תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The eccentricity-mass distribution of exoplanets: signatures of different formation mechanisms?
We examine the distributions of eccentricity and host star metallicityof exoplanets as a function of their mass. Planets with M sin i ⪆ 4MJ have an eccentricity distribution consistent with that ofbinary stars, while planets with M sin i ⪉ 4 MJ are lesseccentric than binary stars and more massive planets. In addition, hoststar metallicities decrease with planet mass. The statisticalsignificance of both of these trends is only marginal with the presentsample of exoplanets. To account for these trends, we hypothesize thatthere are two populations of gaseous planets: the low-mass populationforms by gas accretion onto a rock-ice core in a circumstellar disk andis more abundant at high metallicities, and the high-mass populationforms directly by fragmentation of a pre-stellar cloud. Planets of thefirst population form in initially circular orbits and grow theireccentricities later, and may have a mass upper limit from the totalmass of the disk that can be accreted by the core. The second populationmay have a mass lower limit resulting from opacity-limitedfragmentation. This would roughly divide the two populations in mass,although they would likely overlap over some mass range. If most objectsin the second population form before the pre-stellar cloud becomeshighly opaque, they would have to be initially located in orbits largerthan ~30 AU, and would need to migrate to the much smaller orbits inwhich they are observed. The higher mean orbital eccentricity of thesecond population might be caused by the larger required intervals ofradial migration, and the brown dwarf desert might be due to theinability of high-mass brown dwarfs to migrate inwards sufficiently inradius.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| 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.
| Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood
A new metallicity distribution and an age-metallicity relation arepresented for 437 nearby F and G turn-off and sub-giant stars selectedfrom radial velocity data of Nidever et al. Photometric metallicitiesare derived from uvby- Hβ photometry, and the stellar ages from theisochrones of Bergbusch & VandenBerg as transformed to uvbyphotometry using the methods of Clem et al.The X (stellar population) criterion of Schuster et al., which combinesboth kinematic and metallicity information, provides 22 thick-discstars. σW= 32 +/- 5 km s-1,= 154 +/- 6 km s-1 and<[M/H]>=-0.55 +/- 0.03 dex for these thick-disc stars, which is inagreement with values from previous studies of the thick disc.α-element abundances which are available for some of thesethick-disc stars show the typical α-element signatures of thethick disc, supporting the classification procedure based on the Xcriterion.Both the scatter in metallicity at a given age and the presence of old,metal-rich stars in the age-metallicity relation make it difficult todecide whether or not an age-metallicity relation exists for the olderthin-disc stars. For ages greater than 3 Gyr, our results agree with theother recent studies that there is almost no correlation between age andmetallicity, Δ([M/Fe])/Δ(age) =-0.01 +/- 0.005 dexGyr-1. For the 22 thick-disc stars there is a range in agesof 7-8 Gyr, but again almost no correlation between age and metallicity.For the subset of main-sequence stars with extra-solar planets, theage-metallicity relation is very similar to that of the total sample,very flat, the main difference being that these stars are mostlymetal-rich, [M/H]>~-0.2 dex. However, two of these stars have[M/H]~-0.6 dex and have been classified as thick-disc stars. As for thetotal sample, the range in ages for these stars with extra-solarplanetary systems is considerable with a nearly uniform distributionover 3 <~ age <~ 13 Gyr.
| α-, r-, and s-process element trends in the Galactic thin and thick disks
From a detailed elemental abundance analysis of 102 F and G dwarf starswe present abundance trends in the Galactic thin and thick disks for 14elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu).Stellar parameters and elemental abundances (except for Y, Ba and Eu)for 66 of the 102 stars were presented in our previous studies (Bensbyet al. [CITE], A&A, 410, 527, [CITE], A&A, 415, 155). The 36stars that are new in this study extend and confirm our previous resultsand allow us to draw further conclusions regarding abundance trends. Thes-process elements Y and Ba, and the r-element Eu have also beenconsidered here for the whole sample for the first time. With this newlarger sample we now have the following results: 1) smooth and distinctabundance trends that for the thin and thick disks are clearlyseparated; 2) the α-element trends for the thick disk show typicalsignatures from the enrichment of SN Ia; 3) the thick disk stellarsample is in the mean older than the thin disk stellar sample; 4) thethick disk abundance trends are invariant with galactocentric radii(R_m); 5) the thick disk abundance trends appear to be invariant withvertical distance (Z_max) from the Galactic plane. Adding furtherevidence from the literaure we argue that a merger/interacting scenariowith a companion galaxy to produce a kinematical heating of the stars(that make up today's thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk. The 102stars have -1 ≲ [Fe/H] ≲ +0.4 and are all in the solarneighbourhood. Based on their kinematics they have been divided into athin disk sample and a thick disk sample consisting of 60 and 38 stars,respectively. The remaining 4 stars have kinematics that make themkinematically intermediate to the two disks. Their chemical abundancesalso place them in between the two disks. Which of the two diskpopulations these 4 stars belong to, or if they form a distinctpopulation of their own, can at the moment not be settled. The 66 starsfrom our previous studies were observed with the FEROS spectrograph onthe ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-mtelescope. Of the 36 new stars presented here 30 were observed with theSOFIN spectrograph on the Nordic Optical Telescope on La Palma, 3 withthe UVES spectrograph on VLT/UT2, and 3 with the FEROS spectrograph onthe ESO 1.5-m telescope. All spectra have high signal-to-noise ratios(typically S/N≳ 250) and high resolution (R 80 000, 45 000,and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).Based on observations collected at the Nordic Optical Telescope on LaPalma, Spain, and at the European Southern Observatories on La Silla andParanal, Chile, Proposals # 65.L-0019(B), 67.B-0108(B), 69.B-0277. FullTables [see full text], [see full text] and [see full text] are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/433/185
| HD 137510: An Oasis in the Brown Dwarf Desert
Since the beginning of precise Doppler surveys, which have had stunningsuccess in detecting extrasolar planetary companions, one surprisingenigma has emerged: the relative paucity of spectroscopic binaries wherethe secondary mass lies in between the stellar and planetary massregime. This gap in the mass function for close-in (a<3-4 AU)companions to solar-type stars is generally referred to as the ``browndwarf desert.'' Here we report the detection of a companion to HD 137510(G0 IV), with a minimum mass of 26MJ, moving in an eccentricorbit (e=0.4) with a period of 798 days and an orbital semimajor axis of1.85 AU. The detection is based on precise differential radial velocitydata obtained by the McDonald Observatory and ThüringerLandessternwarte Tautenburg planet search programs.Based on observations made at McDonald Observatory and theThüringer Landessternwarte Tautenburg.
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Abundance Analysis of Planetary Host Stars. I. Differential Iron Abundances
We present atmospheric parameters and iron abundances derived fromhigh-resolution spectra for three samples of dwarf stars: stars that areknown to host close-in giant planets (CGP), stars for which radialvelocity data exclude the presence of a close-in giant planetarycompanion (no-CGP), as well as a random sample of dwarfs with a spectraltype and magnitude distribution similar to that of the planetary hoststars (control). All stars have been observed with the same instrumentand have been analyzed using the same model atmospheres, atomic data,and equivalent width modeling program. Abundances have been deriveddifferentially to the Sun, using a solar spectrum obtained with Callistoas the reflector with the same instrumentation. We find that the ironabundances of CGP dwarfs are on average 0.22 dex greater than that ofno-CGP dwarfs. The iron abundance distributions of both the CGP andno-CGP dwarfs are different than that of the control dwarfs, while thecombined iron abundances have a distribution that is very similar tothat of the control dwarfs. All four samples (CGP, no-CGP, combined, andcontrol) have different effective temperature distributions. We showthat metal enrichment occurs only for CGP dwarfs with temperatures justbelow solar and ~300 K higher than solar, whereas the abundancedifference is insignificant at Teff around 6000 K.
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| Chemical Abundances of 15 Extrasolar Planet Host Stars
Based on high-resolution, high signal-to-noise ratio spectra, we havederived accurate metallicities for 15 extrasolar planet host stars, aswell as differential abundances for 16 other elements. The companions ofthree stars, HD 29587, HD 98230, and possibly HD 190228, have been shownto have masses in the brown dwarf regime and give an average value ofthe metallicities [Fe/H]=-0.44+/-0.17. If we exclude these three stars,the rest of the sample stars are metal-rich relative to the Sun,covering the range from -0.12 to 0.41, with an average [Fe/H] value of0.16+/-0.17. The stars with planets show a slight overabundance patternfor [Na/Fe], [Mg/Fe], [Al/Fe], and [Sc/Fe]. On the other hand, [C/Fe],[O/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe], [V/Fe], [Cr/Fe], [Ni/Fe], and [Ba/Fe]are approximately solar in the sample. The stars with planets also showunderabundances for [K/Fe] and slight overabundances for [Mn/Fe], butstars with brown dwarfs show a contrary trend for these two elements. Aninteresting result is that sulfur displays enhanced values of [S/Fe],ranging from 0.04 to 0.40 through the sample. These results are used toinvestigate the connection between giant planets and high metallicityand to probe the influence of this process on the other elements. Basedon observations collected at the National Astronomical Observatories(Xinglong, China).
| Equivalent Widths of 15 Extrasolar-Planet Host Stars
Not Available
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| Planet Host Stars: Mass, Age and Kinematics
We determine the mass, age and kinematics of 51 extra-solar planet hoststars. The results are then used to search for signs of connection ofthe data with metallicity and to investigate the population nature. Wefind that the increase in mean metallicity with stellar mass is similarto that in normal field stars, so it seems unsuitable to use thisrelation as a constraint on the theory of planet formation. The age andkinematic distributions seem to favour the metallicity of extra-solarplanet host stars being initial. Although the kinematic data of thesestars indicate their origin from two populations -- the thin and thethick disks, kinematics may not help in the maintenance of the planetaround the host. Stars with planets, brown dwarfs or stellar companionsare sorted into three groups and re-investigated separately for theirformation mechanism. The main results indicate that stars withM2 < 25 MJ have [Fe/H] > -0.1 and a wideperiod range, but there are no other differences. Thus, there does notseem to be any physically distinguishable characteristics among thethree star groups.
| Analysis of the Hipparcos Observations of the Extrasolar Planets and the Brown Dwarf Candidates
We analyzed the Hipparcos astrometric observations of 47 stars that werediscovered to harbor giant planets and 14 stars with brown dwarfsecondary candidates. The Hipparcos measurements were used together withthe corresponding stellar radial velocity data to derive an astrometricorbit for each system. To find out the significance of the derivedastrometric orbits, we applied a ``permutation'' technique by which weanalyzed the permuted Hipparcos data to get false orbits. The sizedistribution of these false orbits indicated the range of possiblyrandom, false orbits that could be derived from the true data. Thesetests could not find any astrometric orbit of the planet candidates withsignificance higher than 99%, suggesting that most if not all orbits arenot real. Instead, we used the Hipparcos data to set upper limits on themasses of the planet candidates. The lowest derived upper limit is thatof 47 UMa-0.014 Msolar, which confirms the planetary natureof its unseen companion. For 13 other planet candidates, the upperlimits exclude the stellar nature of their companions, although browndwarf secondaries are still an option. These negate the idea that all ormost of the extrasolar planets are disguised stellar secondaries. Of the14 brown dwarf candidates, our analysis reproduced the results ofHalbwachs et al., who derived significant astrometric orbits for sixsystems that imply secondaries with stellar masses. We show that anotherstar, HD 164427, which was discovered only very recently, also has asecondary with stellar mass. Our findings support Halbwachs et al.'sconclusion about the possible existence of the ``brown dwarf desert''that separates the planets and the stellar secondaries.
| Migration of giant planets in planetesimal discs
Planets orbiting a planetesimal circumstellar disc can migrate inwardfrom their initial positions because of dynamical friction betweenplanets and planetesimals. The migration rate depends on the disc massand on its time evolution. Planets that are embedded in long-livedplanetesimal discs, having total mass of10-4-0.01Msolar, can migrate inward a largedistance and can survive only if the inner disc is truncated or as aresult of tidal interaction with the star. In this case the semimajoraxis, a, of the planetary orbit is less than 0.1au. Orbits with larger aare obtained for smaller values of the disc mass or for a rapidevolution (depletion) of the disc. This model may explain not onlyseveral of the orbital features of the giant planets that have beendiscovered in recent years orbiting nearby stars, but also themetallicity enhancement found in several stars associated withshort-period planets.
| Two Substellar Companions Orbiting HD 168443
Precise Doppler measurements during 4.4 yr from the Keck/HIRESspectrometer reveal two superimposed Keplerian velocity variations forHD 168443 (G6 IV). A simultaneous orbital fit to both companions yieldscompanion masses of Msini=7.7 and 17.2 MJUP, orbital periodsof P=58 days and 4.8 yr, semimajor axes of a=0.29 and 2.9 AU, andeccentricities of e=0.53 and 0.20. An upper limit to the mass of theouter companion of 42 MJUP is derived from the lack ofastrometric wobble. The outer companion was not detected with Keckadaptive optics in the near-IR. Dynamical simulations show that thesystem is remarkably stable for all possible masses of both companions.The two orbiting companions have masses that are probably near andslightly above the upper end of the observed mass distribution of``planets'' at 10 MJUP. Formation in a protoplanetary diskseems plausible. But these objects present a puzzle about theirformation and dynamical history, as well as about their possible kinshipwith planetary systems and triple-star systems. Based on observationsobtained at the W. M. Keck Observatory, which is operated jointly by theUniversity of California and the California Institute of Technology.
| The companion of HD 190228: Planet or brown dwarf?
A detailed abundance analysis has been carried out from high-resolution,high signal-to-noise spectra of the G5IV star HD190228, which is announced to harbor an extrasolar planet withMPsin i of 5.0 MJ and an orbital period of 1127days. Based on the model atmosphere of Teff=5180 K, {log g}=3.7, xit =1.3 km s-1, we obtained [Fe/H]=-0.40,which puts it on the metal-poor tail of the metallicity distribution ofthe so far discovered 48 planet-harboring stars. The relative abundance,[X/Fe], indicates an overabundance of light elements (O, Na, Mg, Al, Si,S, Sc) by 0.1-0.2 dex and the solar abundance of heavier elements (K,Ca, Ti, V, Cr, Mn, Ni, Ba). These elements show no conspicuousanomalies. The solar [C/Fe] seems to be slightly smaller than theaverage (but within the scatter) of field stars of the same [Fe/H], andthere is no clear trend of [X/H] with condensation temperature of theelement. Neither the process of planet formation nor the stellardilution during the subgiant stage seems to have polluted its chemicalcomposition. The initially low metallicity may be explained by thesuggestion that HD 190228 is accompanied by a browndwarf instead of a planet. Based on observation carried out at BeijingAstronomical Observatory (Xinglong, PR China).
| Exploring the brown dwarf desert with Hipparcos
The orbital elements of 11 spectroscopic binaries with brown dwarfcandidates (M2 sin i between 0.01 and 0.08 Msun)are combined with the Hipparcos observations in order to deriveastrometric orbits. Estimations of the masses of the secondarycomponents are thus calculated. It appears that 5 secondary masses aremore than 2 sigmaM2 above the limit of 0.08Msun, and are therefore not brown dwarfs. 2 other stars arestill discarded at the 1 sigmaM2 level, 1 browndwarf is accepted with a low confidence, and we are finally left with 3viable candidates which must be studied by other means. A statisticalapproach is developed, based on the relation between the semi-major axesof the photocentric orbit, a_0, their errors, sigma a_0, andthe frequency distribution of the mass ratios, q. It is investigatedwhether the set of values of a_0 and sigma a_0 obtained forthe sample is compatible with different frequency distributions of q. Itis concluded that a minimum actually exists for M2 betweenabout 0.01 and 0.1 Msun for companions of solar-type stars.This feature could correspond to the transition between giant planetsand stellar companions. Due to the relatively large frequency of singlebrown dwarfs found recently in open clusters, it is concluded that thedistribution of the masses of the secondary components in binary systemsdoes not correspond to the IMF, at least for masses below thehydrogen-ignition limit. Based on photoelectric radial-velocitymeasurements collected at Haute-Provence observatory and on observationsmade with the ESA Hipparcos astrometry satellite.
| Observations of Brown Dwarfs
The brown dwarfs occupy the gap between the least massive star and themost massive planet. They begin as dimly stellar in appearance andexperience fusion (of at least deuterium) in their interiors. But theyare never able to stabilize their luminosity or temperature and growever fainter and cooler with time. For that reason, they can be viewedas a constituent of baryonic "dark matter." Indeed, we currently have ahard time directly seeing an old brown dwarf beyond 100 pc. After 20years of searching and false starts, the first confirmed brown dwarfswere announced in 1995. This was due to a combination of increasedsensitivity, better search strategies, and new means of distinguishingsubstellar from stellar objects. Since then, a great deal of progresshas been made on the observational front. We are now in a position tosay a substantial amount about actual brown dwarfs. We have a rough ideaof how many of them occur as solitary objects and how many are found inbinary systems. We have obtained the first glimpse of atmospheresintermediate in temperature between stars and planets, in which dustformation is a crucial process. This has led to the proposal of thefirst new spectral classes in several decades and the need for newdiagnostics for classification and setting the temperature scale. Thefirst hints on the substellar mass function are in hand, although allcurrent masses depend on models. It appears that numerically, browndwarfs may well be almost as common as stars (though they appear not tocontain a dynamically interesting amount of mass).
| On the Nature of Low-Mass Companions to Solar-like Stars
Low-mass companions (mass <70 Jupiter masses) to solar-like stars arecompared statistically to stellar-mass secondaries in binaries ofsimilar primary spectral types and orbital scales, based largely on thesurvey of Duquennoy & Mayor. To within the limits imposed byobservational constraints, the orbital properties of these low-masscompanions (LMCs) are statistically indistinguishable from those ofbinary systems. In both populations, orbital periods (P), semimajor axes(a), angular momenta (L), and binding energies (U) are all distributedapproximately as f(x)~x-1 for x=P,a,L,U. In both populations,eccentricities are broadly distributed approximately asf(e)~e-0.5, with no significant correlation with otherorbital elements, apart from a marked circularization of close orbits.The distribution of LMC masses is approximately a power law with indexbetween -1 and -2 there is ambiguous evidence in the data for a massspectrum bimodality about approximately 10 Jupiter masses. In bothpopulations the joint distributions of mass with all orbital propertiesare largely scattergrams, with no statistically significantcorrelations. The overall statistical properties of LMCs are suggestiveof a common formation mechanism with binary star systems. The similarform of the distributions of all orbital dynamic quantities in bothpopulations may result from postformation dissipative orbital decay.
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| A New Channel for the Detection of Planetary Systems through Microlensing. II. Repeating Events
In the companion paper we began the task of systematically studying thedetection of planets in wide orbits (a>1.5R_E) via microlensingsurveys. In this paper we continue, focusing on repeating events.Repeating events are those in which a distant source is lensed by two ormore masses in the planetary system. We find that, if all planetarysystems are similar to our own solar system, reasonable extensions ofthe present observing strategies would allow us to detect 3-6 repeatingevents per year along the direction to the Bulge. Indeed, if planetarysystems with multiple planets are common, then future monitoringprograms that lead to the discovery of thousands of stellar-lens eventswill likely discover events in which several different planets within asingle system serve as lenses, with light curves exhibiting multiplerepetitions. In this paper we discuss observing strategies to maximizethe discovery of all wide-orbit planet-lens events. We also compare thelikely detection rates of planets in wide orbits to those of planetslocated in the zone for resonant lensing. We find that, depending on thevalues of the planet masses and stellar radii of the lensed sources(which determine whether or not finite-source-size effects areimportant), and also on the sensitivity of the photometry used byobservers, the detection of planets in wide orbits may be the primaryroute to the discovery of planets via microlensing . We also discuss howthe combination of resonant and wide-orbit events can help us to learnabout the distribution of planetary system properties. In addition, bydetermining the fraction of short-duration events due to planets, weindirectly derive information about the fraction of all short-durationevents that may be due to low-mass MACHOs.
| A New Channel for the Detection of Planetary Systems through Microlensing. I. Isolated Events due to Planet Lenses
We propose and evaluate the feasibility of a new strategy to search forplanets via microlensing observations. This new strategy is designed todetect planets in ``wide'' orbits, i.e., with orbital separation, a,greater than ~1.5R_E. Planets in wide orbits may provide the dominantchannel for the discovery of planets via microlensing, particularlylow-mass (e.g., Earth-mass) planets. This paper concentrates on eventsin which a single planet serves as a lens, leading to an isolated eventof short duration. We point out that a distribution of events due tolensing by stars with wide-orbit planets is necessarily accompanied by adistribution of shorter duration events. The fraction of events in thelatter distribution is proportional to the average value of q^1/2, whereq is the ratio between planet and stellar masses. The position of thepeak or peaks also provides a measure of the mass ratios typical ofplanetary systems. We study detection strategies that can optimize ourability to discover isolated short-duration events due to lensing byplanets and find that monitoring employing sensitive photometry isparticularly useful. If planetary systems similar to our own are common,even modest changes in detection strategy should lead to the discovery afew isolated events of short duration every year. We therefore alsoaddress the issue of the contamination due to stellar populations of anymicrolensing signal due to low-mass MACHOs. We describe how, even forisolated events of short duration, it will be possible to test thehypothesis that the lens was a planet instead of a low-mass MACHO, ifthe central star of the planetary system contributes a measurablefraction of the baseline flux.
| The mass distribution of extrasolar planet candidates and low-mass secondaries.
Not Available
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | פרסאוס |
| התרוממות ימנית: | 04h41m36.32s |
| סירוב: | +42°07'06.5" |
| גודל גלוי: | 7.287 |
| מרחק: | 28.321 פארסק |
| תנועה נכונה: | 535.4 |
| תנועה נכונה: | -417.3 |
| B-T magnitude: | 8.025 |
| V-T magnitude: | 7.348 |
קטלוגים וכינוים:
|
