Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Non-LTE effects on the lead and thorium abundance determinations for cool stars
Context. Knowing accurate lead abundances of metal-poor stars providesconstraints on the Pb production mechanisms in the early Galaxy.Accurately deriving thorium abundances permits a nucleo-chronometric agedetermination of the star. Aims: We aim to improve thecalculation of the Pb i and Th ii lines in stellar atmospheres based onnon-local thermodynamic equilibrium (non-LTE) line formation, and toevaluate the influence of departures from LTE on Pb and Th abundancedeterminations for a range of stellar parameters by varying themetallicity from the solar value down to [Fe/H] = -3. Methods: Wepresent comprehensive model atoms for Pb i and Th ii and describecalculations of the Pb i energy levels and oscillator strengths. Results: The main non-LTE mechanism for Pb i is the ultravioletoverionization. We find that non-LTE leads to systematically depletedtotal absorption in the Pb i lines and accordingly, positive abundancecorrections. The departures from LTE increase with decreasingmetallicity. Using the semi-empirical model atmosphere HM74, wedetermine the lead non-LTE abundance for the Sun to be log?Pb, &sun; = 2.09, in agreement with the meteoriticlead abundance. We revised the Pb and Eu abundances of the two stronglyr-process enhanced stars CS 31082-001 and HE1523-0901 and the metal-poor stellar sample. Our new resultsprovide strong evidence of universal Pb-to-Eu relative r-process yieldsduring the course of Galactic evolution. The stars in the metallicityrange -2.3 < [Fe/H] < -1.4 have Pb/Eu abundance ratios that are,on average, 0.51 dex higher than those of strongly r-process enhancedstars. We conclude that the s-process production of lead started asearly as the time when Galactic metallicity had reached [Fe/H] = -2.3.The average Pb/Eu abundance ratio of the mildly metal-poor stars, with-1.4 ? [Fe/H] ? -0.59, is very close to the corresponding SolarSystem value, in line with the theoretical predictions that AGB starswith [Fe/H] ? -1 provided the largest contribution to the solarabundance of s-nuclei of lead. The departures from LTE for Th ii arecaused by the pumping transitions from the levels with Eexc< 1 eV. Non-LTE leads to weakened Th ii lines and positive abundancecorrections. Overall, the abundance correction does not exceed 0.2 dexwhen collisions with H i atoms are taken into account in statisticalequilibrium calculations.
| First stars. XIV. Sulfur abundances in extremely metal-poor stars
Context. Precise S abundances are important in the study of the earlychemical evolution of the Galaxy. In particular the site of theformation remains uncertain because, at low metallicity, the trend ofthis ?-element versus [Fe/H] remains unclear. Moreover, althoughsulfur is not bound significantly in dust grains in the ISM, it seems tobehave differently in DLAs and old metal-poor stars. Aims: Weattempt a precise measurement of the S abundance in a sample ofextremely metal-poor stars observed with the ESO VLT equipped with UVES,taking into account NLTE and 3D effects. Methods: The NLTEprofiles of the lines of multiplet 1 of S I were computed with a versionof the program MULTI, including opacity sources from ATLAS9 and based ona new model atom for S. These profiles were fitted to the observedspectra. Results: We find that sulfur in EMP stars behaves likethe other ?-elements, with [S/Fe] remaining approximately constantbelow [Fe/H] = -3. However, [S/Mg] seems to decrease slightly withincreasing [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S,and K are most closely matched by the SN model yields by Heger &Woosley. The [S/Zn] ratio in EMP stars is solar, as also found in DLAs.We derive an upper limit to the sulfur abundance [S/Fe] < +0.5 forthe ultra metal-poor star CS 22949-037. This, along with a previouslyreported measurement of zinc, argues against the conjecture that thelight-element abundance pattern of this star (and by analogy, the hyperiron-poor stars HE 0107-5240 and HE 1327-2326) would be due to dustdepletion.Based on observations obtained with the ESO Very Large Telescope atParanal (Large Programme "First Stars", ID 165, N-0276, P.I.: Cayrel.
| The Ubiquity of the Rapid Neutron-capture Process
To better characterize the abundance patterns produced by the r-process,we have derived new abundances or upper limits for the heavy elementszinc (Zn, Z= 30), yttrium (Y, Z= 39), lanthanum (La, Z= 57), europium(Eu, Z= 63), and lead (Pb, Z= 82). Our sample of 161 metal-poor starsincludes new measurements from 88 high-resolution and highsignal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 mSmith Telescope at the McDonald Observatory, and other abundances areadopted from the literature. We use models of the s-process inasymptotic giant branch stars to characterize the high Pb/Eu ratiosproduced in the s-process at low metallicity, and our new observationsthen allow us to identify a sample of stars with no detectable s-processmaterial. In these stars, we find no significant increase in the Pb/Euratios with increasing metallicity. This suggests that s-processmaterial was not widely dispersed until the overall Galactic metallicitygrew considerably, perhaps even as high as [Fe/H] =-1.4, in contrastwith earlier studies that suggested a much lower mean metallicity. Weidentify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor starswith [Eu/Fe] <+0.6 attributable to the r-process, suggesting thatthere is no unique "pure" r-process elemental ratio among pairs of rareearth elements. We confirm earlier detections of an anti-correlationbetween Y/Eu and Eu/Fe bookended by stars strongly enriched in ther-process (e.g., CS 22892-052) and those with deficiencies of the heavyelements (e.g., HD 122563). We can reproduce the range of Y/Eu ratiosusing simulations of high-entropy neutrino winds of core-collapsesupernovae that include charged-particle and neutron-capture componentsof r-process nucleosynthesis. The heavy element abundance patterns inmost metal-poor stars do not resemble that of CS 22892-052, but thepresence of heavy elements such as Ba in nearly all metal-poor starswithout s-process enrichment suggests that the r-process is a commonphenomenon.This paper includes data taken at The McDonald Observatory of TheUniversity of Texas at Austin.
| The ability of intermediate-band Strömgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
Context. Several large scale photometric and spectroscopic surveys arebeing undertaken to provide a more detailed picture of the Milky Way.Given the necessity of generalisation in the determination of, e.g.,stellar parameters when tens and hundred of thousands of stars areconsidered it remains important to provide independent, detailed studiesto verify the methods used in the surveys. Aims: Our first aim isto critically evaluate available calibrations for deriving [M/H] fromStrömgren photometry. Secondly, we develop the standard sequencesfor dwarf stars to reflect their inherent metallicity dependence.Finally, we test how well metallicities derived from ugriz photometryreproduce metallicities derived from the well-tested system ofStrömgren photometry. Methods: We evaluate availablemetallicity calibrations based on Strömgren uvby photometry fordwarf stars using a catalogue of stars with both uvby photometry andspectroscopically determined iron abundances ([Fe/H]). The catalogue wascreated for this project. Using this catalogue, we also evaluateavailable calibrations that determine log g. A larger catalogue, inwhich metallicity is determined directly from uvby photometry, is usedto trace metallicity-dependent standard sequences for dwarf stars. Wealso perform comparisons, for both dwarf and giant stars, ofmetallicities derived from ugriz photometry with metallicities derivedfrom Strömgren photometry. Results: We provide a homogenisedcatalogue of 451 dwarf stars with 0.3 < (b-y)0 < 1.0.All stars in the catalogue have uvby photometry and [Fe/H] determinedfrom spectra with high resolution and high signal-to-noise ratios (S/N).Using this catalogue, we test how well various photometric metallicitycalibrations reproduce the spectroscopically determined [Fe/H]. Usingthe preferred metallicity calibration for dwarf stars, we derive newstandard sequences in the c1,0 versus (b-y)0 planeand in the c1,0 versus (v-y)0 plane for dwarfstars with 0.40 < (b-y)0 < 0.95 and 1.10 <(v-y)0 < 2.38. Conclusions: We recommend thecalibrations of Ramírez & Meléndez (2005) in derivingmetallicities from Strömgren photometry and find that intermediateband photometry, such as Strömgren photometry, more accurately thanbroad band photometry reproduces spectroscopically determined [Fe/H].Strömgren photometry is also better at differentiating betweendwarf and giant stars. We conclude that additional investigations of thedifferences between metallicities derived from ugriz photometry andintermediate-band photometry, such as Strömgren photometry, arerequired.Appendix A is only available in electronic form at http://www.aanda.org, Appendix B isavailable in electronic form at http://www.aanda.org and at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A40
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| A New Technique for Determining Europium Abundances in Solar-Metallicity Stars
We present a new technique for measuring the abundance of europium, arepresentative r-process element, in solar-metallicity stars. Ouralgorithm compares LTE synthetic spectra with high-resolutionobservational spectra using a ?-minimization routine. The analysisis fully automated, and therefore allows consistent measurement ofblended lines even across very large stellar samples. We compare ourresults with literature europium abundance measurements and find them tobe consistent; we also find that our method generates smaller errors.
| A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity (RV)measurements of 160 metal-poor stars observed with HIRES on the Keck 1telescope. We report on variability and long-term velocity trends foreach star in our sample. We identify several long-term, low-amplitude RVvariables worthy of followup with direct imaging techniques. We placelower limits on the detectable companion mass as a function of orbitalperiod. Our survey would have detected, with a 99.5% confidence level,over 95% of all companions on low-eccentricity orbits with velocitysemiamplitude K gsim 100 m s–1, orMp sin i gsim 3.0 M J(P/yr)(1/3), fororbital periods P lsim 3 yr. None of the stars in our sampleexhibits RV variations compatible with the presence of Jovian planetswith periods shorter than the survey duration. The resulting averagefrequency of gas giants orbiting metal-poor dwarfs with–2.0lsim[Fe/H]lsim–0.6 is fp < 0.67% (at the1σ confidence level). We examine the implications of this nullresult in the context of the observed correlation between the rate ofoccurrence of giant planets and the metallicity of their main-sequencesolar-type stellar hosts. By combining our data set with the Fischer& Valenti (2005) uniform sample, we confirm that the likelihood of astar to harbor a planet more massive than Jupiter within 2 AU is asteeply rising function of the host's metallicity. However, the data forstars with –1.0lsim[Fe/H]lsim0.0 are compatible, in a statisticalsense, with a constant occurrence rate fp sime 1%. Ourresults can usefully inform theoretical studies of the process ofgiant-planet formation across two orders of magnitude in metallicity.
| Kinematics and metallicity analysis for nearby F, G and K stars
A sample containing 1 026 stars of spectral types F, G, and K, mainlydwarfs, from the solar neighbourhood with available space velocities andmetallicities is treated. The treatment comprises a statistical analysisof the metallicity and velocity data and calculation of galactocentricorbits. Sample stars identified as members of the galactic halo aredetached from the rest of the sample based on the values of theirmetallicities, velocity components and galactocentric orbits. Inidentifying halo stars a new, kinematical, criterion is proposed. Exceptone, these halo stars are the metal-poorest ones in the sample. Besides,they have very high velocities with respect to LSR. On the other hand,the separation between the thin disc and thick one is done statisticallybased on LSR space velocities, membership probability (Schwarzschilddistribution with assumed parameters) and galactocentric orbits. In themetallicity these two groups are not much different. For each of thethree subsamples the mean motion and velocity ellipsoid are calculated.The elements of the velocity ellipsoids agree well with the values foundin the literature, especially for the thin disc. The fractions of thesubsystems found for the present sample are: thin disc 93%, thick disc6%, halo 1%. The sample stars established to be members of the thin discare examined for existence of star streams. Traces of both, known andunknown, star streams are not found.
| Speckle interferometry of metal-poor stars in the solar neighborhood. II
The results of speckle interferometric observations of 115 metal-poorstars ([m/H] < ‑1) within 250 pc from the Sun and with propermotions µ ≳ 0.2″/yr, made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences,are reported. Close companions with separations ranging from0.034″ to 1″ were observed for 12 objects—G76-21,G59-1, G63-46, G135-16, G168-42, G141-47, G142-44, G190-10, G28-43,G217-8, G130-7, and G89-14—eight of them are astrometricallyresolved for the first time. The newly resolved systems include onetriple star—G190-10. If combined with spectroscopic and visualdata, our results imply a single:binary:triple:quadruple star ratio of147:64:9:1 for a sample of 221 primary components of halo and thick-diskstars.
| Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog
We derive detailed theoretical models for 1074 nearby stars from theSPOCS (Spectroscopic Properties of Cool Stars) Catalog. The Californiaand Carnegie Planet Search has obtained high-quality (R~=70,000-90,000,S/N~=300-500) echelle spectra of over 1000 nearby stars taken with theHamilton spectrograph at Lick Observatory, the HIRES spectrograph atKeck, and UCLES at the Anglo Australian Observatory. A uniform analysisof the high-resolution spectra has yielded precise stellar parameters(Teff, logg, vsini, [M/H], and individual elementalabundances for Fe, Ni, Si, Na, and Ti), enabling systematic erroranalyses and accurate theoretical stellar modeling. We have created alarge database of theoretical stellar evolution tracks using the YaleStellar Evolution Code (YREC) to match the observed parameters of theSPOCS stars. Our very dense grids of evolutionary tracks eliminate theneed for interpolation between stellar evolutionary tracks and allowprecise determinations of physical stellar parameters (mass, age,radius, size and mass of the convective zone, surface gravity, etc.).Combining our stellar models with the observed stellar atmosphericparameters and uncertainties, we compute the likelihood for each set ofstellar model parameters separated by uniform time steps along thestellar evolutionary tracks. The computed likelihoods are used for aBayesian analysis to derive posterior probability distribution functionsfor the physical stellar parameters of interest. We provide a catalog ofphysical parameters for 1074 stars that are based on a uniform set ofhigh-quality spectral observations, a uniform spectral reductionprocedure, and a uniform set of stellar evolutionary models. We explorethis catalog for various possible correlations between stellar andplanetary properties, which may help constrain the formation anddynamical histories of other planetary systems.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| 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.
| 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.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Heavy elements and chemical enrichment in globular clusters
High resolution (R 40 000) and high S/N spectra have been acquiredwith UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for severalmain sequence turnoff stars (V 17 mag) and subgiants at the baseof the Red Giant Branch (V 16 mag) in three globular clusters (NGC6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities(respectively [Fe/H] ≃ -2.0; -1.5; -0.7). Spectra for a sample of25 field halo subdwarves have also been taken with equal resolution, buthigher S/N. These data have been used to determine the abundances ofseveral neutron-capture elements in these three clusters: strontium,yttrium, barium and europium. This is the first abundance determinationof these heavy elements for such unevolved stars in these three globularclusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundanceratios, have been used to test the self-enrichment scenario. Acomparison is done with field halo stars and other well known Galacticglobular clusters in which heavy elements have already been measured inthe past, at least in bright giants (V 11-12 mag). Our resultsshow clearly that globular clusters have been uniformly enriched by r-and s-process syntheses, and that most of them seem to follow exactlythe same abundance patterns as field halo stars, which discards the``classical'' self-enrichment scenario for the origin of metallicitiesand heavy elements in globular clusters.Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO-LP 165.L-0263).
| The distance to NGC 5904 (M 5) via the subdwarf main sequence fitting method
We present a determination of the distance modulus of the globularcluster NGC 5904 (M 5), obtained by means of the subdwarf main-sequencefitting on the (V,V-I) color-magnitude diagram. The subdwarf sample hasbeen selected from the HIPPARCOS catalog in a metallicity rangehomogeneous with the cluster ([Fe/H] ≃ -1.1). Both the cluster andthe subdwarfs have been observed with the sametelescope+instrument+filters setup (namely, ESO-NTT equipped with theSUSI2 camera), in order to preserve homogeneity and reduce systematicuncertainties. A set of archival HST data has then been used to obtain adeep and precise ridge line. These have been accurately calibrated inthe ground photometric system by using the NTT data and used to fit thecluster distance modulus. By adopting the most commonly accepted valuesfor the reddening, E(B-V) = 0.035 and 0.03, we obtain respectivelyμ0 = 14.44 ± 0.09 ± 0.07 andμ0 = 14.41 ± 0.09 ± 0.07, in agreement withrecent determinations.Based on data collected at ESO-La Silla, Chile, (GTO 63.L-0717) and fromHST archival data (GO 8310).
| 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.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete 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/419/167
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Magnesium Isotopic Abundance Ratios in Cool Stars
From high-resolution spectra of 61 cool dwarfs and giants, Mg isotopicabundance ratios 24Mg:25Mg:26Mg arederived from spectral synthesis of the MgH A-X lines near 5140 Å.Our sample spans the range -2.5<=[Fe/H]<=0.1, including the firstmeasurements of Mg isotope ratios in stars with metallicities below[Fe/H]=-2.0. We confirm the decrease in 25Mg/24Mgand 26Mg/24Mg with decreasing [Fe/H], as predictedby recent models of Galactic chemical evolution in which the Mg isotopesare produced in massive stars. A subset of kinematically identifiedthin-disk stars have Mg isotope ratios in excellent agreement with thepredictions. Within the measurement uncertainties, these thin-disk starsshow no scatter about the predictions. Several of our stars are likelymembers of the thick disk, and their high Mg isotopic ratios may reflectthe nucleosynthetic history of the thick disk, which is distinct fromthe predictions for, and observations of, the thin disk. For thick-diskand halo stars we find a scatter in 25Mg/24Mg and26Mg/24Mg exceeding our measurement uncertaintiesand increasing with increasing metallicity. Our data suggest that anadditional source of 25Mg and 26Mg is required.Intermediate-mass asymptotic giant branch stars are likely candidates.
| Abundances for metal-poor stars with accurate parallaxes. I. Basic data
We present element-to-element abundance ratios measured from highdispersion spectra for 150 field subdwarfs and early subgiants withaccurate Hipparcos parallaxes (errors <20%). For 50 stars new spectrawere obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 mtelescope, and SARG at TNG. Additionally, literature equivalent widthswere taken from the works by Nissen & Schuster, Fulbright, andProchaska et al. to complement our data. The whole sample includes boththick disk and halo stars (and a few thin disk stars); most stars havemetallicities in the range -2<[Fe/H]<-0.6. We found our data, thatof Nissen & Schuster, and that of Prochaska to be of comparablequality; results from Fulbright scatter a bit more, but they are stillof very good quality and are extremely useful due to the large size ofhis sample. The results of the present analysis will be used inforthcoming papers to discuss the chemical properties of thedissipational collapse and accretion components of our Galaxy.Based in part on data collected at the European Southern Observatory,Chile, at the MacDonald Observatory, Texas, USA, and at the TelescopioNazionale Galileo, Canary Island, INAF,Italy-Spain.}\fnmsep\thanks{Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia\resizebox{8.8cm}{2.2mm}htpp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/187}
| Intermediate-metallicity, high-velocity stars and Galactic chemical evolution
High signal-to-noise ratio spectra were obtained of 10high-proper-motion stars having -1 <~[Fe/H] < 0, and a comparablenumber of disc stars. All but two of the high-proper-motion stars wereconfirmed to have [Fe/H] > -1.0, some approaching solar metallicity,but, even so, earlier measurements overestimated the metallicities andvelocities of some of these stars. Models of stellar populations wereused to assign membership probabilities to the Galactic components towhich the high-velocity stars might belong. Many were found to be moreprobably thick-disc than halo objects, despite their large spacemotions, and two might be associated with the inner Galaxy. It may benecessary to reassess contamination of previous halo samples, such asthose used to define the metallicity distribution, to account forcontamination by high-velocity thick-disc stars, and to considerpossible subcomponents of the halo.The change in [α/Fe] ratios at [Fe/H]~=-1.0 is often used toconstrain the degree and timing of Type Ia supernova nucleosynthesis inGalactic chemical-evolution models. [Ti/Fe] values were measured foreight of the high-velocity stars. Both high- and low-[Ti/Fe] halo starsexist; likewise high- and low-[Ti/Fe] thick-disc stars exist. Weconclude that the [Ti/Fe]`break' is not well defined for a givenpopulation; nor is there a simple, continuous evolutionary sequencethrough the break. Implications for the interpretation of the[α/Fe] break in terms of SN Ia time-scales and progenitors arediscussed. The range of [Ti/Fe] found for high-velocity (low rotation)thick-disc stars contrasts with that for the low-velocity (highrotation) thick-disc sample studied by Prochaska et al.
| A Search for Cool Subdwarfs: Stellar Parameters for 134 Candidates
The results of a search for cool subdwarfs are presented. Kinematic (U,V, and W) and stellar parameters (Teff, logg, [Fe/H], andξt) are derived for 134 candidate subdwarfs based onhigh-resolution spectra. The observed stars span 4200K
| 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.
| 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.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Ofiuco |
| Ascensión Recta: | 18h12m21.88s |
| Declinación: | +05°24'04.4" |
| Magnitud Aparente: | 10.344 |
| Movimiento Propio en Ascensión Recta: | -498.6 |
| Movimiento Propio en Declinación: | -645.9 |
| B-T magnitude: | 11.189 |
| V-T magnitude: | 10.414 |
Catálogos y designaciones:
|
INHABITED SKY
