Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Accurate masses of low mass stars GJ 765.2AB (0.83 Mȯ + 0.76 Mȯ)
Context: Because of the lack of precise masses, the coverage of themain-sequence empirical mass-luminosity relation for stars in the massrange from 0.6 Mȯ to 0.9 Mȯ isincomplete. The nearby K-type visual and spectroscopic binary GJ 765.2 =MLR 224 is a good candidate for new reliable points in this significantpart of the relation. Aims: We have found a combined orbital solutionfor the pair and derived physical properties of the components usinginterferometric and spectroscopic data. Methods: Thediffraction-limited speckle observations were mostly collected at the 6m BTA telescope, and the velocities of the components were obtainedusing the CORAVEL radial velocity scanner on the Swiss 1 m telescope. Results: In a combined solution, the orbital period is found to be11.919 yr. The masses of the GJ 765.2 components areMA=0.831± 0.020 Mȯ andMB=0.763± 0.019 Mȯ. The obtainedorbital parallax of the system, π_orb=31.0±0.5 mas, is 7percent lower than the Hipparcos value. The absolute V magnitudes of thestars, derived from the measured speckle magnitude differences, are:MVA=5.99±0.04 andMVB=6.64±0.05. The effective temperaturesof the components, T_effA=5060±130 K andT_effB=4690±160 K, follow from the V-K and J-K colorindices. The star metallicity value, estimated from the 6 m telescopespectrum, is [M/H]=-0.35±0.15 dex. Conclusions: .The presentedindividual masses have 2.4% and 2.5% relative accuracies. Therefore, thecomponents of GJ 765.2 rank among a dozen stars with masses accurate towithin a few percent in the mass range 0.6-0.9 Mȯ. Theexisting data on the kinematics of GJ 765.2 and its chromosphericactivity indicate that the binary belongs to the middle age (3-4×109 yr) thin disk population of the galaxy.Based on observations made with the 6 m BTA telescope, which is operatedby the Special Astrophysical Observatory, Russia, and the Observatoirede Haute-Provence, operated by the Centre National de la RechercheScientifique de France. Tables 1 and 2 are only available in electronicform at http://www.aanda.org
| uvby-β photometry of high-velocity and metal-poor stars. XI. Ages of halo and old disk stars
New uvby-β data are provided for 442 high-velocity and metal-poorstars; 90 of these stars have been observed previously by us, and 352are new. When combined with our previous two photometric catalogues, thedata base is now made up of 1533 high-velocity and metal-poor stars, allwith uvby-β photometry and complete kinematic data, such as propermotions and radial velocities taken from the literature. Hipparcos, plusa new photometric calibration for Mv also based on theHipparcos parallaxes, provide distances for nearly all of these stars;our previous photometric calibrations give values for E(b-y) and [Fe/H].The [Fe/H], V(rot) diagram allows us to separate these stars intodifferent Galactic stellar population groups, such as old-thin-disk,thick-disk, and halo. The X histogram, where X is our stellar-populationdiscriminator combining V(rot) and [Fe/H], and contour plots for the[Fe/H], V(rot) diagram both indicate two probable components to thethick disk. These population groups and Galactic components are studiedin the (b-y)0, Mv diagram, compared to theisochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), toderive stellar ages. The two thick-disk groups have the meancharacteristics: ([Fe/H], V(rot), Age, σW') ≈ (-0.7dex, 120 km s-1, 12.5 Gyr, 62.0 km s-1), and≈(-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups,-2.31 ≤ [Fe/H] ≤ -1.31, show a mean age of 13.0 ± 0.2(mean error) Gyr, giving a mean difference from the WMAP results for theage of the Universe of 0.7 ± 0.3 Gyr. These results for the agesand components of the thick disk and for the age of the Galactic halofield stars are discussed in terms of various models and ideas for theformation of galaxies and their stellar populations.
| 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.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| 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.
| Statistical properties of solar-type close binaries
Two Coravel radial velocity surveys dedicated to F7-K field dwarfs andto open clusters are merged in order to investigate the statisticalproperties of binaries with periods up to 10 years. Thanks to theaccurate trigonometric parallaxes provided by Hipparcos, an unbiasedsample of spectroscopic binaries (SB) is selected. After correction forthe uncertainties of the measurements, the following results areobtained: 1. The distribution of mass ratios exhibits a peak forequal-mass binaries (twins), which is higher for short-period binariesthan for long-period binaries. 2. Apart from the twins, the distributionof mass ratios exhibits a broad peak from 0.2 to 0.6. 3. The orbitaleccentricities of twins are slightly smaller than those of otherbinaries. 4. An excess of SB is observed with periods shorter than about50 days in comparison with the Duquennoy and Mayor log-normaldistribution of periods. These features suggest that close binary starsare generated by two different processes. A possible difference couldcome from the accretion onto the binary, for instance from a commonenvelope or from a circumbinary disk. Alternatively, twins could comefrom dynamic evolution of multiple systems. It is not clear whether theformation models are already sufficiently elaborated to reproduce ourstatistics.
| The Visual Orbits of the Spectroscopic Binaries HD 6118 and HD 27483 from the Palomar Testbed Interferometer
We present optical interferometric observations of two double-linedspectroscopic binaries, HD 6118 and HD 27483, taken with the PalomarTestbed Interferometer (PTI) in the K band. HD 6118 is one of the mosteccentric spectroscopic binaries, and HD 27483 is a spectroscopic binaryin the Hyades open cluster. The data collected with PTI in 2001-2002allow us to determine astrometric orbits, which, when combined with theradial velocity measurements, determine all orbital parameters of thesystems. The masses of the components are 2.65+/-0.27 and2.36+/-0.24Msolar for HD 6118 and 1.38+/-0.13 and1.39+/-0.13Msolar for HD 27483. The apparent semimajor axisof HD 27483 is only 1.2 mas, making it the closest binary successfullyobserved with an optical interferometer.
| 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
| An Assessment of Dynamical Mass Constraints on Pre-Main-Sequence Evolutionary Tracks
We have assembled a database of stars having both masses determined frommeasured orbital dynamics and sufficient spectral and photometricinformation for their placement on a theoretical H-R diagram. Our sampleconsists of 115 low-mass (M<2.0 Msolar) stars, 27pre-main-sequence and 88 main-sequence. We use a variety of availablepre-main-sequence evolutionary calculations to test the consistency ofpredicted stellar masses with dynamically determined masses. Despitesubstantial improvements in model physics over the past decade, largesystematic discrepancies still exist between empirical and theoreticallyderived masses. For main-sequence stars, all models considered predictmasses consistent with dynamical values above 1.2 Msolar andsome models predict consistent masses at solar or slightly lower masses,but no models predict consistent masses below 0.5 Msolar,with all models systematically underpredicting such low masses by5%-20%. The failure at low masses stems from the poor match of mostmodels to the empirical main sequence below temperatures of 3800 K, atwhich molecules become the dominant source of opacity and convection isthe dominant mode of energy transport. For the pre-main-sequence samplewe find similar trends. There is generally good agreement betweenpredicted and dynamical masses above 1.2 Msolar for allmodels. Below 1.2 Msolar and down to 0.3 Msolar(the lowest mass testable), most evolutionary models systematicallyunderpredict the dynamically determined masses by 10%-30%, on average,with the Lyon group models predicting marginally consistent masses inthe mean, although with large scatter. Over all mass ranges, theusefulness of dynamical mass constraints for pre-main-sequence stars isin many cases limited by the random errors caused by poorly determinedluminosities and especially temperatures of young stars. Adopting awarmer-than-dwarf temperature scale would help reconcile the systematicpre-main-sequence offset at the lowest masses, but the case for this isnot compelling, given the similar warm offset at older ages between mostsets of tracks and the empirical main sequence. Over all age ranges, thesystematic discrepancies between track-predicted and dynamicallydetermined masses appear to be dominated by inaccuracies in thetreatment of convection and in the adopted opacities.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| The Mass Ratio Distribution in Main-Sequence Spectroscopic Binaries Measured by Infrared Spectroscopy
We report infrared spectroscopic observations of a large well-definedsample of main-sequence, single-lined spectroscopic binaries to detectthe secondaries and derive the mass ratio distribution of short-periodbinaries. The sample consists of 51 Galactic disk spectroscopic binariesfound in the Carney and Latham high proper motion survey, with primarymasses in the range 0.6-0.85 Msolar. Our infraredobservations detect the secondaries in 32 systems, two of which havemass ratios, q=M2/M1, as low as ~0.20. Togetherwith 11 systems previously identified as double-lined binaries byvisible light spectroscopy, we have a complete sample of 62 binaries, ofwhich 43 are double lined. The mass ratio distribution is approximatelyconstant over the range q=1.0-0.3. The distribution appears to rise atlower q values, but the uncertainties are sufficiently large that wecannot rule out a distribution that remains constant. The massdistribution derived for the secondaries in our sample and that of theextrasolar planets apparently represent two distinct populations.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| On the Mass-Ratio Distribution of Spectroscopic Binaries
In this paper we derive the mass-ratio and secondary-mass distributionsof a large, well-defined, complete sample of 129 spectroscopic binarieswith periods between 1 and 2500 days. The binaries, whose orbits werepublished recently, were detected by a systematic radial-velocity surveyof a sample of more than 1400 large proper motion stars. Three featuresstand out in the mass-ratio distribution: a rise as the mass ratio goesdown to q~0.2, a sharp drop below q~0.2, and a smaller peak at q~0.8.Another way to characterize the results is to state that thedistribution includes two ``populations,'' one with a high asymmetricpeak at q~0.2 and another with a smaller peak at q~0.8, while theminimum between the two populations is centered at q~0.55. The size ofthe binary sample allows us to divide it into two subsamples and lookfor differences in the mass-ratio distributions of the two subsamples.We performed two different divisions: one into Galactic halo versus diskpopulations, and the other into high- and low-mass primary stars (aboveand below 0.67 Msolar). The former division yieldsdifferences with moderate statistical significance of 88%, while thelatter is more significant at a level of 97%. Our analysis suggests thatthe rise toward low mass ratios does not appear in the mass-ratiodistribution of the halo binaries. The other separation shows a broadpeak at mass ratio of q~0.8-1 for the subsample of binaries withlow-mass primaries but no corresponding peak in the subsample withhigh-mass primaries. We discuss our findings and their application totheories of binary formation.
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| 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}
| Optical interferometry in astronomy
Here I review the current state of the field of optical stellarinterferometry, concentrating on ground-based work although a briefreport of space interferometry missions is included. We pause both toreflect on decades of immense progress in the field as well as toprepare for a new generation of large interferometers just now beingcommissioned (most notably, the CHARA, Keck and VLT Interferometers).First, this review summarizes the basic principles behind stellarinterferometry needed by the lay-physicist and general astronomer tounderstand the scientific potential as well as technical challenges ofinterferometry. Next, the basic design principles of practicalinterferometers are discussed, using the experience of past and existingfacilities to illustrate important points. Here there is significantdiscussion of current trends in the field, including the new facilitiesunder construction and advanced technologies being debuted. This decadehas seen the influence of stellar interferometry extend beyond classicalregimes of stellar diameters and binary orbits to new areas such asmapping the accretion discs around young stars, novel calibration of thecepheid period-luminosity relation, and imaging of stellar surfaces. Thethird section is devoted to the major scientific results frominterferometry, grouped into natural categories reflecting these currentdevelopments. Lastly, I consider the future of interferometry,highlighting the kinds of new science promised by the interferometerscoming on-line in the next few years. I also discuss the longer-termfuture of optical interferometry, including the prospects for spaceinterferometry and the possibilities of large-scale ground-basedprojects. Critical technological developments are still needed to makethese projects attractive and affordable.
| 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.
| 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
| Testing Models of Stellar Evolution for Metal-poor Stars: An Interferometric-spectroscopic Orbit for the Binary HD 195987
We report spectroscopic and interferometric observations of themoderately metal-poor double-lined binary system HD 195987, with anorbital period of 57.3 days. By combining our radial-velocity andvisibility measurements, we determine the orbital elements and deriveabsolute masses for the components of MA=0.844+/-0.018Msolar and MB=0.6650+/-0.0079 Msolar,with relative errors of 2% and 1%, respectively. We also determine theorbital parallax, πorb=46.08+/-0.27 mas, corresponding toa distance of 21.70+/-0.13 pc. The parallax and the measured brightnessdifference between the stars in V, H, and K yield the component absolutemagnitudes in those bands. We also estimate the effective temperaturesof the stars as TAeff=5200+/-100 K andTBeff=4200+/-200 K. Together with detailedchemical abundance analyses from the literature giving [Fe/H]~-0.5(corrected for binarity) and [α/Fe]=+0.36, we use these physicalproperties to test current models of stellar evolution for metal-poorstars. Among the four that we considered, we find that no single modelfits all observed properties at the measured composition, although weidentify the assumptions in each one that account for the discrepancy,and we conclude that a model with the proper combination of assumptionsshould be able to reproduce all the radiative properties. Theindications from the isochrone fits and the pattern of enhancement ofthe metals in HD 195987 are consistent with this being a thick diskobject, with an age of 10-12 Gyr.
| A Survey of Proper-Motion Stars. XV. Orbital Solutions for 34 Double-lined Spectroscopic Binaries
We present orbital solutions for 34 double-lined spectroscopic binariesfound in the Carney-Latham sample of 1464 stars selected for high propermotion. We use TODCOR, a two-dimensional correlation technique, toextract the velocities for the primary and secondary stars and theirlight ratio. For our single-order echelle spectra, obtained with theCenter for Astrophysics Digital Speedometers, we find that we can reachsecondaries that are as much as 2 mag fainter than their primaries. Theratio of the primary to secondary velocity residuals from the orbitalfit equals approximately the secondary-to-primary light ratio, as wouldbe expected for the photon-limited case. We use our mass and lightratios to evaluate the mass-luminosity relation for metal-poormain-sequence dwarfs in the mass range 0.55-0.8 Msolar. Weassume an L~Mβ relation and find that the exponentat around 5200 Å is 7.4+/-0.6. We find this is in good agreementwith the slope of the corresponding theoretical MV-M 14Gyr isochrones from the VandenBerg & Bell models for metal-poorstars. Some of the results presented here used observations made withthe Multiple Mirror Telescope, a facility operated jointly by theUniversity of Arizona and the Smithsonian Institution.
| Abundances and Kinematics of Field Stars. II. Kinematics and Abundance Relationships
As an investigation of the origin of ``α-poor'' halo stars, weanalyze kinematic and abundance data for 73 intermediate-metallicitystars (-1>[Fe/H]>=-2) selected from Paper I of this series. We findevidence for a connection between the kinematics and the enhancement ofcertain element-to-iron ([X/Fe]) ratios in these stars. Statisticallysignificant correlations were found between [X/Fe] and galacticrest-frame velocities (vRF) for Na, Mg, Al, Si, Ca, and Ni,with marginally significant correlations existing for Ti and Y as well.We also find that the [X/Fe] ratios for these elements all correlatewith a similar level of significance with [Na/Fe]. Finally, we comparethe abundances of these halo stars against those of stars in nearbydwarf spheroidal (dSph) galaxies. We find significant differencesbetween the abundance ratios in the dSph stars and halo stars of similarmetallicity. From this result, it is unlikely that the halo stars in thesolar neighborhood, including even the ``α-poor'' stars, were oncemembers of disrupted dSph galaxies similar to those studied to date.
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| 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
| Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,high signal-to-noise ratio survey of 168 stars, most of which aremetal-poor dwarfs. We follow a self-consistent LTE analysis technique todetermine the stellar parameters and abundances, and we estimate theeffects of random and systematic uncertainties on the resultingabundances. Element-to-iron ratios are derived for key α-, odd-Z,Fe-peak, and r- and s-process elements. Effects of non-LTE on theanalysis of Fe I lines are shown to be very small on average.Spectroscopically determined surface gravities are derived that arequite close to those obtained from Hipparcos parallaxes.
| A Suggested Search for Partial Eclipses in the Nearby Binary Star Gliese 793 .1 (HD 195987)
Not Available
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cigno |
| Ascensione retta: | 20h32m51.64s |
| Declinazione: | +41°53'54.5" |
| Magnitudine apparente: | 7.068 |
| Distanza: | 22.227 parsec |
| Moto proprio RA: | -154.3 |
| Moto proprio Dec: | 454.5 |
| B-T magnitude: | 8.063 |
| V-T magnitude: | 7.151 |
Cataloghi e designazioni:
|
