내용
사진
사진 업로드
DSS Images Other Images
관련 글
An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities(vsini) of a sample of 45 candidate field horizontal-branch (HB) starsspanning a wide range of effective temperatures, from red HB stars withTeff~=5000K to blue HB stars with Teff of 17,000K.Among the cooler blue HB stars (Teff=7500-11500 K), weconfirm prior studies showing that, although a majority of stars rotateat vsini<15kms-1, there exists a subset of ``fastrotators'' with vsini as high as 30-35 km s-1. All but one ofthe red HB stars in our sample have vsini<10kms-1, and noanalogous rotation bimodality is evident. We also identify anarrow-lined hot star (Teff~=16,000K) with enhancedphotospheric metal abundances and helium depletion, similar to theabundance patterns found among hot BHB stars in globular clusters, andfour other stars that may also belong in this category. We discussdetails of the spectral line fitting procedure that we use to deducevsini and explore how measurements of field HB star rotation may shedlight on the issue of HB star rotation in globular clusters.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars
We present a detailed analysis of the space motions of 1203solar-neighborhood stars with metal abundances [Fe/H]<=-0.6, on thebasis of a catalog, of metal-poor stars selected without kinematic biasrecently revised and supplemented by Beers et al. This sample, havingavailable proper motions, radial velocities, and distance estimates forstars with a wide range of metal abundances, is by far the largest suchcatalog to be assembled to date. We show that the stars in our samplewith [Fe/H]<=-2.2, which likely represent a ``pure'' halo component,are characterized by a radially elongated velocity ellipsoid(σU,σV,σW)=(141+/-11,106+/-9, 94+/-8) km s-1 and small prograde rotation=30 to 50 km s-1, consistent withprevious analysis of this sample by Beers and Sommer-Larsen based onradial velocity information alone. In contrast to the previous analysis,we find a decrease in with increasingdistance from the Galactic plane for stars that are likely to be membersof the halo population(Δ/Δ|Z|=-52+/-6 km s-1kpc-1), which may represent the signature of a dissipativelyformed flattened inner halo. Unlike essentially all previouskinematically selected catalogs, the metal-poor stars in our sampleexhibit a diverse distribution of orbital eccentricities, e, with noapparent correlation between [Fe/H] and e. This demonstrates, clearlyand convincingly, that the evidence offered in 1962 by Eggen,Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, anapparent correlation between the orbital eccentricity of halo stars withmetallicity, is basically the result of their proper-motion selectionbias. However, even in our nonkinematically selected sample, we haveidentified a small concentration of high-e stars at [Fe/H]~-1.7, whichmay originate, in part, from infalling gas during the early formation ofthe Galaxy. We find no evidence for an additional thick disk componentfor stellar abundances [Fe/H]<=-2.2. The kinematics of theintermediate-abundance stars close to the Galactic plane are, in part,affected by the presence of a rapidly rotating thick disk component with ~=200 km s-1 (with a verticalvelocity gradient on the order ofΔ/Δ|Z|=-30+/-3 km s-1kpc-1) and velocity ellipsoid (σU,σV, σW)=(46+/-4, 50+/-4, 35+/-3) kms-1. The fraction of low-metallicity stars in the solarneighborhood that are members of the thick disk population is estimatedas ~10% for -2.2<[Fe/H]<=-1.7 and ~30% for -1.7<[Fe/H]<=-1.We obtain an estimate of the radial scale length of the metal-weak thickdisk of 4.5+/-0.6 kpc. We also analyze the global kinematics of thestars constituting the halo component of the Galaxy. The outer part ofthe halo, which we take to be represented by local stars on orbitsreaching more than 5 kpc from the Galactic plane, exhibits no systematicrotation. In particular, we show that previous suggestions of thepresence of a ``counter-rotating high halo'' are not supported by ouranalysis. The density distribution of the outer halo is nearly sphericaland exhibits a power-law profile that is accurately described asρ~R-3.55+/-0.13. The inner part of the halo ischaracterized by a prograde rotation and a highly flattened densitydistribution. We find no distinct boundary between the inner and outerhalo. We confirm the clumping in angular-momentum phase space of a smallnumber of local metal-poor stars noted in 1999 by Helmi et al. We alsoidentify an additional elongated feature in angular-momentum phase spaceextending from the clump to regions with high azimuthal rotation. Thenumber of members in the detected clump is not significantly increasedfrom that reported by Helmi et al., even though the total number of thesample stars we consider is almost triple that of the previousinvestigation. We conclude that the fraction of halo stars that may havearisen from the precursor object of this clump may be smaller than 10%of the present Galactic halo, as previously suggested. The implicationsof our results for the formation of the Galaxy are discussed, inparticular in the context of the currently favored cold dark mattertheory of hierarchical galaxy formation.
| Ca II H and K Photometry on the UVBY System. III. The Metallicity Calibration for the Red Giants
New photometry on the uvby Ca system is presented for over 300 stars.When combined with previous data, the sample is used to calibrate themetallicity dependence of the hk index for cooler, evolved stars. Themetallicity scale is based upon the standardized merger of spectroscopicabundances from 38 studies since 1983, providing an overlap of 122evolved stars with the photometric catalog. The hk index producesreliable abundances for stars in the [Fe/H] range from -0.8 to -3.4,losing sensitivity among cooler stars due to saturation effects athigher [Fe/H], as expected.
| Distribution and Studies of the Infrared Stellar Population in the Galaxy. VI. The Halo
We present infrared J, H and K observations of 69 local galactic halostars. We produce the two colour (JHK) and the colour magnitude (Kversus (J-K)) diagrams for this stellar sample and compare them with thesame diagrams for the stellar populations in the globular clusters M3,M13, M92 and 47 Tucanae and in the old open cluster M67; we also comparethese diagrams with those for the stellar population in the galacticbulge.
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
| Post-main-sequence and POST red giant branch variables with pulsation periods less than one day
Post-main-sequence (mass 1 to 3 solar masses) and post-giant branch (0.5to 1 solar mass) pulsators are discussed on the basis of four color andH beta light curves published elsewhere. The post-main-sequencevariables, called ultrashort period cepheid (USPC) (delta Sct), pulsatein the fundamental and first harmonic modes of radial pulsation and, inmany cases, in nonradial modes. The variables for which photometryallows accurate, luminosity estimates and are known to pulsatesimultaneously in the fundamental and first harmonic or in thefundamental mode alone, define a PL relation (MV = -2.80 logP - 0.60, fundamental). It is notable that the slope of this relation isin the range of slopes found for classical cepheids. Accurate Vphotometry is lacking for many of the variables known as 'anomalouscepheids', but the available data divide them into low mass,pseudocepheids (BL Her and W Vir stars) and post-main-sequence USPC(delta Sct) variables. Four USPC in NGC 5053 and six in NGC 6466, forwhich accurate photometry is available, give remarkably consistentmoduli of 16.06 +/- 0.05 and 15.98 +/- 0.08 mag, respectively, for theclusters, in which they are blue stragglers similar to SX Phe inKapteyn's star group. The assumption that the four post-giant branchvariables, called VSPC (RR Lyr), S Ari, SU Dra, and ST Leo in Kapteyn'sstar group and RR Lyr in the Groombridge 1830 group, are physicalmembers of these groups and share their V-velocities, leads to acalibration of the photometry for the derivation of reddening,luminosity, and heavy element abundance of 45 field variables. Theresulting reddenings are consistent with values obtained by othermethods and the metallicities are consistent with the most accuratelyavailable spectroscopic determinations of delta S and of Ca II K. Theluminosities of the bulk of the variables confirm Sandage's (1993)relation between MV and (Fe/H). Four or five of the fieldvariables are probably binary, including BB Vir which Kinman &Carreta (1992) have independently noted as double. The PL relation forUSPC (delta Sct) variables intersects the horizontal branch (HB) near P= 0.3d and at least two field very short period cepheid (VSPC) (RR Lyr)star, FW Lup (0.484 d) and ST Pic (0.486 d) may be first overtonepulsators of the USPC (delta Sct) variety. A dozen field VSPC (RR Lyr)stars populate a (Fe/H), MV relation with the same slope asthe other stars but displaced 0.7 mag toward higher luminosities. Theonly cluster variable found to populate this diplaced relation is No. 9in 47 Tuc, although ST Vir, which may be a member of the Arcturus group,should also be considered. The elevated luminosities are unlikely to becaused by either evolution or errors in the photometric indices. Apossible source of these apparently young VSPC(RR Lyr) variables withhalo metallicity is in second (or third) generation globular clustersformed during an episodic collapse of the galaxy that produced metalpoor stars but in a dynamical situation that hastened the disruption ofthe clusters, currently formed, before the still older globularclusters, created under conditions that have kept them in a moredisruptive free environment.
| Astrometric and astrophysical discontinuities between the galactic old disk and halo stellar populations
Intermediate band, RI, and DDO photometry of the weak-lined stars in thefirst three volumes of the Michigan catalogs of spectral type arediscussed on the basis of luminosity and heavy element abundance. Theinterface between the old disk (Fe/H greater than -0.8 dex) and halo(Fe/H less than -1.2 dex) populations represents discontinuities in boththe stellar motions and the stellar physics. The CN strengths of bothevolved and unevolved halo stars decrease with decreasing temperature,in a mirror image of the increase with decreasing temperature for thedisk objects. The result for the halo giants has been attributed to deepmixing in the stellar atmospheres but the similar result for unevolveddwarfs indicates a difference in formation rather than in evolutionaryprocess of the two populations.
| Estimation of stellar metal abundance. I - Calibration of the CA II K index
A method for estimating the stellar metal abundances is proposed whichcompares measures of the equivalent width of a single feature inmoderate resolution (1 A) optical spectra of stars, the Ca II K line at3933 A, with models of the predicted line strength as a function of thebroadband B-V color and Fe/H. The approach is capable of providingestimates of stellar metallicity over the range -4.5 to -1.0 with ascatter of about 0.15 dex for dwarfs and giants in the color range0.33-0.85. For cooler stars, with B-V in the range 0.85-1.1, the scattermay be as large as 0.19 dex. The calibration of the Ca II K index withFe/H is discussed, and average radial velocities and abundances arepresented for several galactic globular clusters.
| Armchair cartography - A map of the Galactic halo based on observations of local, metal-poor stars
The velocity distribution of metal-poor halo stars in the solarneighborhood is studied to extract data on the global spatial andkinematic properties of the Galactic stellar halo. A global model of thesolar neighborhood stars is constructed from observed positions andthree-dimensional velocity of local, metal-poor halo stars in terms of adiscrete sum of orbits. The characteristics of the reconstructed haloare examined and used to study the evolution of the halo subsystems.
| Three-dimensional classification of 78 metal-deficient giants in the Vinius photometric system
Not Available
| Population studies. II - Kinematics as a function of abundance and galactocentric position for (Fe/H) of -0.6 or less
A catalog is presented of some 1200 Galactic objects which have radialvelocities and (Fe/H) abundances of -0.6 or less. These data areanalyzed to yield information on the kinematic properties of the olderpopulations of the Galaxy and on the interdependence between kinematicsand abundance. It is found that the kinematics of the availablekinematically selected stars differ from those of the nonkinematicallyselected objects. No evidence is found for any significant difference inthe kinematic properties of the various halo subgroups, nor for anydependence of kinematics on abundance. While the rotation of the halo issmall at about 37 km/s for (Fe/H) of -1.2 or less, it rises quickly forhigher abundances to a value of about 160 km/s at (Fe/H) = 0.6. Objectsin the abundance range -0.9 to -0.6 appear to belong predominantly to apopulation possessing the kinematic characteristics of a thick disk. Theimplications of these findings for the suggestion that globular clustersbelong to the same population as the noncluster objects, for the originof the thick disk, and for the mass of the Galaxy are discussed.
| Population studies. III - Further comparison of spectroscopically and kinematically selected halo samples
A comparison is made between the kinematically selected sample of halostars of Eggen, Lynden-Bell, and Sandage (1962, ELS) and the more recentspectroscopically selected sample of such objects obtained by Eggen(1984) from the Michigan Spectral Types Catalogs. Data are presented onthe abundances, absolute magnitudes, and kinematics of 29 halo stars,and the eccentricity distributions in the samples of ELS and Eggen arecompared. The results are found to be consistent with the view that thekinematically selected sample analyzed by ELS is biased againstlow-abundance, low-eccentricity stars.
| The kinematics of halo red giants
The present 337 radial velocities were obtained with typical accuraciesof 0.7 cm km/sec for 85 metal-poor field red giants, selected from thekinematically unbiased samples of Bond (1980) and Bidelman and MacConnel(1973). The multiply-observed stars suggest the field halo binaryfraction exceeds 10 percent. Using these velocities and those publishedby others, a sample of 174 red giants with Fe/H of not more than -1.5 isobtained. Their mean motion with respect to the local standard of restis -206 + or - 23 km/sec, and the velocity dispersions are sigma (R) of154 + or - 18 km/sec, sigma(theta) of 102 + or - 27 km/sec, andsigma(phi) of 107 + or - 15 km/sec. Using photometrically derivedabsolute magnitudes and published proper motions, orbital eccentricitiesare computed for 72 stars not already considered in a similar study ofsouthern stars by Norris et al. (1985). A few stars with e of less than0.4 are found.
| Photoelectric photometry of metal-deficient giants of spectral types G and K in the Vilnius system
Not Available
| Population studies. I - The Bidelman-MacConnell 'weak-metal' stars
BRVI and DDO photometry are presented for 309 Bidelman-MacConnell'weak-metal' stars. Radial velocities are calculated for most of thestars having Fe/H abundances of no more than -0.8. The photometricobservations were carried out using the 0.6-meter and 1.0-metertelescopes of the Siding Spring Observatory. Photometric taxonomy wasused to classify the stars as dwarfs, giants, red-horizontal branchstars, and ultraviolet-bright stars, respectively. It is found that 35percent of the stars are giants; 50 percent are dwarfs; and 5 percentbelong to the red-horizontal branch group. The role of selection effectsin investigations of the formation of the Galaxy is discussed on thebasis of the photometric observations and the observational constraintsproposed by Eggen et al. (1962).
| Weak-lined stars in the Michigan spectral survey. I - A-, F-, and G-type stars
Data from intermediate-band and H-beta photometric observations of 89early-type stars classified as weak-lined by Bidelman and MacConnell(1973), Houk and Cowley (1975), or Houk (1978 and 1982) are presented intables and graphs and analyzed. The group is found to comprise 55 diskstars and 34 halo stars (Fe/H less than -0.65), including 21 near themain-sequence turnoff, seven red horizontal-branch stars, one bluehorizontal-branch star, one suprahorizontal-branch object, and four bluestragglers; five of the newly identified halo stars are shown to haveradial velocities in excess of 100 km/s. Special consideration is givento the 17 early-type members of Kapetyn's star group (Eggen, 1977), andthe mean luminosity and Fe/H ratio of the four very-short-periodCepheids of this group are determined as 0.75 mag and -1.7.
| Southern subdwarf photometry
UBV photometry and normalized UV excesses are reported for 176 southernmetal-poor stars selected from the objective-prism survey of Bidelmanand MacConnell (1973) as well as 49 other metal-deficient starsidentified in other surveys. Photometry is also presented for 32 otherstars lying near the 225 program stars (although not explicitlyidentified as such in the text, the program stars are apparentlysubdwarfs and subdwarf candidates). Previously determined spectral typesand degrees of line weakening are given for the 225 stars, andmetallicities are estimated on the basis of the degrees of lineweakening. It is noted that 33 F and G stars with extreme meannormalized UV excesses of approximately 0.22 mag have a mean Fe/H valueof -1.4 and probably represent halo subdwarfs, while 82 F and G starswith moderate UV excesses are mostly old disk stars.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
