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The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopicsubtypes, and stellar population subcomponents of the white dwarfswithin 20 pc of the Sun. We find no convincing evidence of halo whitedwarfs in the total 20 pc sample of 129 white dwarfs nor is thereconvincing evidence of genuine thick disk subcomponent members within 20parsecs. Virtually, the entire 20 pc sample likely belongs to the thindisk. The total DA to non-DA ratio of the 20 pc sample is 1.6, amanifestation of deepening envelope convection which transforms DA starswith sufficiently thin H surface layers into non-DAs. The addition offive new stars to the 20 pc sample yields a revised local space densityof white dwarfs of (4.9 ± 0.5) × 10–3pc–3 and a corresponding mass density of (3.3 ±0.3) × 10–3 M sunpc–3. We find that at least 15% of the white dwarfswithin 20 parsecs of the Sun (the DAZ and DZ stars) have photosphericmetals that possibly originate from accretion of circumstellar material(debris disks) around them. If this interpretation is correct, thissuggests the possibility that the same percentage have planets orasteroid-like bodies orbiting them.
| Planets and Debris Disks: Results from a Spitzer/MIPS Search for Infrared Excess
Using the MIPS camera on the Spitzer Space Telescope, we have searchedfor debris disks around 104 stars known from radial velocity studies tohave one or more planets. Combining this new data with 42 alreadypublished observations of planet-bearing stars, we find that 14 of the146 systems have IR excess at 24 and/or 70 μm. Only one star, HD69830, has IR excess exclusively at 24 μm, indicative of warm dust inthe inner system analogous to that produced by collisions in the solarsystem's asteroid belt. For the other 13 stars with IR excess theemission is stronger at 70 μm, consistent with cool dust (<100 K)located beyond 10 AU, well outside of the orbital location of the knownplanets. Selection effects inhibit detection of faint disks around theplanet-bearing stars (e.g., the stars tend to be more distant),resulting in a lower detection rate for IR excess than in acorresponding control sample of nearby stars not known to have planets(9% ± 3% versus 14% ± 3%). Even taking into account theselection bias, we find that the difference between the dust emissionaround stars with planets and stars without known planets is notstatistically significant.
| Survey of Nearby FGK Stars at 160 μm with Spitzer
The Spitzer Space Telescope has advanced debris disk sciencetremendously with a wealth of information on debris disks around nearbyA, F, G, K, and M stars at 24 and 70 μm with the MIPS photometerand at 8-34 μm with IRS. Here we present 160 μmobservations of a small subset of these stars. At this wavelength, thestellar photospheric emission is negligible and any detected emissioncorresponds to cold dust in extended Kuiper Belt analogs. However, theSpitzer 160 μm observations are limited in sensitivity by thelarge beam size which results in significant "noise" due to cirrus andextragalactic confusion. In addition, the 160 μm measurementssuffer from the added complication of a light leak next to the star'sposition whose flux is proportional to the near-infrared flux of thestar. We are able to remove the contamination from the leak and report160 μm measurements or upper limits for 24 stars. Three stars(HD 10647, HD 207129, and HD 115617) have excesses at 160 μmthat we use to constrain the properties of the debris disks around them.A more detailed model of the spectral energy distribution of HD 10647reveals that the 70 and 160 μm emission could be due to smallwater ice particles at a distance of 100 AU, consistent with HubbleSpace Telescope optical imaging of circumstellar material in the system.
| No Transition Disk? Infrared Excess, PAH, H2, and X-Rays from the Weak-Lined T Tauri Star DoAr 21
As part of a program to understand disk dispersal and the interplaybetween circumstellar disks and X-ray emission, we present newhigh-resolution mid-infrared (IR) imaging, high-resolution opticalspectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined TTauri star DoAr 21. DoAr 21 (age <106 yr and mass ~2.2 Msun based on evolutionary tracks) is a strong X-ray emitter,with conflicting evidence in the literature about its disk properties.It shows weak but broad Hα emission (reported here for the firsttime since the 1950s); polarimetric variability; polycyclic aromatichydrocarbon (PAH) and H2 emission; and a strong, spatiallyresolved 24 μm excess in archival Spitzer photometry. Geminisub-arcsecond-resolution 9-18 μm images show that there is little orno excess mid-IR emission within 100 AU of the star; the excess emissionis extended over several arcseconds and is quite asymmetric. Theextended emission is bright in the ultraviolet (UV)-excited λ =11.3 μm PAH emission feature. A new high-resolution X-ray gratingspectrum from Chandra shows that the stellar X-ray emission is very hardand dominated by continuum emission; it is well fit by amulti-temperature thermal model, typical of hard coronal sources, andshows no evidence of unusually high densities. A flare during the X-rayobservation shows a temperature approaching 108 K. We arguethat the far-UV emission from the transition region is sufficient toexcite the observed extended PAH and continuum emission, and that theH2 emission may be similarly extended and excited. While thisextended emission may be a disk in the final stages of clearing, it alsocould be more akin to a small-scale photodissociation region than aprotoplanetary disk, highlighting both the very young ages(<106 yr) at which some stars are found without disks andthe extreme radiation environment around even late-typepre-main-sequence stars.
| Planetesimal Accretion in Binary Systems: Role of the Companion's Orbital Inclination
Recent observations show that planets can reside in close binary systemswith stellar separation of only ~20 AU. However, planet formationin such close binary systems is a challenge to current theory. One ofthe major theoretical problems occurs in the intermediatestage—planetesimals accretion into planetary embryos—duringwhich the companion's perturbations can stir up the relative velocities(utriV) of planetesimals and thus slow down or even cease their growth.Recent studies have shown that conditions could be even worse foraccretion if the gas-disk evolution was included. However, all previousstudies assumed a two-dimensional disk and a coplanar binary orbit.Extending previous studies by including a three-dimensional gas disk andan inclined binary orbit with small relative inclination ofiB = 0fdg1-5°, we numerically investigate the conditionsfor planetesimal accretion at 1-2 AU, an extension of the habitable zone(~1-1.3 AU), around α Centauri A in this paper. Inclusion of thebinary inclination leads to the following: (1) differential orbitalphasing is realized in the three-dimensional space, and thusdifferent-sized bodies are separated from each other, (2) total impactrate is lower, and impacts mainly occur between similar-sized bodies,(3) accretion is more favored, but the balance between accretion anderosion remains uncertain, and the "possible accretion region" extendsup to 2 AU when assuming an optimistic Q* (critical specific energy thatleads to catastrophic fragmentation), and (4) impact velocities (utriV)are significantly reduced but still much larger than their escapevelocities, which infers that planetesimals grow by means of type IIrunaway mode. As a conclusion, the inclusion of a small binaryinclination is a promising mechanism that favors accretion, opening apossibility that planet formation in close binary systems can go throughthe difficult stage of planetesimals accretion into planetary embryos.
| The Stability and Dynamics of Planets in Tight Binary Systems
Planets have been observed in tight binary systems with separations lessthan 20 AU. A likely formation scenario for such systems involves adynamical capture, after which high relative inclinations are likely andmay lead to Kozai oscillations. We numerically investigate the fate ofan initially coplanar double-planet system in a class of binaries withseparation ranging between 12 and 20 AU. Dynamical integrations ofrepresentative four-body systems are performed, each including a hotJupiter and a second planet on a wider orbit. We find that, althoughsuch systems can remain stable at low relative inclinations(lsim40°), high relative inclinations are likely to lead toinstabilities. This can be avoided if the planets are placed in aKozai-stable zone within which mutual gravitational perturbations cansuppress the Kozai mechanism. We investigate the possibility of inducingKozai oscillations in the inner orbit by a weak coupling mechanismbetween the planets in which the coplanarity is broken due to adifferential nodal precession. Propagating perturbations from thestellar companion through a planetary system in this manner can havedramatic effects on the dynamical evolution of planetary systems,especially in tight binaries and can offer a reasonable explanation foreccentricity trends among planets observed in binary systems. We findthat inducing such oscillations into the orbit of a hot Jupiter is morelikely in tight binaries and an upper limit can be set on the binaryseparation above which these oscillations are not observed.
| Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars
We present interferometric angular sizes for 12 stars with knownplanetary companions, for comparison with 28 additional main-sequencestars not known to host planets. For all objects we estimate bolometricfluxes and reddenings through spectral-energy distribution (SED) fits,and in conjunction with the angular sizes, measurements of effectivetemperature. The angular sizes of these stars are sufficiently smallthat the fundamental resolution limits of our primary instrument, thePalomar Testbed Interferometer, are investigated at thesub-milliarcsecond level and empirically established based upon knownperformance limits. We demonstrate that the effective temperature scaleas a function of dereddened (V – K)0 color isstatistically identical for stars with and without planets. A usefulbyproduct of this investigation is a direct calibration of the TEFF scale for solarlike stars, as a function of both spectraltype and (V – K)0 color, with an precision of\overline{\Delta T}_{\it {(V-K)}_0} = 138\,K over the range (V –K)0 = 0.0-4.0 and \overline{\Delta T}_{SpType} = 105\,K forthe range F6V-G5V. Additionally, in an Appendix we provide SED fits forthe 166 stars with known planets which have sufficient photometryavailable in the literature for such fits; this derived "XO-Rad"database includes homogeneous estimates of bolometric flux, reddening,and angular size.
| The multiplicity of exoplanet host stars. New low-mass stellar companions of the exoplanet host stars HD 125612 and HD 212301
Aims: We present new results from our ongoing multiplicity study ofexoplanet host stars, carried out with SofI/NTT. We provide the mostrecent list of confirmed binary and triple star systems that harborexoplanets. Methods: We use direct imaging to identify widestellar and substellar companions as co-moving objects to the observedexoplanet host stars, whose masses and spectral types are determinedwith follow-up photometry and spectroscopy. Results: We found twonew co-moving companions of the exoplanet host stars HD 125612 and HD212301. HD 125612 B is a wide M 4 dwarf (0.18 M_ȯ) companion of theexoplanet host star HD 125612, located about 1.5 arcmin (~4750 AU ofprojected separation) south-east of its primary. In contrast, HD 212301B is a close M 3 dwarf (0.35 M_ȯ), which is found about 4.4 arcsec(~230 AU of projected separation) north-west of its primary. Conclusions: The binaries HD 125612 AB and HD 212301 AB are newmembers in the continuously growing list of exoplanet host star systemsof which 43 are presently known. Hence, the multiplicity rate ofexoplanet host stars is about 17%.Based on observations obtained on La Silla in ESO programs079.C-0099(A), 080.C-0312(A).
| The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs
Cool L- and T-type objects were discovered first as companions to starsin 1988 and 1995, respectively. A certain example of the even coolerY-type spectral class (T_eff ⪉ 500 K) has not been seen. Recentinfrared-imaging observations of stars and brown dwarfs indicate thatsubstellar companions with large semi-major axes and with masses lessthan the brown dwarf/giant planet dividing line ( 13.5{M}_J) are rare.Theoretical considerations of the Jeans mass fragmentation of molecularclouds are consistent with this minimum mass cutoff and also with thesemi-major axis (hundreds of AU) characteristic of the lowest massimaged companions. As a consequence, Y-class companions with largesemi-major axes should be scarce around stars <2 Gyr old, and alsoaround substellar primaries of all ages. By focusing on brown dwarfcompanions to young stellar primaries, it is possible to derive a firstestimate of the brown dwarf IMF over the entire range of brown dwarfmasses (13{M}J to 79{M}_J) - the number of companion browndwarfs is proportional to the mass to the -1.2±0.2 power.
| Stellar wobble caused by a binary system: Can it really be mistaken as an extra-solar planet?
The traditional method for detecting extra-solar planets relies onmeasuring a small stellar wobble which is assumed to be caused by aplanetorbiting the star. Recently, it has been suggested that a similarstellar wobblecould be caused by a close binary system. Here weshow that, although the effect of a close binary system can at firstsight bemistaken as a planetary companion to the star, more careful analysis oftheobservational data should allow us to distinguish between the twoeffects.
| Extrasolar Giant Planets and X-Ray Activity
We have carried out a survey of X-ray emission from stars with giantplanets, combining both archival and targeted surveys. Over 230 starshave been currently identified as possessing planets, and roughlyone-third of these have been detected in X-rays. We carry out detailedstatistical analysis on a volume-limited sample of main-sequence starsystems with detected planets, comparing subsamples of stars that haveclose-in planets with stars that have more distant planets. Thisanalysis reveals strong evidence that stars with close-in giant planetsare on average more X-ray active by a factor of ~4 than those withplanets that are more distant. This result persists for various sampleselections. We find that even after accounting for observational samplebias, a significant residual difference still remains. Thisobservational result is consistent with the hypothesis that giantplanets in close proximity to the primary stars influence the stellarmagnetic activity.
| The instability transition for the restricted 3-body problem. I. Theoretical approach
Aims: We study the onset of orbital instability for a small object,identified as a planet, that is part of a stellar binary system withproperties equivalent to the restricted three body problem. Methods: Our study is based on both analytical and numerical means andmakes use of a rotating (synodic) coordinate system keeping both binarystars at rest. This allows us to define a constant of motion (Jacobi'sconstant), which is used to describe the permissible region of motionfor the planet. We illustrate the transition to instability by depictingsets of time-dependent simulations with star-planet systems of differentmass and distance ratios. Results: Our method utilizes theexistence of an absolute stability limit. As the system parameters arevaried, the permissible region of motion passes through the threecollinear equilibrium points, which significantly changes the type ofplanetary orbit. Our simulations feature various illustrative examplesof instability transitions. Conclusions: Our study allows us toidentify systems of absolute stability, where the stability limit doesnot depend on the specifics or duration of time-dependent simulations.We also find evidence of a quasi-stability region, superimposed on theregion of instability, where the planetary orbits show quasi-periodicbehavior. The analytically deduced onset of instability is found to beconsistent with the behavior of the depicted time-dependent models,although the manifestation of long-term orbital stability will requiremore detailed studies.
| Planet formation in α Centauri A revisited: not so accretion friendly after all
We numerically explore planet formation around α Centauri A byfocusing on the crucial planetesimals-to-embryos phase. Our approach issignificantly improved with respect to the earlier work of Marzari &Scholl, since our deterministic N-body code computing the relativevelocities between test planetesimals handles bodies with differentsize. Due to this step-up, we can derive the accretion versusfragmentation trend of a planetesimal population having any given sizedistribution. This is a critical aspect of planet formation in binariessince the pericenter alignment of planetesimal orbits due to thegravitational perturbations of the companion star and to gas frictionstrongly depends on size. Contrary to Marzari & Scholl, we findthat, for the nominal case of a Minimum-Mass Solar Nebula gas disc, theregion beyond ~0.5au from the primary is strongly hostile toplanetesimal accretion. In this area, impact velocities betweendifferent-sized bodies are increased, by the differential orbitalphasing, to values too high to allow mutual accretion. For any realisticsize distribution for the planetesimal population, thisaccretion-inhibiting effect is the dominant collision outcome and theaccretion process is halted. Results are relatively robust with respectto the profile and density of the gas disc. Except for an unrealisticalmost gas-free case, the inner `accretion-safe' area never extendsbeyond 0.75au. We conclude that planet formation is very difficult inthe terrestrial region around α Centauri A, unless it started fromfast-formed very large (>30km) planetesimals. Notwithstanding theseunlikely initial conditions, the only possible explanation for thepresence of planets around 1au from the star would be the hypotheticaloutward migration of planets formed closer to the star or a differentorbital configuration in the binary's early history. Our conclusionsdiffer from those of several studies focusing on the laterembryos-to-planets stage, confirming that the planetesimals-to-embryosphase is more affected by binary perturbations.
| Beyond the Iron Peak: r- and s-Process Elemental Abundances in Stars with Planets
We present elemental abundances of 118 stars (28 of which are knownextrasolar planetary host stars) observed as part of theAnglo-Australian Planet Search. Abundances of O, Mg, Cr, Y, Zr, Ba, Nd,and Eu (along with previously published abundances for C and Si) arepresented. This study is one of the first to specifically examineplanetary host stars for the heavy elements produced by neutron capturereactions. We find that the abundances in host stars are chemicallydifferent from both the standard solar abundances and the abundances innon-host stars in all elements studied, with enrichments over non-hoststars ranging from 0.06 dex (for O) to 0.11 dex (for Cr and Y). Suchabundance trends are in agreement with other previous studies of fieldstars and lead us to conclude that the chemical anomalies observed inplanetary host stars are the result of normal galactic chemicalevolution processes. Based on this observation, we conclude that theobserved chemical traits of planetary host stars are primordial inorigin, coming from the original nebula and not from a ``pollution''process occurring during or after formation, and that planet formationoccurs naturally with the evolution of stellar material.
| Planet formation in binary stars: the case of γ Cephei
Context: Over 30 planetary systems have been discovered to reside inbinary stars. As some of the binary separations are smaller than 20astronomical units (AU) the gravitational perturbation of the secondarystar has a very strong influence on the planet formation process, as ittruncates the protoplanetary disc, possibly shortens its lifetime, andstirs up the embedded planetesimals. Due to its small semi-major axis(18.5 AU) and relatively large eccentricity e = 0.35 the binary starγ Cephei represents a particularly challenging example worthy ofstudy in greater detail. Aims: In the present study we model theorbital evolution and growth of embedded protoplanetary cores of about30 earth masses in the putative protoplanetary disc surrounding theprimary star in the γ Cep system. Methods: We assumecoplanarity of the disc, binary, and planet and perform two-dimensionalhydrodynamic simulations of embedded cores in a protoplanetary discperturbed by a secondary companion. Before embedding the planet, theequilibrium structure of the disc for the observed binary parameters ofγ Cep is determined. We initiate the embedded planets in the discon circular orbits with different initial distances from the primary. Results: The presence of the eccentric secondary star perturbs thedisc periodically and generates strong spiral arms at periapse thatpropagate toward the disc centre. The disc perturbations then weaken asthe secondary approaches apoapse. The disc also becomes slightlyeccentric (e_disc ≈ 0.1-0.15), and displays a slow retrogradeprecession in the inertial frame. Embedded cores interact with theeccentric disc, and are periodically disturbed both by the strong spiralshocks and the eccentric binary. For all initial separations considered(from 2.5 to 3.5 AU) the cores migrate inward. In contrast, theireccentricity evolution depends primarily on the starting position in thedisc. For initial semi-major axes ap ≳ 2.7 we find astrong increase in the planetary eccentricity despite the presence ofinward migration. Only cores that are initially far from the disc outeredge (ap ≲ 2.7 AU) have a bounded orbital eccentricitythat converges, after mass accretion, roughly to the value of the planetobserved in the γ Cep system. Conclusions: Even though aclose binary system such as γ Cep still presents a challenge toplanet formation theory, we have shown that under the condition thatprotoplanetary cores can form at around 2.5 AU, it is possible to evolveand grow such a core to form a planet with a final configuration similarto what is observed.
| Formation and Detectability of Terrestrial Planets around α Centauri B
We simulate the formation of planetary systems around α CentauriB. The N-body accretionary evolution of a Σ~r-1 diskpopulated with 400-900 lunar-mass protoplanets is followed for 200 Myr.All simulations lead to the formation of multiple-planet systems with atleast one planet in the 1-2 M⊕ mass range at 0.5-1.5AU. We examine the detectability of our simulated planetary systems bygenerating synthetic radial velocity observations including noise basedon the radial velocity residuals to the recently published three planetfit to the nearby K0 V star HD 69830. Using these syntheticobservations, we find that we can reliably detect a 1.8M⊕ planet in the habitable zone of α Centauri Bafter only 3 years of high cadence observations. We also find that theplanet is detectable even if the radial velocity precision is 3 ms-1, as long as the noise spectrum is white. Our results showthat the greatest uncertainty in our ability to detect rocky planets inthe α Centauri system is the unknown magnitude of ultralowfrequency stellar noise.
| Planetesimal and gas dynamics in binaries
Observations of extrasolar planets reveal that planets can be found inclose binary systems, where the semimajor axis of the binary orbit isless than 20 au. The existence of these planets challenges planetformation theory because the strong gravitational perturbations due tothe companion increase encounter velocities between planetesimals andmake it difficult for them to grow through accreting collisions. Westudy planetesimal encounter velocities in binary systems, where theplanetesimals are embedded in a circumprimary gas disc that is allowedto evolve under the influence of the gravitational perturbations of thecompanion star. We use the RODEO (Roe Solver for Disc Embedded Objects)method to evolve the vertically integrated Navier-Stokes equations forthe gas disc. Embedded within this disc is a population of planetesimalsof various sizes that evolve under the influence of the gravitationalforces of both stars and friction with the gas. The equations of motionfor the planetesimals are integrated using a fourth-order symplecticalgorithm. We find that the encounter velocities between planetesimalsof different size strongly depend on the gas disc eccentricity.Depending on the amount of wave damping, we find two possible states ofthe gas disc: a quiet state, where the disc eccentricity reaches asteady state that is determined by the forcing of the binary, for whichthe encounter velocities do not differ by more than a factor of 2 fromthe case of a circular gas disc, and an excited state, for which the gasdisc obtains a large free eccentricity, which drives up the encountervelocities more substantially. In both cases, the inclusion of the fullgas dynamics increases the encounter velocity compared to the case of astatic, circular gas disc. Full numerical parameter exploration is stillimpossible, but we derive analytical formulae to estimate encountervelocities between bodies of different sizes given the gas disceccentricity. The gas dynamical evolution of a protoplanetary disc in abinary system tends to make planetesimal accretion even more difficultthan in a static, axisymmetric gas disc.
| On the formation and migration of giant planets in circumbinary discs
Aims. We present the results of hydrodynamic simulations of theformation and subsequent orbital evolution of giant planets embedded ina circumbinary disc. The aim is to examine whether or not giant planetscan be found to orbit stably in close binary systems. Methods: Weperformed numerical simulations using a grid-based hydrodynamics code.We assume that a 20 M_⊕ core has migrated to the edge of the innercavity formed by the binary where it remains trapped by corotationtorques. This core is then allowed to accrete gas from the disc, and westudy its orbital evolution as it grows in mass. For each of the twoaccretion time scales we considered, we performed three simulations. Intwo of the three simulations, we stopped the accretion onto the planetonce its mass became characteristic of that of Saturn or Jupiter. In theremaining case, the planet accreted disc material freely in such a waythat its mass became higher than Jupiter's. Results: The simulationsshow different outcomes depending on the final mass mp of thegiant. For m_p=1~MS (where MS is Saturn's mass),we find that the planet migrates inward through its interaction with thedisc until its eccentricity becomes high enough to induce a torquereversal. The planet then migrates outward, and the system remainsstable on long time scales. For mp ≥ 1~MJ(where MJ is Jupiter's mass) we observed two differentoutcomes. In each case the planet enters the 4:1 resonance with thebinary, and resonant interaction drives up the eccentricity of theplanet until it undergoes a close encounter with the secondary star,leading to scattering. The result can either be ejection from the systemor scattering out into the disc followed by a prolonged period ofoutward migration. These results suggest that circumbinary planets aremore likely to be quite common in the Saturn-mass range. Jupiter-masscircumbinary planets are likely to be less common because of their lessstable evolution, but if present are likely to orbit at large distancesfrom the central binary.
| A peculiarity of metal-poor stars with planets?
Stars with planets at intermediate metallicities ([-0.7, -0.2] dex)exhibit properties that differ from the general field stars. Thirteenstars with planets reported in this metallicity range belong to thethick disc, while only one planet have been detected among stars of thethin disc. Although this statistics is weak, it contradicts the knowncorrelation between the presence of planet and metallicity. We relatethis finding to the specific property of the thin disc in thismetallicity range, where stars are shown to rotate around the Galaxyfaster than the Sun. Their orbital parameters are conveniently explainedif they are contaminants coming from the outer Galactic disc, as aresult of radial mixing. This must be considered together with the factthat metal-rich stars ([Fe/H]>+0.1 dex) found in the solarneighbourhood, which are the hosts of most of the detected planets, aresuspected of being wanderers from the inner Galactic disc. It is thenquestionned why stars that originate in the inner and outer thin discshow respectively the highest and lowest rate of detected planets. It issuggested that the presence of giant planets might be primarily afunction of a parameter linked to galactocentric radius, but notmetallicity. Combined with the existing radial metallicity gradient,then radial mixing explains the correlation at high metallicity observedlocally, but also the peculiarity found at low metallicity, which cannotbe accounted for by a simple correlation between metallicity and planetprobability.
| Debris Disks around Sun-like Stars
We have observed nearly 200 FGK stars at 24 and 70 μm with theSpitzer Space Telescope. We identify excess infrared emission, includinga number of cases where the observed flux is more than 10 times brighterthan the predicted photospheric flux, and interpret these signatures asevidence of debris disks in those systems. We combine this sample of FGKstars with similar published results to produce a sample of more than350 main sequence AFGKM stars. The incidence of debris disks is4.2+2.0-1.1% at 24 μm for a sample of 213Sun-like (FG) stars and 16.4+2.8-2.9% at 70 μmfor 225 Sun-like (FG) stars. We find that the excess rates for A, F, G,and K stars are statistically indistinguishable, but with a suggestionof decreasing excess rate toward the later spectral types; this may bean age effect. The lack of strong trend among FGK stars of comparableages is surprising, given the factor of 50 change in stellar luminosityacross this spectral range. We also find that the incidence of debrisdisks declines very slowly beyond ages of 1 billion years.
| On the evolution of multiple low mass planets embedded in a circumbinary disc
Context: Previous work has shown that the tidal interaction between abinary system and a circumbinary disc leads to the formation of a largeinner cavity in the disc. Subsequent formation and inward migration of alow mass planet causes it to become trapped at the cavity edge, where itorbits until further mass growth or disc dispersal. The question of howsystems of multiple planets in circumbinary discs evolve has not yetbeen addressed. Aims: We present the results of hydrodynamicsimulations of multiple low mass planets embedded in a circumbinarydisc. The aim is to examine their long term evolution as they approachand become trapped at the edge of the tidally truncated inner cavity. Methods: A grid-based hydrodynamics code was used to compute simulationsof 2D circumbinary disc models with embedded planets. The 3D evolutionof the planet orbits was computed, and inclination damping due to thedisc was calculated using prescribed forces. We present a suite ofsimulations which study the evolution of pairs of planets migrating inthe disc. We also present the results of hydrodynamic simulations offive-planet systems, and study their long term evolution after discdispersal using a N-body code. Results: For the two-planet simulationswe assume that the innermost planet has migrated to the edge of theinner cavity and remains trapped there, and study the subsequentevolution of the system as the outermost planet migrates inward. We findthat the outcomes largely depend on the mass ratio q=m_i/m_o, wheremi (m_o) is the mass of the innermost (outermost) planet. Forq < 1, planets usually undergo dynamical scattering or orbitalexchange. For values of q > 1 the systems reach equilibriumconfigurations in which the planets are locked into mean motionresonances, and remain trapped at the edge of the inner cavity withoutfurther migration. Most simulations of five-planet systems we performedresulted in collisions and scattering events, such that only a singleplanet remained in orbit about the binary. In one case however, amultiplanet resonant system was found to be dynamically stable over longtime scales, suggesting that such systems may be observed in planetsearches focussed on close binary systems.
| Stringent Criteria for Stable and Unstable Planetary Orbits in Stellar Binary Systems
The existence of planets in stellar binary (and higher order) systemshas now been confirmed by many observations. The stability of planetaryorbits in these systems has been extensively studied, but no precisestability criteria have so far been introduced. Therefore, there is anurgent need for developing stringent mathematical criteria that allow usto precisely determine whether a planetary orbit in a binary system isstable or unstable. In this Letter, such criteria are defined using theconcept of Jacobi's integral and Jacobi's constant. These criteria areused to contest previous results on planetary orbital stability inbinary systems.
| Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors
We here develop an improved way of using a rotating star as a clock, setit using the Sun, and demonstrate that it keeps time well. Thistechnique, called gyrochronology, derives ages for low-massmain-sequence stars using only their rotation periods and colors. Thetechnique is developed here and used to derive ages for illustrativegroups of nearby field stars with measured rotation periods. We firstdemonstrate the reality of the interface sequence, the unifying featureof the rotational observations of cluster and field stars that makes thetechnique possible, and extend it beyond the proposal of Skumanich byspecifying the mass dependence of rotation for these stars. We delineatewhich stars it cannot currently be used on. We then calibrate the agedependence using the Sun. The errors are propagated to understand theirdependence on color and period. Representative age errors associatedwith the technique are estimated at ~15% (plus possible systematicerrors) for late F, G, K, and early M stars. Gyro ages for the MountWilson stars are shown to be in good agreement with chromospheric agesfor all but the bluest stars, and probably superior. Gyro ages are thencalculated for each of the active main-sequence field stars studied byStrassmeier and collaborators. These are shown to have a median age of365 Myr. The sample of single field stars assembled by Pizzolato andcollaborators is then assessed and shown to have gyro ages ranging fromunder 100 Myr to several Gyr, with a median age of 1.2 Gyr. Finally, wedemonstrate that the individual components of the three wide binariesξ Boo AB, 61 Cyg AB, and α Cen AB yield substantially the samegyro ages.
| Characterization of the long-period companions of the exoplanet host stars: HD 196885, HD 1237 and HD 27442. VLT/NACO and SINFONI near-infrared, follow-up imaging and spectroscopy
Aims:We present the results of near-infrared, follow-up imaging andspectroscopic observations at VLT, aimed at characterizing thelong-period companions of the exoplanet host stars HD 196885, HD 1237and HD 27442. The three companions were previously discovered in thecourse of our CFHT and VLT coronographic imaging survey dedicated to thesearch for faint companions of exoplanet host stars. Methods: Weused the NACO near-infrared adaptive optics instrument to obtainastrometric follow-up observations of HD 196885 A and B. The long-slitspectroscopic mode of NACO and the integral field spectrograph SINFONIwere used to carry out a low-resolution spectral characterization of thethree companions HD 196885 B, HD 1237 B and HD 27442 B between 1.4 and2.5 μm. Results: We can now confirm that the companion HD196885 B is comoving with its primary exoplanet host star, as previouslyshown for HD 1237 B and HD 27442 B. We find that both companions HD196885 B and HD 1237 B are low-mass stars of spectral type M1±1Vand M4±1V respectively. HD 196885 AB is one of the closer (~23AU) resolved binaries known to host an exoplanet. This system is thenideal for carrying out a combined radial velocity and astrometricinvestigation of the possible impact of the binary companion on theplanetary system formation and evolution. Finally, we confirm viaspectroscopy that HD 27442 B is a white dwarf companion, the third oneto be discovered orbiting an exoplanet host star, following HD 147513and Gliese 86. The detection of the broad Brγ line of hydrogenindicates a white dwarf atmosphere dominated by hydrogen.Based on ESO observing programs 075.C-0825(A), 275.B-5057A and077.C-0444(A).
| Habitable Planet Formation in Binary Planetary Systems
Recent radial velocity observations have indicated that Jovian-typeplanets can exist in moderately close binary star systems. Numericalsimulations of the dynamical stability of terrestrial-class planets insuch environments have shown that, in addition to their giant planets,these systems can also harbor Earth-like objects. In this paper we studythe late stage of terrestrial planet formation in such binary planetarysystems, and present the results of the simulations of the formation ofEarth-like bodies in their habitable zones. We consider a circumprimarydisk of Moon- to Mars-sized objects and numerically integrate the orbitsof these bodies at the presence of the Jovian-type planet of the systemand for different values of the mass, semimajor axis, and orbitaleccentricity of the secondary star. Results indicate that Earth-likeobjects, with substantial amounts of water, can form in the habitablezone of the primary star. Simulations also indicate that by transferringangular momentum from the secondary star to protoplanetary objects, thegiant planet of the system plays a key role in the radial mixing ofthese bodies and the water contents of the final terrestrial planets. Wewill discuss the results of our simulation and show that the formationof habitable planets in binary planetary systems is more probable inbinaries with moderate to large perihelia.
| On the migration of protoplanets embedded in circumbinary disks
Aims.We present the results of hydrodynamical simulations of low massprotoplanets embedded in circumbinary accretion disks. The aim is toexamine the migration and long term orbital evolution of theprotoplanets, in order to establish the stability properties of planetsthat form in circumbinary disks. Methods: Simulations were performedusing a grid-based hydrodynamics code. First we present a set ofcalculations that study how a binary interacts with a circumbinary disk.We evolve the system for ~ 105 binary orbits, which is thetime needed for the system to reach a quasi-equilibrium state. From thistime onward the apsidal lines of the disk and the binary are aligned,and the binary eccentricity remains essentially unchanged with a valueof eb ~ 0.08. Once this stationary state is obtained, weembed a low mass protoplanet in the disk and let it evolve under theaction of the binary and disk forces. We consider protoplanets withmasses of mp = 5, 10 and 20 M_⊕. Results: In eachcase, we find that inward migration of the protoplanet is stopped at theedge of the tidally truncated cavity formed by the binary. This effectis due to positive corotation torques, which can counterbalance the netnegative Lindblad torques in disk regions where the surface densityprofile has a sufficiently large positive gradient. Halting of migrationoccurs in a region of long-term stability, suggesting that low masscircumbinary planets may be common, and that gas giant circumbinaryplanets should be able to form in circumbinary disks.
| Planet dispersal in binary systems during transient multiple star phases
Aims:If a significant fraction of binary star systems spent some time asinclined triple systems, either during their formation process or as theoutcome of several close dynamical encounters in a crowded stellarenvironemnt, then the number of planets in binaries would besignificantly lower than around single stars. The stellar chaotic phasepreceding the instability of the triple system and the wide oscillationsin eccentricity and inclination of the companion star due to the highmutual inclination between the companion and the singleton would quicklyeject planets orbiting the binary in S-type orbits. Methods: Weperform numerical simulations of the dynamical evolution of hierarchicaltriple star systems with planets hosted around the primary star of theinner binary. Different values of mutual inclination, binary separationand singleton initial semimajor axis are explored in a statistical way.Results: We find that a significant mutual inclinationim between the singleton and the binary is a key factor forinstability of the planetary system. When im is larger than~40° the fraction of planets in the binary surviving the chaoticphase of the triple declines dramatically. The combination ofeccentricity and inclination oscillations of the binary companioninduced by the secular perturbations of the singleton and the sequenceof close encounters preceding the ejection of one star fully destabilizea planetary system extending beyond 1 AU from the star. Forim around 90° the percentage of surviving planets islower than 20% for all binaries with a semimajor axis smaller than 200AU. Conclusions: The frequency of planets in binaries with lowseparation may be strongly reduced by the residence of the pair in thepast in a temporary inclined hierarchical triple.
| The NaI D resonance lines in main-sequence late-type stars
We study the sodium D lines (D1: 5895.92Å D2: 5889.95Å) inlate-type dwarf stars. The stars have spectral types between F6 and M5.5(B - V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82and 0.6. We obtained medium-resolution echelle spectra using the 2.15-mtelescope at the Argentinian observatory Complejo Astronómico ElLeoncito (CASLEO). The observations have been performed periodicallysince 1999. The spectra were calibrated in wavelength and in flux. Adefinition of the pseudo-continuum level is found for all ourobservations. We also define a continuum level for calibration purposes.The equivalent width of the D lines is computed in detail for all ourspectra and related to the colour index (B - V) of the stars. Whenpossible, we perform a careful comparison with previous studies.Finally, we construct a spectral index (R'D) as the ratiobetween the flux in the D lines and the bolometric flux. We find that,once corrected for the photospheric contribution, this index can be usedas a chromospheric activity indicator in stars with a high level ofactivity. Additionally, we find that combining some of our results, weobtain a method to calibrate in flux stars of unknown colour.
| The multiplicity of exoplanet host stars. Spectroscopic confirmation of the companions GJ 3021 B and HD 27442 B, one new planet host triple-star system, and global statistics
Aims.We present new results from our ongoing multiplicity study ofexoplanet host stars and present a list of 29 confirmed planet hostmultiple-star systems. Furthermore, we discuss the properties of thesestellar systems and compare the properties of exoplanets detected inthese systems with those of planets orbiting single stars. Methods: Weused direct imaging to search for wide stellar and substellar companionsof exoplanet host stars. With infrared and/or optical spectroscopy, wedetermined the spectral properties of the newly-found co-movingcompanions. Results: We obtained infrared H- and K-band spectra of theco-moving companion GJ 3021 B. The infrared spectra and the apparentH-band photometry of the companion is consistent with an M3-M5 dwarf atthe distance of the exoplanet host star. HD 40979 AB is a wide planethost stellar system, with a separation of ~ 6400 AU. The companion tothe exoplanet host star turned out to be a close stellar pair with aprojected separation of ~130 AU, hence, this system is a new member ofthose rare planet host triple-star systems of which only three othersystems are presently known. HD 27442 AB is a wide binary system listedin the Washington Double Star Catalogue, whose common proper motion wasrecently confirmed. This system is composed of the subgiant HD 27442 Ahosting the exoplanet, and its faint companion HD 27442 B. The visibleand infrared J-, H-, and K_S-band photometry of HD 27442 B at thedistance of the primary star shows that the companion is probably awhite dwarf. Our multi-epochs SofI imaging observations confirm thisresult and even refine the suggested physical characteristics of HD27442 B. This companion should be a relatively young, hot white dwarfwith an effective temperature of ~14 400 K, and cooling age of ~220 Myr.Finally, we could unambiguously confirm the white dwarf nature of HD27442 B with follow-up optical and infrared spectroscopy. The spectra ofthe companion show Hydrogen absorption features of the Balmer, Paschen,and Bracket series. With its subgiant primary and the white-dwarfcompanion, the HD 27442 AB system is the most evolved planet hoststellar system known today. The mass-period and eccentricity-periodcorrelation of planets around single stars and those residing inmultiple-star systems seem different for the short-period planets. Thedistribution functions of planet orbital elements (P, e) are identical,while the mass-distribution (m sin(i)) exhibits one difference. Whileboth planet populations exhibit a peak in their mass-distribution atabout 1 M_Jup, the frequency of more massive planets continuallydecreases in single-star systems, whereas the mass-distribution ofplanets residing in multiple-star systems exhibits a further peak atabout 4 M_Jup. This indicates that the mass-distributions of the twoplanet populations might differ in the intermediate mass-range between 2and 6 M_Jup.Based on observations obtained on La Silla in ESO programs 70.C-0116(A),71.C-0140(A), 73.C-0103(A), and on Paranal in ESO runs 074.C-0144(B),074.C-0144(C), 073.C-0370(A), on Mauna Kea in UKIRT program U/02A/16, aswell as at the Munich LMU University Observatory on Mount Wendelstein.
| Hα and the Ca II H and K lines as activity proxies for late-type stars
Context: The main chromospheric activity indicator is the S index, whichis the ratio of the flux in the core of the Ca II H and K lines to thecontinuum nearby, and is well studied for stars from F to K. Anotherchromospheric proxy is the Hα line, which is believed to betightly correlated with the Ca II index. Aims: In this work wecharacterize both chromospheric activity indicators, the one associatedwith the H and K Ca II lines and the other with Hα, for the wholerange of late type stars, from F to M. Methods: We present periodicmedium-resolution echelle observations covering the complete visualrange, taken at the CASLEO Argentinean Observatory over 7 years. We usea total of 917 flux-calibrated spectra for 109 stars that range from F6to M5. We statistically study these two indicators for stars ofdifferent activity levels and spectral types. Results: We directlyderive the conversion factor that translates the known S index to fluxin the Ca II cores, and extend its calibration to a wider spectralrange. We investigate the relation between the activity measurements inthe calcium and hydrogen lines, and found that the usual correlationobserved is the product of the dependence of each flux on stellarcolour, and not the product of similar activity phenomena.Tables 1 and 2 and full Figs. 1 and 6 are only available in electronicform at http://www.aanda.org
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Observation and Astrometry data
| Constellation: | Eridanus |
| Right ascension: | 02h10m25.98s |
| Declination: | -50°49'25.3" |
| Apparent magnitude: | 6.124 |
| Proper motion RA: | 2150.3 |
| Proper motion Dec: | 673.2 |
| B-T magnitude: | 7.148 |
| V-T magnitude: | 6.209 |
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