Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| Kinematic structure of the corona of the Ursa Major flow found using proper motions and radial velocities of single stars
Aims.We study the kinematic structure of peripheral areas of the UrsaMajoris stream (Sirius supercluster). Methods.We use diagrams ofindividual stellar apexes developed by us and the classical technique ofproper motion diagrams generalized to a star sample distributed over thesky. Results.Out of 128 cluster members we have identified threecorona (sub)structures comprised of 13, 13 and 8 stars. Thesubstructures have a spatial extension comparable to the size of thecorona. Kinematically, these groups are distinguished by their propermotions, radial velocities and by the directions of their spatialmotion. Coordinates of their apexes significantly differ from those ofthe apexes of the stream and its nucleus. Our analysis shows that thesesubstructures do not belong to known kinematic groups, such as Hyades orCastor. We find kinematic inhomogeneity of the corona of the UMa stream.
| Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar-mass companions have focusedprimarily on stars older than 1 Gyr. Increased levels of stellaractivity in young stars hinders the detection of solar system analogs,and therefore until recently there has been a prejudice againstinclusion of young stars in radial velocity surveys. Adaptive opticssurveys of young stars have given us insight into the multiplicity ofyoung stars, but only for massive, distant companions. Understanding thelimit of the radial velocity technique, restricted to high-mass,close-orbiting planets and brown dwarfs, we began a survey of youngstars of various ages. While the number of stars needed to carry outfull analysis of the problems of planetary and brown dwarf populationand evolution is large, the beginning of such a sample is included here.We report on 61 young stars ranging in age from the β Pictorisassociation (~12 Myr) to the Ursa Major association (~300 Myr). Thisinitial search resulted in no stars showing evidence of companionslarger than ~1MJup-2MJup in short-period orbits atthe 3 σ level. We also present derived stellar parameters, as mosthave unpublished values. The chemical homogeneity of a cluster, andpresumably of an association, may help to constrain true membership, sowe present [Fe/H] abundances for the stars in our sample.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| High-Resolution Spectroscopy of Ursa Major Moving Group Stars
We use new and extant literature spectroscopy to address abundances andmembership for UMa moving group stars. We first compare the UMa, Coma,and Hyades H-R diagrams via a homogeneous set of isochrones and findthat these three aggregates are essentially coeval; this (near)coevality can explain the indistinguishable distributions of UMa andHyades dwarfs in the chromospheric emission versus color plane. Ourspectroscopy of cool UMa dwarfs reveals striking abundanceanomalies-trends with Teff, ionization state, and excitationpotential-like those recently seen in young, cool M34, Pleaides, andHyades dwarfs. In particular, the trend of rising λ7774-based O Iabundance with declining Teff is markedly subdued in UMacompared to the Pleiades, suggesting a dependence on age or metallicity.Recent photometric metallicity estimates for several UMa dwarfs aremarkedly low compared to the group's canonical metallicity, and similardeviants are seen among cool Hyads as well. Our spectroscopy does notconfirm these curious photometric estimates, which seem to be calledinto question for cool dwarfs. Despite disparate sources of Li data, ourhomogeneous analysis indicates that UMa members evince remarkably smallscatter in the Li-Teff plane for Teff>=5200 K.Significant star-to-star scatter suggested by previous studies is seenfor cooler stars. Comparison with the consistently determined HyadesLi-Teff trend reveals differences that are qualitativelyconsistent with this cluster's larger [Fe/H] (and perhaps slightlygreater age). However, quantitative comparison with standard stellarmodels indicates the differences are smaller than expected, suggestingthe action of a fourth parameter beyond age, mass, and [Fe/H]controlling Li depletion. The UMa-Coma cool star Li abundances may showa slight 0.2 dex difference; however, this may be mass-independent andthus more consistent with a modest initial Li abundance difference.This paper includes data taken at the McDonald Observatory of theUniversity of Texas at Austin.Based on observations obtained at Kitt Peak National Observatory, adivision of the National Optical Astronomy Observatories, which isoperated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science 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
| Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group
Utilizing Hipparcos parallaxes, original radial velocities and recentliterature values, new Ca II H and K emission measurements,literature-based abundance estimates, and updated photometry (includingrecent resolved measurements of close doubles), we revisit the UrsaMajor moving group membership status of some 220 stars to produce afinal clean list of nearly 60 assured members, based on kinematic andphotometric criteria. Scatter in the velocity dispersions and H-Rdiagram is correlated with trial activity-based membership assignments,indicating the usefulness of criteria based on photometric andchromospheric emission to examine membership. Closer inspection,however, shows that activity is considerably more robust at excludingmembership, failing to do so only for <=15% of objects, perhapsconsiderably less. Our UMa members demonstrate nonzero vertex deviationin the Bottlinger diagram, behavior seen in older and recent studies ofnearby young disk stars and perhaps related to Galactic spiralstructure. Comparison of isochrones and our final UMa group membersindicates an age of 500+/-100 Myr, some 200 Myr older than thecanonically quoted UMa age. Our UMa kinematic/photometric members' meanchromospheric emission levels, rotational velocities, and scattertherein are indistinguishable from values in the Hyades and smaller thanthose evinced by members of the younger Pleiades and M34 clusters,suggesting these characteristics decline rapidly with age over 200-500Myr. None of our UMa members demonstrate inordinately low absolutevalues of chromospheric emission, but several may show residual fluxes afactor of >=2 below a Hyades-defined lower envelope. If one defines aMaunder-like minimum in a relative sense, then the UMa results maysuggest that solar-type stars spend 10% of their entire main-sequencelives in periods of precipitously low activity, which is consistent withestimates from older field stars. As related asides, we note six evolvedstars (among our UMa nonmembers) with distinctive kinematics that liealong a 2 Gyr isochrone and appear to be late-type counterparts to diskF stars defining intermediate-age star streams in previous studies,identify a small number of potentially very young but isolated fieldstars, note that active stars (whether UMa members or not) in our samplelie very close to the solar composition zero-age main sequence, unlikeHipparcos-based positions in the H-R diagram of Pleiades dwarfs, andargue that some extant transformations of activity indices are notadequate for cool dwarfs, for which Ca II infrared triplet emissionseems to be a better proxy than Hα-based values for Ca II H and Kindices.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| Mining the Metal-rich Stars for Planets
We examine the correlation between stellar metallicity and the presenceof short-period planets. It appears that approximately 1% of dwarf starsin the solar neighborhood harbor short-period planets characterized bynear-circular orbits and orbital periods P<20 days. However, amongthe most metal-rich stars (defined as having [Fe/H]>0.2 dex), itappears that the fraction increases to 10%. Using the Hipparcos databaseand the Hauck & Mermilliod compilation of Strömgren uvbyphotometry, we identify a sample of 206 metal-rich stars of spectraltype K, G and F which have an enhanced probability of harboringshort-period planets. Many of these stars would be excellent candidatesfor addition to radial velocity surveys. We have searched the Hipparcosepoch photometry for transiting planets within our 206 star catalog. Wefind that the quality of the Hipparcos data is not high enough to permitunambiguous transit detections. It is, however, possible to identifycandidate transit periods. We then discuss various ramifications of thestellar metallicity-planet connection. First, we show that there ispreliminary evidence for increasing metallicity with increasing stellarmass among known planet-bearing stars. This trend can be explained by ascenario in which planet-bearing stars accrete an average of 30M⊕ of rocky material after the gaseous protoplanetarydisk phase has ended. We present dynamical calculations which suggestthat a survey of metallicities of spectroscopic binary stars can be usedto understand the root cause of the stellar metallicity-planetconnection.
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Idra |
| Ascensione retta: | 09h26m42.83s |
| Declinazione: | -14°29'26.7" |
| Magnitudine apparente: | 7.899 |
| Distanza: | 39.888 parsec |
| Moto proprio RA: | 37.2 |
| Moto proprio Dec: | -129.7 |
| B-T magnitude: | 8.782 |
| V-T magnitude: | 7.972 |
Cataloghi e designazioni:
|
