl, P. Thebault

Discs in binaries have a complex behaviour because of the perturbations of the companion star. Planet formation in binary-star systems both depend on the companion star parameters and on the properties of the circumstellar disc. An eccentric disc may increase the impact velocity of planetesimals and therefore jeopardize the accumulation process. We model the evolution of discs in close binaries including the effects of self-gravity and adopting different prescriptions to model the disc's radiative properties. We focus on the dynamical properties and evolutionary tracks of the discs. We use the hydrodynamical code FARGO and we include in the energy equation heating and cooling effects. Radiative discs have a lower disc eccentricity compared to locally isothermal discs with same temperature profile. As a consequence, we do not observe the formation of an internal elliptical low density region as in locally isothermal disc models. However, the disc eccentricity depends on the disc mass through the opacities. Akin to locally isothermal disc models, self-gravity forces the disc's longitude of pericenter to librate about a fixed orientation with respect to the binary apsidal line (\pi). The disc's radiative properties play an important role in the evolution of discs in binaries. A radiative disc has an overall shape and internal structure that are significantly different compared to a locally isothermal disc with same temperature profile. This is an important finding both for describing the evolutionary track of the disc during its progressive mass loss, and for planet formation since the internal structure of the disc is relevant for planetesimals growth in binary systems. The non-symmetrical distribution of mass in these discs causes large eccentricities for planetesimals that may affect their growth.

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Posts: 131433
Date:
Circumbinary Planets
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Title: The Kepler Search for Circumbinary Planets
Authors: William F. Welsh, Kepler Team

We present the latest results from the Kepler search for planets in binary star systems, including transiting and non-transiting cases. In many cases, eclipse timing variations (ETVs) indicate the presence of a third body in the system. If the O-C diagram exhibits a small amplitude and short-period signal, that third body may be planetary. Kepler-16, along with several other candidate systems, will be discussed. Kepler was selected as the 10th mission of the Discovery Program. Funding for this mission is provided by NASA, Science Mission Directorate. The authors also acknowledge support from the Kepler Participating Scientists Program via NASA grant NNX08AR14G.

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Posts: 131433
Date:
Double sun sunset
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Title: GSC 07396-00759 = V4046 Sgr C[D]: a Wide-separation Companion to the Close T Tauri Binary System V4046 Sgr AB
Authors: J. H. Kastner (1), G. G. Sacco (1), R. Montez Jr. (1), D. P. Huenemoerder (2), H. Shi (2), E. Alecian (3), C. Argiroffi (4,5), M. Audard (6,7), J. Bouvier (8), F. Damiani (5), J.-F. Donati (9), S. G. Gregory (10), M. Güdel (11), G. A. J. Hussain (12), A. Maggio (5), T. Montmerle (13) ((1) Center for Imaging Science, Rochester Institute of Technology, (2) MIT, Kavli Institute for Astrophysics and Space Research, (3) Observatoire de Paris, (4) Univ. di Palermo, (5) INAF - Osservatorio Astronomico di Palermo, (6) University of Geneva, (7) Observatoire de Genéve, (8) UJF-Grenoble, (9) CNRS & Univ. de Toulouse, (10) California Institute of Technology, (11) University of Vienna, (12) ESO, Garching, (13) Institut d'Astrophysique de Paris)

We explore the possibility that GSC 07396-00759 (spectral type M1e) is a widely separated (~2.82', or projected separation ~12,350 AU) companion to the "old" (age ~12 Myr) classical T Tauri binary system V4046 Sgr AB, as suggested by the proximity and similar space motions of the two systems. If the two systems are equidistant and coeval, then GSC 07396--00759, like V4046 Sgr AB, must be a spectroscopic binary with nearly equal-mass components, and V4046 Sgr must be at least ~8 Myr old. Analysis of a serendipitous Chandra X-ray gratings spectrum and light curve as well as XMM-Newton light curves and CCD spectra of GSC 07396-00759 obtained during long exposures targeting V4046 Sgr AB reveals a relatively hard (T_X ~ 10^7 K) X-ray spectrum, strong flaring, and relatively low-density plasma. These X-ray characteristics of GCS 07396--00759 are indicative of a high level of coronal activity, consistent with its apparent weak-lined T Tauri star status. Interactions between V4046 Sgr AB and GCS 07396-00759 when the two systems were more closely bound may be responsible for (a) their dissolution ~10^6 yr ago, (b) the present tight, circular orbit of V4046 Sgr AB, and (c) the persistence of the gaseous circumbinary disk still orbiting V4046 Sgr AB.

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