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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.
| IRAS Detections of Metal-poor Red Giants
A number of relatively bright metal-poor red giants from the HD and BDcatalogs are found to have been detected by the IRAS satellite. Data forthese stars have been retrieved from the IRAS Point Source Catalog (PSC)and/or the Faint Source Catalog (FSC). The majority of metal-poor giantsin these samples fall along relatively well-defined sequences in plotsof V-[12] versus B-V and V-I; for these stars, the 12 μm fluxdetected is presumed to arise from the photosphere. Only a subset ofstars detected at 12 μm were detected at 25 μm these are displayedin a plot of [12]-[25] versus V-[12]. There are a small number of giantsthat exhibit notable 12 and/or 25 μm excesses relative to the meansequences defined by the bulk of the sample. Those stars with the mostunambiguous evidence for infrared excesses are variable stars, eitherlong-period or semiregular variables or RV Tauri stars. As such, thosestars exhibiting infrared excesses in the metal-poor giant sample arelikely in the asymptotic giant branch (AGB) or post-AGB phase ofevolution. There is no clear evidence for nonvariable first-ascent redgiants having been detected among the infrared-excess stars. In fact,some metal-poor red giants known to exhibit outflows in theirchromospheres do not show infrared excesses. A Population II starascending the red giant branch for the first time appears to have toolow a mass-loss rate to be recognizable as an infrared-excess star inthe IRAS PSC or FSC.
| 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.
| U, V, W velocity components for the old disk using radial velocities of 1295 stars in the three cardinal Galactic directions
New radial velocities are presented for 1295 stars chosen at random nearthe three cardinal Galactic directions of l = 180 deg, b = 0; l = 90deg, b = 0 deg; and b = 90 deg, giving the distribution in U, V, and W,respectively, from the radial velocities alone. The measurements weremade with the coude spectrograph of the Mount Wilson 100 in. Hookerreflector. The purpose of the program is to set limits on the densitynormalization in the solar neighborhood of the old thin disk, the oldthick disk, and the halo. Many more high-velocity stars are present inthe unbiased sample than expected from previous estimates of thenormalization. The data suggest the density ratios in the solarneighborhood to be about 90 percent, 10 percent, and about 0.5 percentfor the thin disk, thick disk, and halo populations, respectively.
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Osservazione e dati astrometrici
| Costellazione: | Toro |
| Ascensione retta: | 05h12m34.10s |
| Declinazione: | +27°36'15.4" |
| Magnitudine apparente: | 8.619 |
| Distanza: | 143.885 parsec |
| Moto proprio RA: | -0.7 |
| Moto proprio Dec: | 2 |
| B-T magnitude: | 10.366 |
| V-T magnitude: | 8.764 |
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