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uvby-? photometry and kinematics of metal-poor stars: A search for moving groups in the Galactic Stellar Halo
The possible presence of moving groups in the local Galactic stellarhalo has been examined based on new uvby-? photometry for a sampleof 143 metal-poor stars joined to the photometry of 1553 stars from theprevious catalogues of Schuster & Nissen (1988), Schuster et al.(1993), and Schuster et al. (2006). This new set of photometric data,together with radial velocities and proper motions taken from theliterature, have allowed us to obtain Galactic space velocities U', V',and W' for nearly the entire sample. With this kinematic information,three diagrams: the [Fe/H] vs V_rot, the Bottlinger (V' vs U') and theToomre (V' vs (U'^2 + W'^2)^{1/2}) have been obtained and used toidentify probable groups of stars in common between them. Onceidentified, these groups were compared to moving groups reported in theliterature. In particular, probable members of the Kapteyn group and? Cen were identified.
| A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. II. On the Frequency of Giant Planets in the Metal-Poor Regime
We present an analysis of three years of precision radial velocity (RV)measurements of 160 metal-poor stars observed with HIRES on the Keck 1telescope. We report on variability and long-term velocity trends foreach star in our sample. We identify several long-term, low-amplitude RVvariables worthy of followup with direct imaging techniques. We placelower limits on the detectable companion mass as a function of orbitalperiod. Our survey would have detected, with a 99.5% confidence level,over 95% of all companions on low-eccentricity orbits with velocitysemiamplitude K gsim 100 m s–1, orMp sin i gsim 3.0 M J(P/yr)(1/3), fororbital periods P lsim 3 yr. None of the stars in our sampleexhibits RV variations compatible with the presence of Jovian planetswith periods shorter than the survey duration. The resulting averagefrequency of gas giants orbiting metal-poor dwarfs with–2.0lsim[Fe/H]lsim–0.6 is fp < 0.67% (at the1σ confidence level). We examine the implications of this nullresult in the context of the observed correlation between the rate ofoccurrence of giant planets and the metallicity of their main-sequencesolar-type stellar hosts. By combining our data set with the Fischer& Valenti (2005) uniform sample, we confirm that the likelihood of astar to harbor a planet more massive than Jupiter within 2 AU is asteeply rising function of the host's metallicity. However, the data forstars with –1.0lsim[Fe/H]lsim0.0 are compatible, in a statisticalsense, with a constant occurrence rate fp sime 1%. Ourresults can usefully inform theoretical studies of the process ofgiant-planet formation across two orders of magnitude in metallicity.
| Subdwarf studies. II - Abundances and kinematics from medium resolution spectra. III - The halo metallicity distribution
Stars previously identified as having UV excesses are observed at 1-Aresolution in the Ca II K-line region. Comparisons of these data withother samples and with Monte Carlo simulations involving a singlecomponent halo have yielded estimates of halo velocity dispersions androtation velocity, corrected for the kinematic biases in the sample. Itis suggested that the data are not consistent with a model in which thehalo formed from star formation in a dissipating, collapsing cloud; theyare, however, reconcilable with the formation of the halo stars bynumerous, independently evolving gas clouds. The metallicitydistribution of a sample of 372 kinematically selected halo stars isthen constructed, with a view to selection effects in the data. Goodagreement is noted between the globular cluster metallicity distributionand a stochastic model with a mean of 10 enrichments/fragment.
| Subdwarf studies. I - UBVRI photometry of NLTT stars
UBVRI photometry is presented for a sample of 1656 southern stars,including 1211 that were previously unmeasured, drown from the NLTTproper-motion catalog. The catalog is shown to be a rich source ofsubdwarfs. The normalized ultraviolet excess delta (U - B)0.6,photometric parallax, and interstellar reddening are calculated for eachstar when possible. Photometric parallaxes are compared withtrigonometric parallaxes from the literature. It is found that theformer do not have systematic errors greater than about 25 percent. Inagreement with other studies, the bluest subdwarfs are found at B - V =0.35. The selection of the program stars on the basis of large reducedproper motions restricted subgiant contamination of the sample to about5 percent and increased the discovery fraction of halo stars relative todisk stars. The claim is made here that the sample can be used toinvestigate the abundance distribution of the halo. The sample includesstars with ultraviolet excesses characteristic of disk abundances butwith velocities up to 150 km/s. These are believed to be stars that,quite expectedly, reside in the high-velocity tail of the disk velocitydistribution.
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