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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.
| 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.
| Elemental abundance survey of the Galactic thick disc
We have performed an abundance analysis for F- and G- dwarfs of theGalactic thick-disc component. A sample of 176 nearby (d<= 150pc)thick-disc candidate stars was chosen from the Hipparcos catalogue andsubjected to a high-resolution spectroscopic analysis. Using accurateradial velocities combined with the Hipparcos astrometry, kinematics (U,V and W) and Galactic orbital parameters were computed. We estimate theprobability for a star to belong to the thin disc, the thick disc or thehalo. With a probability P>= 70 per cent taken as certain membership,we assigned 95 stars to the thick disc, 13 to the thin disc, and 20 tothe halo. The remaining 48 stars in the sample cannot be assigned withreasonable certainty to one of the three components.Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, Y, Ba, Ce, Nd and Eu have been obtained. The abundances for thethick-disc stars are compared with those for the thin-disc members fromReddy et al. The ratios of α-elements (O, Mg, Si, Ca and Ti) toiron for thick-disc stars show a clear enhancement compared to thin-discmembers in the range -0.3 < [Fe/H] < -1.2. There are also otherelements - Al, Sc, V, Co, and possibly Zn - which show enhanced ratiosto iron in the thick disc relative to the thin disc. The abundances ofNa, Cr, Mn, Ni and Cu (relative to Fe) are very similar for thin- andthick-disc stars. The dispersion in abundance ratios [X/Fe] at given[Fe/H] for thick-disc stars is consistent with the expected scatter dueto measurement errors, suggesting a lack of `cosmic' scatter.A few stars classified as members of the thick disc by our kinematiccriteria show thin-disc abundances. These stars, which appear older thanmost thin-disc stars, are also, on average, younger than the thick-discpopulation. They may have originated early in the thin-disc history, andbeen subsequently scattered to hotter orbits by collisions. The thickdisc may not include stars with [Fe/H] > -0.3. The observedcompositions of the thin and thick discs seem to be consistent with themodels of galaxy formation by hierarchical clustering in a Lambda colddark matter (ΛCDM) universe.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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
| 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.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| Spectroscopic binary orbits from photoelectric radial velocities. Paper 173: HD 111306, HD 113023, HD 117901, and HD 142474.
Not Available
| 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
| High-resolution spectral analysis of the mildly metal-poor stars II. Chemical abundances
Based on the first paper, here the model parameters of a set of samplestars and the chemical abundances for nine elements are presented. Therelations between the elemental abundances and [Fe/H] are alsodiscussed. On average, [Na/Fe] tends to -0.01dex and shows solarpattern. The ? elements, Si and Ca, show the same abundance patterns,while [Ti/Fe] has a large scatter. The abundances of these threeelements all decrease with [Fe/H] increasing. The iron-peak elements V,Cr, Ni show a large scatter at different metallicity, and [Ca/Fe] isoverabundant in all the sample stars, while [Mn/Fe] is definitelyunder-abundant and increases with the metallicity increasing.
| High-resolution spectral analysis of the mildly metal-poor stars I. Spectral data and equivalent widths
The high resolution, high signal to noise ratio spectra of a sample ofmildly metal-poor stars were obtained with the Coude EchelleSpectrograph attached to the 2.16m telescope at Beijing AstronomicalObservatory (Xinglong, China). The spectra are reduced with the MIDASpackage and the equivalent widths of seven stars were measured. Theerror of the equivalent widths is estimated by comparing them with theother independent measurements made by the other authors. The standarddeviations are presented in the end.
| 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.
| The low-mass white dwarf companion to β Crateris
We present FUSE H Lyman series spectroscopy of the hot white dwarfcompanion to the 4th magnitude A1III star βCrt, which shows that ithas an unusually low mass, MWD=0.43Msolar, and hasalmost certainly evolved through binary interaction. This system couldbe a long-sought remnant of Algol-type evolution, although radialvelocity measurements appear to show that the pair are not close.Instead, micro-variations in the proper motion of βCrt as measuredby Hipparcos suggest that the period could be as high as ~10yr. However,a low-mass white dwarf in a system with a period >~3yr is difficultto explain by conventional models for binary evolution. We speculate onalternative models for the evolution of this system which involve aneccentric binary or multiple components.
| 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.
| 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).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| Planets without stars : the probable abundance, nature, and significance of ISPs.
Not Available
| Galactic [O/Fe] and [C/Fe] Ratios: The Influence of New Stellar Parameters
We consider the effects of recent NLTE gravities and Fe abundances onstellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce oreliminate the well-known discrepancy between CH- and C I-based Cabundances in metal-poor stars and previously seen trends ofatomic-based [C/Fe] and [O/Fe] with Teff. With the NLTEparameters, the metal-poor molecular-based [C/Fe] ratio maintains itsincrease with declining [Fe/H] this may also be demonstrated by therevised atomic-based ratios. [O/Fe] values derived from OH and O Ifeatures are considerably reduced and typically show improved agreementbut are 0.1-0.2 dex larger than those exhibited by the Lick-Texassyndicate's recent [O I]-based giant determinations. The revised [O/Fe]ratios still show an increase down to at least [Fe/H]~-2 we suggest thatrecent field giant data show an increase with similar slope. Evenadopting uniform NLTE parameters, study-to-study abundance differencescan be significant; moreover, different NLTE studies yield differinggravities and Fe abundances even after taking Teffdifferences into account. Comparison of metal-poor giant gravities andcluster abundances with isochrones, trigonometric gravities, andnear-turnoff cluster abundances yields conflicting indications aboutwhether the evolved gravities might be underestimated as suggested formetal-poor dwarfs. Regardless, we argue that even extreme gravityrevisions do not affect the [O/Fe]-[Fe/H] relation derived from theextant results. Combining what we believe the most reliable giant anddwarf data considered here, we find[O/Fe]=-0.184(+/-0.022)×[Fe/H]+0.019 with an rms scatter of only0.13 dex; there is no indication of a break or slope change atintermediate [Fe/H]. The gentle slope is in very reasonable agreementwith some chemical evolution models employing yields with small mass andmetallicity dependences. Finally, two notes are made concerning Naabundance-spatial position and element-to-element correlations in M13giants.
| 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.
| 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.
| Stellar Iron Abundances: Non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II inthe atmosphere of late-type stars. We used atomic models for Fe I and FeII having, respectively, 256 and 190 levels, as well as 2117 and 3443radiative transitions. Photoionization cross sections are from the IronProject. These atomic models were used to investigate non-LTE (NLTE)effects in iron abundances of late-type stars with different atmosphericparameters. We found that most Fe I lines in metal-poor stars are formedin conditions far from LTE. We derived metallicity corrections of about0.3 dex with respect to LTE values for the case of stars with[Fe/H]~-3.0. Fe II is found not to be affected by significant NLTEeffects. The main NLTE effect invoked in the case of Fe I isoverionization by ultraviolet radiation; thus classical ionizationequilibrium is far from being satisfied. An important consequence isthat surface gravities derived by LTE analysis are in error and shouldbe corrected before final abundance corrections. This apparently solvesthe observed discrepancy between spectroscopic surface gravities derivedby LTE analyses and those derived from Hipparcos parallaxes. A table ofNLTE [Fe/H] and log g values for a sample of metal-poor late-type starsis given.
| The mass distribution of extrasolar planet candidates and low-mass secondaries.
Not Available
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Habitable Moons
Not Available
| MEETING ASTROPHYSICS:Enhanced: Protostars and Planets
Not Available
| Formation of undermassive single white dwarfs and the influence of planets on late stellar evolution
We propose a scenario to form low-mass, single, slow rotating whitedwarfs from a solar-like star accompanied by a massive planet, or abrown dwarf, in a relatively close orbit (e.g. HD 89707). Such whitedwarfs were recently found by Maxted & Marsh (1998). When thesolar-like star ascends the giant branch it captures the planet and thesubsequent spiral-in phase expels the envelope of the giant leaving alow-mass helium white dwarf remnant. In case the planet evaporizes, orfills its own Roche-lobe, the outcome is a single undermassive whitedwarf. The observed distribution of planetary systems supports theapplicability of this scenario.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Északi Vízikígyó |
| Rektaszcenzió: | 10h20m49.98s |
| Deklináció: | -15°28'47.7" |
| Vizuális fényesség: | 7.191 |
| Távolság: | 34.614 parszek |
| RA sajátmozgás: | -221 |
| Dec sajátmozgás: | 294.3 |
| B-T magnitude: | 7.829 |
| V-T magnitude: | 7.244 |
Katalógusok és elnevezések:
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INHABITED SKY
