Ýçindekiler
Görüntüler
| - Hiçbir görüntü bulunamadý - |
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| High-Resolution Spectroscopy of some Active Southern Stars
High-resolution échelle spectra of 42 nearby southern solar-typestars have been obtained, in a search for young, single, active, andrapidly rotating sun-like stars suitable for Doppler imaging and ZeemanDoppler imaging studies. As a result of this survey, 13 stars weredetermined to be youthful with ages less than 600Myr (Hyades age) andeight of these were found to have projected rotational velocities inexcess of 15kms-1. In addition, five spectroscopic binarysystems were identified. Of those stars observed for this survey, HD106506 is the most outstanding target for mapping active regions. It isan apparently young and single star with rapid rotation (v sin i~80kms-1), strong Hα chromospheric activity (logR'Hα~-4.2), and deformation of the spectral lineprofiles indicating the presence of large starspots.
| 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
| Nearby young stars
We present the results of an extensive all-sky survey of nearby stars ofspectral type F8 or later in a systematic search of young (zero-age mainsequence) objects. Our sample has been derived by cross-correlating theROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754candidates distributed more or less randomly over the sky. Follow-upspectroscopy of these candidate objects has been performed on 748 ofthem. We have discovered a tight kinematic group of ten stars withextremely high lithium equivalent widths that are presumably youngerthan the Pleiades, but again distributed rather uniformly over the sky.Furthermore, about 43 per cent of our candidates have detectable levelsof lithium, thus indicating that these are relatively young objects withages not significantly above the Pleiades age.Based on observations collected at the European Southern Observatory,Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D-0236(A)).
| Fast-rotating nearby solar-type stars sin i and X-ray luminosities relationships. II. Li abundances, v sin i and X-ray luminosities relationships
We present an analysis of our high-resolution spectroscopic andhigh-precision UBV(RI)_c photometric observations of a sample of 110nearby late-F and G-type stars selected for their large rotationalvelocity. The relationships between Li abundance, X-ray luminosity, andvsin i are investigated. We find that, as expected, the stars in oursample show statistically higher Li abundance and activity level thanfield star samples with similar characteristics, but slower rotation.Surprisingly, however, we also find four rapidly-rotating singlemain-sequence stars with very low Li abundance. For both single andbinary stars we find a large spread of Li abundance for stars withrotation lower than about 18 km s-1. The well-establishedcorrelation between X-ray luminosity and rotation rate is clearlyobserved. All single unevolved solar type stars with vsin i larger than18 km s-1 are strong X-ray emitters and have high Liabundance. Finally, we find also five evolved stars with very low Liabundance that are still rather fast rotators. The results from oursample confirm the presence of young very active stars close to the Sun,in agreement with recent findings from EUV and X-ray surveys, althoughour sample does not show such extreme characteristics as those selectedfrom EUV and X-ray surveys at the current flux limits.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables, Figures and the complete data set are 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/397/987
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
| A Multiplicity Survey of Chromospherically Active and Inactive Stars
Surveys of three samples of solar-type stars, segregated bychromospheric emission level, were made to determine their multiplicityfractions and to investigate the evolution of multiplicity with age. Intotal, 245 stars were searched for companions with DeltaV <= 3.0 andseparations of 0.035" to 1.08" using optical speckle interferometry. Byincorporating the visual micrometer survey for duplicity of theLamontHussey Observatory, the angular coverage was extended to 5.0" withno change in the DeltaV limit. This magnitude difference allows massratios of 0.63 and larger to be detected throughout a search region of2-127 AU for the stars observed. The 84 primaries observed in thechromospherically active sample are presumably part of a youngpopulation and are found to have a multiplicity fraction of 17.9% +/-4.6%. The sample of 118 inactive, presumably older, primaries wereselected and observed using identical methods and are found to have amultiplicity fraction of only 8.5% +/- 2.7%. Given the known linkbetween chromospheric activity and age, these results tentatively implya decreasing stellar multiplicity fraction from 1 to 4 Gyr, theapproximate ages of the two samples. Finally, only two of the 14 veryactive primaries observed were found to have a companion meeting thesurvey detection parameters. In this case, many of the systems areeither very young, or close, RS CVn type multiples that are unresolvableusing the techniques employed here.
| The Problem of HIPPARCOS Distances to Open Clusters. II. Constraints from Nearby Field Stars
This paper examines the discrepancy between distances to nearby openclusters as determined by parallaxes from Hipparcos compared totraditional main-sequence fitting. The biggest difference is seen forthe Pleiades, and our hypothesis is that if the Hipparcos distance tothe Pleiades is correct, then similar subluminous zero-age main-sequence(ZAMS) stars should exist elsewhere, including in the immediate solarneighborhood. We examine a color-magnitude diagram of very young andnearby solar-type stars and show that none of them lie below thetraditional ZAMS, despite the fact that the Hipparcos Pleiades parallaxwould place its members 0.3 mag below that ZAMS. We also presentanalyses and observations of solar-type stars that do lie below theZAMS, and we show that they are subluminous because of low metallicityand that they have the kinematics of old stars.
| Metallicity effects on the chromospheric activity-age relation for late-type dwarfs
We show that there is a relationship between the age excess, defined asthe difference between the stellar isochrone and chromospheric ages, andthe metallicity as measured by the index [Fe/H] for late-type dwarfs.The chromospheric age tends to be lower than the isochrone age formetal-poor stars, and the opposite occurs for metal-rich objects. Wesuggest that this could be an effect of neglecting the metallicitydependence of the calibrated chromospheric emission-age relation. Wepropose a correction to account for this dependence. We also investigatethe metallicity distributions of these stars, and show that there aredistinct trends according to the chromospheric activity level. Inactivestars have a metallicity distribution which resembles the metallicitydistribution of solar neighbourhood stars, while active stars appear tobe concentrated in an activity strip on the logR'_HKx[Fe/H] diagram. Weprovide some explanations for these trends, and show that thechromospheric emission-age relation probably has different slopes on thetwo sides of the Vaughan-Preston gap.
| A Survey of Ca II H and K Chromospheric Emission in Southern Solar-Type Stars
More than 800 southern stars within 50 pc have been observed forchromospheric emission in the cores of the Ca II H and K lines. Most ofthe sample targets were chosen to be G dwarfs on the basis of colors andspectral types. The bimodal distribution in stellar activity first notedin a sample of northern stars by Vaughan and Preston in 1980 isconfirmed, and the percentage of active stars, about 30%, is remarkablyconsistent between the northern and southern surveys. This is especiallycompelling given that we have used an entirely different instrumentalsetup and stellar sample than used in the previous study. Comparisons tothe Sun, a relatively inactive star, show that most nearby solar-typestars have a similar activity level, and presumably a similar age. Weidentify two additional subsamples of stars -- a very active group, anda very inactive group. The very active group may be made up of youngstars near the Sun, accounting for only a few percent of the sample, andappears to be less than ~0.1 Gyr old. Included in this high-activitytail of the distribution, however, is a subset of very close binaries ofthe RS CVn or W UMa types. The remaining members of this population maybe undetected close binaries or very young single stars. The veryinactive group of stars, contributting ~5%--10% to the total sample, maybe those caught in a Maunder Minimum type phase. If the observations ofthe survey stars are considered to be a sequence of snapshots of the Sunduring its life, we might expect that the Sun will spend about 10% ofthe remainder of its main sequence life in a Maunder Minimum phase.
| 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
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Erbogan |
| Sað Açýklýk: | 13h05m16.81s |
| Yükselim: | -50°51'23.8" |
| Görünürdeki Parlaklýk: | 8.319 |
| Uzaklýk: | 45.228 parsek |
| özdevim Sað Açýklýk: | -135.5 |
| özdevim Yükselim: | -1.1 |
| B-T magnitude: | 9.152 |
| V-T magnitude: | 8.388 |
Kataloglar ve belirtme:
|
INHABITED SKY
