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The impact of stellar duplicity on planet occurrence and properties. I. Observational results of a VLT/NACO search for stellar companions to 130 nearby stars with and without planets
Context: Although it is commonly agreed that the presence of a closestellar companion is likely to affect planet formation and evolution,the precise effects and their actual impact on planet occurrence arestill debated. Different conclusions have been reached on thetheoretical side, while observational constraints are sparse, aconsequence of the discrimination against close binaries in Dopplerplanet searches. Accordingly, basic questions such as how hospitablebinaries are to planets and how binary separation and mass ratio impacton planet formation, remain poorly known. Aims: In an effort to bringobservational constraints on the occurrence and properties of planets inbinaries and multiple stars, we have been conducting a dedicatedinvestigation, the results of which will be presented in this series. Methods: Our investigation follows two different approaches, one basedon radial-velocity monitoring, the other based on direct imaging. Inthis first paper, we present the observational results from oursystematic adaptive optics search with VLT/NACO for close stellarcompanions to 130 nearby stars, 57 with planets and 73 without, forcomparison. The inclusion of a control sub-sample is a unique feature ofour program that will enable a meaningful and rigorous comparisonbetween the properties of planet-host stars and the properties of fieldstars subject to the same selection effects against close binaries, butshowing no evidence for planetary companions. Results: Our data reveal95 companion candidates found in the vicinity of 33 of our targets.Nineteen of these candidates are true companions and 2 are likely boundobjects. Among planet-host stars, we discovered a tight pair of very lowmass companions to HD 65216 (projected separation of 255 AU), an early Mcompanion to HD 177830 (projected separation of 97 AU), and we resolvedthe previously known companion to HD 196050 into a close pair of Mdwarfs. Our data additionally confirm the bound nature of the companionsto HD 142, HD 16141, and HD 46375. Among control stars, we detected truecompanions to HD 7895, HD 24331, HD 31412, HD 40397, HD 43834, HD 70923,HD 78351, HD 104263, HD 129642, HD 154682, and HD 223913, and likelybound companions to HD 82241 and HD 134180. Most of these objects are Mdwarfs and have projected separations between 7 and 505 AU.Based on observations collected at the ESO VLT Yepun telescope,proposals 70.C-0557, 71.C-0125, 73.C-0124, 74.C-0048, 75.C-0069, and76.C-0057. Tables 1, 2, and Figs. 3, 4 are only available in electronicform at http://www.aanda.org
| Parent stars of extrasolar planets - VIII. Chemical abundances for 18 elements in 31 stars
We present the results of detailed spectroscopic abundance analyses for18 elements in 31 nearby stars with planets (SWPs). The resultingabundances are combined with other similar studies of nearby SWPs andcompared to a sample of nearby stars without detected planets. We findsome evidence for abundance differences between these two samples forAl, Si and Ti. Some of our results are in conflict with a recent studyof SWPs in the SPOCS data base. We encourage continued study of theabundance patterns of SWPs to resolve these discrepancies.
| Planetary Formation Scenarios Revisited: Core-Accretion versus Disk Instability
The core-accretion and disk instability models have so far been used toexplain planetary formation. These models have different conditions,such as planet mass, disk mass, and metallicity for formation of gasgiants. The core-accretion model has a metallicity condition([Fe/H]>-1.17 in the case of G-type stars), and the mass of planetsformed is less than 6 times that of the Jupiter mass MJ. Onthe other hand, the disk instability model does not have the metallicitycondition, but requires the disk to be 15 times more massive than theminimum mass solar nebulae model. The mass of planets formed is morethan 2 MJ. These results are compared to the 161 detectedplanets for each spectral type of the central stars. The results showthat 90% of the detected planets are consistent with the core-accretionmodel regardless of the spectral type. The remaining 10% are not in theregion explained by the core-accretion model, but are explained by thedisk instability model. We derived the metallicity dependence of theformation probability of gas giants for the core-accretion model.Comparing the result with the observed fraction having gas giants, theyare found to be consistent. On the other hand, the observation cannot beexplained by the disk instability model, because the condition for gasgiant formation is independent of the metallicity. Consequently, most ofplanets detected so far are thought to have been formed by thecore-accretion process, and the rest by the disk instability process.
| Formation of Earth-like Planets During and After Giant Planet Migration
Close-in giant planets are thought to have formed in the cold outerregions of planetary systems and migrated inward, passing through theorbital parameter space occupied by the terrestrial planets in our ownsolar system. We present dynamical simulations of the effects of amigrating giant planet on a disk of protoplanetary material and thesubsequent evolution of the planetary system. We numerically investigatethe dynamics of postmigration planetary systems over 200 million yearsusing models with a single migrating giant planet, one migrating and onenonmigrating giant planet, and excluding the effects of a gas disk.Material that is shepherded in front of the migrating giant planet bymoving mean motion resonances accretes into ``hot Earths,'' but survivalof these bodies is strongly dependent on dynamical damping. Furthermore,a significant amount of material scattered outward by the giant planetsurvives in highly excited orbits; the orbits of these scattered bodiesare then damped by gas drag and dynamical friction over the remainingaccretion time. In all simulations Earth-mass planets accrete onapproximately 100 Myr timescales, often with orbits in the habitablezone. These planets range in mass and water content, with bothquantities increasing with the presence of a gas disk and decreasingwith the presence of an outer giant planet. We use scaling arguments andprevious results to derive a simple recipe that constrains which giantplanet systems are able to form and harbor Earth-like planets in thehabitable zone, demonstrating that roughly one-third of the knownplanetary systems are potentially habitable.
| Stable satellites around extrasolar giant planets
In this work, we study the stability of hypothetical satellites ofextrasolar planets. Through numerical simulations of the restrictedelliptic three-body problem we found the borders of the stable regionsaround the secondary body. From the empirical results, we derivedanalytical expressions of the critical semimajor axis beyond which thesatellites would not remain stable. The expressions are given as afunction of the eccentricities of the planet, eP, and of thesatellite, esat. In the case of prograde satellites, thecritical semimajor axis, in the units of Hill's radius, is given byaE ~ 0.4895 (1.0000 - 1.0305eP -0.2738esat). In the case of retrograde satellites, it isgiven by aE ~ 0.9309 (1.0000 - 1.0764eP -0.9812esat). We also computed the satellite stability region(aE) for a set of extrasolar planets. The results indicatethat extrasolar planets in the habitable zone could harbour theEarth-like satellites.
| Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties
Habitable planets are likely to be broadly Earth-like in composition,mass, and size. Masses are likely to be within a factor of a few of theEarth's mass. Currently, we do not have sufficiently sensitivetechniques to detect Earth-mass planets, except in rare circumstances.It is thus necessary to model the known exoplanetary systems. Inparticular, we need to establish whether Earth-mass planets could bepresent in the classical habitable zone (HZ) or whether the giantplanets that we know to be present would have gravitationally ejectedEarth-mass planets or prevented their formation. We have answered thisquestion by applying computer models to the 152 exoplanetary systemsknown by 2006 April 18 that are sufficiently well characterized for ouranalysis. For systems in which there is a giant planet, inside the HZ,which must have arrived there by migration, there are two cases: (1)where the migration of the giant planet across the HZ has not ruled outthe existence of Earth-mass planets in the HZ; and (2) where themigration has ruled out existence. For each case, we have determined theproportion of the systems that could contain habitable Earth-massplanets today, and the proportion for which this has been the case forat least the past 1000 Myr (excluding any early heavy bombardment). Forcase 1 we get 60% and 50%, respectively, and for case 2 we get 7% and7%, respectively.
| The abundance distribution of stars with planets
We present the results of a uniform, high-precision spectroscopicmetallicity study of 136 G-type stars from the Anglo-Australian PlanetSearch, 20 of which are known to harbour extrasolar planets (as at 2005July). Abundances in Fe, C, Na, Al, Si, Ca, Ti and Ni are presented,along with Strömgen photometric metallicities. This study is one ofseveral recent studies examining the metallicities of a sample ofplanet-host and non-planet-host stars that were obtained from a singlesample, and analysed in an identical manner, providing an unbiasedestimate of the metallicity trends for planet-bearing stars. We findthat non-parametric tests of the distribution of metallicities forplanet-host and non-planet-host stars are significantly different at alevel of 99.4 per cent confidence. We confirm the previously observedtrend for planet-host stars to have higher mean metallicities thannon-planet-host stars, with a mean metallicity for planet-host stars of[Fe/H] = 0.06 +/- 0.03dex compared with [Fe/H] = -0.09 +/- 0.01dex fornon-host-stars in our sample. This enrichment is also seen in the otherelements studied. Based on our findings, we suggest that this observedenhancement is more likely a relic of the original gas cloud from whichthe star and its planets formed, rather than being due to `pollution' ofthe stellar photosphere.
| Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems
We present results of a reconnaissance for stellar companions to all 131radial velocity-detected candidate extrasolar planetary systems known asof 2005 July 1. Common proper-motion companions were investigated usingthe multiepoch STScI Digitized Sky Surveys and confirmed by matching thetrigonometric parallax distances of the primaries to companion distancesestimated photometrically. We also attempt to confirm or refutecompanions listed in the Washington Double Star Catalog, in the Catalogsof Nearby Stars Series by Gliese and Jahreiß, in Hipparcosresults, and in Duquennoy & Mayor's radial velocity survey. Ourfindings indicate that a lower limit of 30 (23%) of the 131 exoplanetsystems have stellar companions. We report new stellar companions to HD38529 and HD 188015 and a new candidate companion to HD 169830. Weconfirm many previously reported stellar companions, including six starsin five systems, that are recognized for the first time as companions toexoplanet hosts. We have found evidence that 20 entries in theWashington Double Star Catalog are not gravitationally bound companions.At least three (HD 178911, 16 Cyg B, and HD 219449), and possibly five(including HD 41004 and HD 38529), of the exoplanet systems reside intriple-star systems. Three exoplanet systems (GJ 86, HD 41004, andγ Cep) have potentially close-in stellar companions, with planetsat roughly Mercury-Mars distances from the host star and stellarcompanions at projected separations of ~20 AU, similar to the Sun-Uranusdistance. Finally, two of the exoplanet systems contain white dwarfcompanions. This comprehensive assessment of exoplanet systems indicatesthat solar systems are found in a variety of stellar multiplicityenvironments-singles, binaries, and triples-and that planets survive thepost-main-sequence evolution of companion stars.
| Catalog of Nearby Exoplanets
We present a catalog of nearby exoplanets. It contains the 172 knownlow-mass companions with orbits established through radial velocity andtransit measurements around stars within 200 pc. We include fivepreviously unpublished exoplanets orbiting the stars HD 11964, HD 66428,HD 99109, HD 107148, and HD 164922. We update orbits for 83 additionalexoplanets, including many whose orbits have not been revised sincetheir announcement, and include radial velocity time series from theLick, Keck, and Anglo-Australian Observatory planet searches. Both thesenew and previously published velocities are more precise here due toimprovements in our data reduction pipeline, which we applied toarchival spectra. We present a brief summary of the global properties ofthe known exoplanets, including their distributions of orbital semimajoraxis, minimum mass, and orbital eccentricity.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. The Keck Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Chemical Composition of the Planet-harboring Star TrES-1
We present a detailed chemical abundance analysis of the parent star ofthe transiting extrasolar planet TrES-1. Based on high-resolution KeckHIRES and Hobby-Eberly Telescope HRS spectra, we have determinedabundances relative to the Sun for 16 elements (Na, Mg, Al, Si, Ca, Sc,Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, and Ba). The resulting averageabundance of <[X/H]>=-0.02+/-0.06 is in good agreement withinitial estimates of solar metallicity based on iron. We compare theelemental abundances of TrES-1 with those of the sample of stars withplanets, searching for possible chemical abundance anomalies. TrES-1appears not to be chemically peculiar in any measurable way. Weinvestigate possible signs of selective accretion of refractory elementsin TrES-1 and other stars with planets and find no statisticallysignificant trends of metallicity [X/H] with condensation temperatureTc. We use published abundances and kinematic information forthe sample of planet-hosting stars (including TrES-1) and severalstatistical indicators to provide an updated classification in terms oftheir likelihood to belong to either the thin disk or the thick disk ofthe Milky Way. TrES-1 is found to be very likely a member of thethin-disk population. By comparing α-element abundances of planethosts and a large control sample of field stars, we also find thatmetal-rich ([Fe/H]>~0.0) stars with planets appear to besystematically underabundant in [α/Fe] by ~0.1 dex with respect tocomparison field stars. The reason for this signature is unclear, butsystematic differences in the analysis procedures adopted by differentgroups cannot be ruled out.
| Abundance ratios of volatile vs. refractory elements in planet-harbouring stars: hints of pollution?
We present the [ X/H] trends as a function of the elemental condensationtemperature TC in 88 planet host stars and in avolume-limited comparison sample of 33 dwarfs without detected planetarycompanions. We gathered homogeneous abundance results for many volatileand refractory elements spanning a wide range of T_C, from a few dozento several hundred kelvin. We investigate possible anomalous trends ofplanet hosts with respect to comparison sample stars to detect evidenceof possible pollution events. No significant differences are found inthe behaviour of stars with and without planets. This is consistent witha "primordial" origin of the metal excess in planet host stars. However,a subgroup of 5 planet host and 1 comparison sample stars stands out ashaving particularly high [ X/H] vs. TC slopes.
| Abundances of refractory elements in the atmospheres of stars with extrasolar planets
Aims.This work presents a uniform and homogeneous study of chemicalabundances of refractory elements in 101 stars with and 93 without knownplanetary companions. We carry out an in-depth investigation of theabundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg and Al. The newcomparison sample, spanning the metallicity range -0.70< [Fe/H]<0.50, fills the gap that previously existed, mainly at highmetallicities, in the number of stars without known planets.Methods.Weused an enlarged set of data including new observations, especially forthe field "single" comparison stars . The line list previously studiedby other authors was improved: on average we analysed 90 spectral linesin every spectrum and carefully measured more than 16 600 equivalentwidths (EW) to calculate the abundances.Results.We investigate possibledifferences between the chemical abundances of the two groups of stars,both with and without planets. The results are globally comparable tothose obtained by other authors, and in most cases the abundance trendsof planet-host stars are very similar to those of the comparison sample.Conclusions.This work represents a step towards the comprehension ofrecently discovered planetary systems. These results could also beuseful for verifying galactic models at high metallicities andconsequently improve our knowledge of stellar nucleosynthesis andgalactic chemical evolution.
| Frequency of Hot Jupiters and Very Hot Jupiters from the OGLE-III Transit Surveys toward the Galactic Bulge and Carina
We derive the frequencies of hot Jupiters (HJs) with 3-5 day periods andvery hot Jupiters (VHJs) with 1-3 day periods by comparing the planetsactually detected in the OGLE-III survey with those predicted by ourmodels. The models are constructed following Gould and Morgan (2003) bypopulating the line of sight with stars drawn from the HipparcosCatalogue. Using these, we demonstrate that the number of stars withsensitivity to HJs and VHJs is only 5-16% of those in the OGLE-IIIfields satisfying the spectroscopic-follow-up limit of V_max < 17.5mag. Hence, the frequencies we derive are much higher than a naiveestimate would indicate. We find that at 90% confidence the fraction ofstars with planets in the two period ranges is (1/320)(1^+1.37_-0.59)for HJs and (1/710)(1^+1.10_-0.54) for VHJs. The HJ rate isstatistically indistinguishable from that found in radial velocity (RV)studies. However, we note that magnitude-limited RV samples are heavilybiased toward metal-rich (hence, planet-bearing) stars, while transitsurveys are not, and therefore we expect that more sensitive transitsurveys should find a deficit of HJs as compared to RV surveys. Thedetection of three transiting VHJs, all with periods less than 2 days,is marginally consistent with the complete absence of such detections inRV surveys. The planets detected are consistent with being uniformlydistributed between 1.00 and 1.25 Jovian radii, but there are too few inthe sample to map this distribution in detail.
| Ground-based direct detection of close-in extra-solar planets with nulling and high order adaptive optics
Ground-based direct detection of extra-solar planets is very challengingdue to high planet to star brightness contrasts. For giant close-inplanets, such as have been discovered by the radial velocity method,closer than 0.1 AU, the reflected light is predicted to be fairly highyielding a contrast ratio ranging from 10-4 to10-5 at near infra-red wavelengths. In this paper, weinvestigate direct detection of reflected light from such planets usingnulling interferometry, and high-order adaptive optics in conjunctionwith large double aperture ground-based telescopes. In thisconfiguration, at least 10-3 suppression of the entirestellar Airy pattern with small loss of planet flux as close as 0.03arcsec is achievable. Distinguishing residual starlight from the planetsignal is achieved by using the center of gravity shift method ormulticolor differential imaging. Using these assumptions, we deriveexposure times from a few minutes to several hours for direct detectionof many of the known extra-solar planets with several short-baselinedouble aperture telescopes such as the Large Binocular Telescope (LBT),the Very Large Telescope (VLT) and the Keck Telescope.
| Report on CCD activities at the Bronberg Observatory (CBA Pretoria) in 2005.
Not Available
| A link between the semimajor axis of extrasolar gas giant planets and stellar metallicity
The fact that most extrasolar planets found to date are orbitingmetal-rich stars lends credence to the core accretion mechanism of gasgiant planet formation over its competitor, the disc instabilitymechanism. However, the core accretion mechanism is not refined to thepoint of explaining orbital parameters such as the unexpected semimajoraxes and eccentricities. We propose a model that correlates themetallicity of the host star with the original semimajor axis of itsmost massive planet, prior to migration, assuming that the coreaccretion scenario governs giant gas planet formation. The modelpredicts that the optimum regions for planetary formation shift inwardsas stellar metallicity decreases, providing an explanation for theobserved absence of long-period planets in metal-poor stars. We compareour predictions with the available data on extrasolar planets for starswith masses similar to the mass of the Sun. A fitting procedure producesan estimate of what we define as the zero-age planetary orbit (ZAPO)curve as a function of the metallicity of the star. The model hints thatthe lack of planets circling metal-poor stars may be partly caused by anenhanced destruction probability during the migration process, becausethe planets lie initially closer to their central star.
| On the ages of exoplanet host stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanethost (EH) stars, observable from the southern hemisphere. We measuredthe chromospheric activity index, R'{_HK}. We compiled previouslypublished values of this index for the observed objects as well as theremaining EH stars in an effort to better smooth temporal variations andderive a more representative value of the average chromospheric activityfor each object. We used the average index to obtain ages for the groupof EH stars. In addition we applied other methods, such as: Isochrone,lithium abundance, metallicity and transverse velocity dispersions, tocompare with the chromospheric results. The kinematic method is a lessreliable age estimator because EH stars lie red-ward of Parenago'sdiscontinuity in the transverse velocity dispersion vs dereddened B-Vdiagram. The chromospheric and isochrone techniques give median ages of5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The medianage of F and G EH stars derived by the isochrone technique is 1-2 Gyrolder than that of identical spectral type nearby stars not known to beassociated with planets. However, the dispersion in both cases is large,about 2-4 Gyr. We searched for correlations between the chromosphericand isochrone ages and L_IR/L* (the excess over the stellarluminosity) and the metallicity of the EH stars. No clear tendency isfound in the first case, whereas the metallicy dispersion seems toslightly increase with age.
| A Bayesian Analysis of Extrasolar Planet Data for HD 73526
A new Bayesian tool for nonlinear model fitting has been developed thatemploys a parallel tempering Markov chain Monte Carlo algorithm with anovel statistical control system. The algorithm has been used toreanalyze precision radial velocity data for HD 73526. For asingle-planet model, three possible orbits were found with periods of127.88+0.37-0.09,190.4+1.8-2.1, and376.2+1.4-4.3 days. The 128 day orbit, with aneccentricity of 0.56+/-0.03, has a maximum value ofprior×likelihood that is 16 times larger than for the next highestsolution at 376 days. However, the 376 day orbit, with an eccentricityof 0.10+0.05-0.10, is formally more probablebecause for this sparse data set there is a much larger volume ofparameter space with a significant probability density in the vicinityof the 376 day peak. The previously reported orbit (Tinney et al. 2003)of 190.5+/-3.0 days corresponds to our least probable orbit. Theanalysis highlights the need for measurements around phase 0.5 for the376 day period.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| Abundances of Na, Mg and Al in stars with giant planets
We present Na, Mg and Al abundances in a set of 98 stars with knowngiant planets, and in a comparison sample of 41 “single”stars. The results show that the [X/H] abundances (with X = Na, Mg andAl) are, on average, higher in stars with giant planets, a resultsimilar to the one found for iron. However, we did not find any strongdifference in the [X/Fe] ratios, for a fixed [Fe/H], between the twosamples of stars in the region where the samples overlap. The data wasused to study the Galactic chemical evolution trends for Na, Mg and Aland to discuss the possible influence of planets on this evolution. Theresults, similar to those obtained by other authors, show that the[X/Fe] ratios all decrease as a function of metallicity up to solarvalues. While for Mg and Al this trend then becomes relatively constant,for Na we find indications of an upturn up to [Fe/H] values close to0.25 dex. For metallicities above this value the [Na/Fe] becomesconstant.
| The Effective Temperature Scale of FGK Stars. I. Determination of Temperatures and Angular Diameters with the Infrared Flux Method
The infrared flux method (IRFM) has been applied to a sample of 135dwarf and 36 giant stars covering the following regions of theatmospheric parameter space: (1) the metal-rich ([Fe/H]>~0) end(consisting mostly of planet-hosting stars), (2) the cool(Teff<~5000 K) metal-poor (-1<~[Fe/H]<~-3) dwarfregion, and (3) the very metal-poor ([Fe/H]<~-2.5) end. These starswere especially selected to cover gaps in previous works onTeff versus color relations, particularly the IRFMTeff scale of A. Alonso and collaborators. Our IRFMimplementation was largely based on the Alonso et al. study (absoluteinfrared flux calibration, bolometric flux calibration, etc.) with theaim of extending the ranges of applicability of their Teffversus color calibrations. In addition, in order to improve the internalaccuracy of the IRFM Teff scale, we recomputed thetemperatures of almost all stars from the Alonso et al. work usingupdated input data. The updated temperatures do not significantly differfrom the original ones, with few exceptions, leaving the Teffscale of Alonso et al. mostly unchanged. Including the stars withupdated temperatures, a large sample of 580 dwarf and 470 giant stars(in the field and in clusters), which cover the ranges3600K<~Teff<~8000K and -4.0<~[Fe/H]<~+0.5, haveTeff homogeneously determined with the IRFM. The meanuncertainty of the temperatures derived is 75 K for dwarfs and 60 K forgiants, which is about 1.3% at solar temperature and 4500 K,respectively. It is shown that the IRFM temperatures are reliable in anabsolute scale given the consistency of the angular diameters resultingfrom the IRFM with those measured by long baseline interferometry, lunaroccultation, and transit observations. Using the measured angulardiameters and bolometric fluxes, a comparison is made between IRFM anddirect temperatures, which shows excellent agreement, with the meandifference being less than 10 K for giants and about 20 K for dwarfstars (the IRFM temperatures being larger in both cases). This resultwas obtained for giants in the ranges 3800K
| Stellar wind regimes of close-in extrasolar planets
Close-in extrasolar planets of Sun-like stars are exposed to stellarwind conditions that differ considerably from those for planets in thesolar system. Unfortunately, these stellar winds belong to the stillunknown parameters of these planetary systems. On the other hand, theyplay a crucial role in a number of star-planet interaction processesthat may lead to observable radiation events. In order to lay afoundation for the investigation of such interaction processes, weestimate stellar wind parameters on the basis of the solar wind model byWeber & Davis and study the implications of the stellar magneticfields. Our results suggest that in contrast to the solar systemplanets, some close-in extrasolar planets may be obstacles in asub-Alfvénic stellar wind flow. In this case, the stellar windmagnetic pressure is comparable to or even larger than the dynamic flowpressure. We discuss possible consequences of these findings for thewind-exoplanet interactions. Further, we derive upper limit estimatesfor the energies such stellar winds can deposit in the exoplanetarymagnetospheres. We finally discuss the implications thesub-Alfvénic environment may have on the star-planet interaction.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo not show dilution of metallicity, (3) no abundance variations for Na,Si, Ti, or Ni are found as a function of condensation temperature, and(4) no correlations between metallicity and orbital period oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| Prospects for Habitable ``Earths'' in Known Exoplanetary Systems
We have examined whether putative Earth-mass planets could remainconfined to the habitable zones (HZs) of the 111 exoplanetary systemsconfirmed by 2004 August. We find that in about half of these systemsthere could be confinement for at least the past 1000 Myr, though insome cases only in variously restricted regions of the HZ. The HZmigrates outward during the main-sequence lifetime, and we find that inabout two-thirds of the systems an Earth-mass planet could be confinedto the HZ for at least 1000 Myr sometime during the main-sequencelifetime. Clearly, these systems should be high on the target list forexploration for terrestrial planets. We have reached our conclusions bydetailed investigations of seven systems, which has resulted in anestimate of the distance from the giant planet within which orbitalstability is unlikely for an Earth-mass planet. This distance is givenby nRH, where RH is the Hill radius of the giantplanet and n is a multiplier that depends on the giant's orbitaleccentricity and on whether the Earth-mass planet is interior orexterior to the giant planet. We have estimated n for each of the sevensystems by launching Earth-mass planets in various orbits and followingtheir fate with a hybrid orbital integrator. We have then evaluated thehabitability of the other exoplanetary systems using nRHderived from the giant's orbital eccentricity without carrying outtime-consuming orbital integrations. A stellar evolution model has beenused to obtain the HZs throughout the main-sequence lifetime.
| Hot Jupiters and Hot Spots: The Short- and Long-Term Chromospheric Activity on Stars with Giant Planets
We monitored the chromospheric activity in the Ca II H and K lines of 13solar-type stars (including the Sun): 8 of them over 3 years at theCanada-France-Hawaii Telescope (CFHT) and 5 in a single run at the VeryLarge Telescope (VLT). A total of 10 of the 13 targets have closeplanetary companions. All of the stars observed at the CFHT showlong-term (months to years) changes in H and K intensity levels. Fourstars display short-term (days) cyclical activity. For two, HD 73256 andκ1 Cet, the activity is likely associated with anactive region rotating with the star; however, the flaring in excess ofthe rotational modulation may be associated with a hot Jupiter. Aplanetary companion remains a possibility for κ1 Cet.For the other two, HD 179949 and υ And, the cyclic variation issynchronized to the hot Jupiter's orbit. For both stars thissynchronicity with the orbit is clearly seen in two out of three epochs.The effect is only marginal in the third epoch at which the seasonallevel of chromospheric activity had changed for both stars. Short-termchromospheric activity appears weakly dependent on the mean K linereversal intensities for the sample of 13 stars. In addition, asuggestive correlation exists between this activity and theMpsini of the star's hot Jupiter. Because of their smallseparation (<=0.1 AU), many of the hot Jupiters lie within theAlfvén radius of their host stars, which allows a direct magneticinteraction with the stellar surface. We discuss the conditions underwhich a planet's magnetic field might induce activity on the stellarsurface and why no such effect was seen for the prime candidate, τBoo. This work opens up the possibility of characterizing planet-starinteractions, with implications for extrasolar planet magnetic fieldsand the energy contribution to stellar atmospheres.Based on observations collected at the Canada-France-Hawaii Telescopeoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii, aswell as data from the European Southern Observatory's Very LargeTelescope, Chile (programme ESO 73.C-0694).
| Quantifying the Uncertainty in the Orbits of Extrasolar Planets
Precise radial velocity measurements have led to the discovery of ~100extrasolar planetary systems. We investigate the uncertainty in theorbital solutions that have been fitted to these observations.Understanding these uncertainties will become more and more important asthe discovery space for extrasolar planets shifts to longer and longerperiods. While detections of short-period planets can be rapidlyrefined, planets with long orbital periods will require observationsspanning decades to constrain the orbital parameters precisely. Alreadyin some cases, multiple distinct orbital solutions provide similarlygood fits, particularly in multiple-planet systems. We present a methodfor quantifying the uncertainties in orbital fits and addressingspecific questions directly from the observational data rather thanrelying on best-fit orbital solutions. This Markov chain Monte Carlo(MCMC) technique has the advantage that it is well suited to thehigh-dimensional parameter spaces necessary for the multiple-planetsystems. We apply the MCMC technique to several extrasolar planetarysystems, assessing the uncertainties in orbital elements for severalsystems. Our MCMC simulations demonstrate that for some systems thereare strong correlations between orbital parameters and/or significantnon-Gaussianities in parameter distributions, even though themeasurement errors are nearly Gaussian. Once these effects areconsidered, the actual uncertainties in orbital elements can besignificantly larger or smaller than the published uncertainties. Wealso present simple applications of our methods, such as predicting thetimes of possible transits for GJ 876.
| Radial Velocity Detectability of Low-Mass Extrasolar Planets in Close Orbits
Detection of Jupiter-mass companions to nearby solar-type stars withprecise radial velocity measurements is now routine, and Doppler surveysare moving toward lower velocity amplitudes. The detection of severalNeptune-mass planets with orbital periods of less than a week has beenreported. The drive toward the search for close-in, Earth-mass planetsis on the agenda. Successful detection or meaningful upper limits willplace important constraints on the process of planet formation. In thispaper, we quantify the statistics of detection of low-mass planets inclose orbits, showing how the detection threshold depends on the numberand timing of the observations. In particular, we consider the case of alow-mass planet close to but not on the 2:1 mean motion resonance with ahot Jupiter. This scenario is a likely product of the core-accretionhypothesis for planet formation coupled with migration of Jupiters inthe protoplanetary disk. It is also advantageous for detection becausethe orbital period is well constrained. We show that the minimumdetectable mass is ~4 M⊕(N/20)-1/2(σ/ms-1)(P/days)1/3(M*/Msolar)2/3for N>=20, where N is the number of observations, P is the orbitalperiod, σ is the quadrature sum of Doppler velocity measurementerrors and stellar jitter, and M* is the stellar mass.Detection of few Earth-mass rocky cores will require ~1 m s-1velocity precision and, most important, a better understanding ofstellar radial velocity ``jitter.''
| Five New Extrasolar Planets
We report multiple Doppler measurements of five nearby FGK main-sequencestars and subgiants obtained during the past 4-6 yr at the KeckObservatory. These stars, namely, HD 183263, HD 117207, HD 188015, HD45350, and HD 99492, all exhibit coherent variations in their Dopplershifts consistent with a planet in Keplerian motion. The five newplanets occupy known realms of planetary parameter space, including awide range of orbital eccentricities, e=0-0.78, and semimajor axes,0.1-3.8 AU, that provide further statistical information about the truedistributions of various properties of planetary systems. One of theplanets, HD 99492b, has a low minimum mass of0.112MJup=36MEarth. Four of the five planets orbitbeyond 1 AU. We describe two quantitative tests of the false alarmprobability for Keplerian interpretations of measured velocities. Themore robust of these involves Monte Carlo realizations of scrambledvelocities as a proxy for noise. Keplerian orbital fits to that``noise'' yield the distribution of χ2ν tocompare with χ2ν from the original(unscrambled) velocities. We establish a 1% false alarm probability asthe criterion for candidate planets. All five of these planet-bearingstars are metal-rich, with [Fe/H]>+0.27, reinforcing the strongcorrelation between planet occurrence and metallicity. From the fullsample of 1330 stars monitored at Keck, Lick, and the Anglo-AustralianTelescope, the shortest orbital period for any planet is 2.64 days,showing that shorter periods occur less frequently than 0.1% in thesolar neighborhood. Photometric observations were acquired for four ofthe five host stars with an automatic telescope at Fairborn Observatory.The lack of brightness variations in phase with the radial velocitiessupports planetary-reflex motion as the cause of the velocityvariations. No transits were observed, but their occurrence is not ruledout by our observations.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
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