The radial velocity behavior and chemical compositions of sixty-two blue metal-poor (BMP) stars have been established from more than 1200 echelle spectra obtained at Las Campanas Observatory from 1992 through 1999. Analysis of survey spectra provides abundances for this sample, which we use to calibrate the K line versus B-V relation. Forty-four of the stars have [Fe/H] < - 1, while eighteen lie on -1 < [Fe/H] < 0. One star, the SX Phe variable CS 22966-043, appears to be the most extreme example of a rare abundance class characterized by alpha-element deficiencies, high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and [Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities, while 42 are definite or probable members of binary systems. The binary fraction of BMP stars appears to be independent of chemical composition.

We have monitored the radial velocities of seven very metal-poor stars with abnormally strong CH G bands. On the basis of their colors four are giants (gCH) and three are subgiants (sgCH). Three of the gCH stars are definite spectroscopic binaries with long periods, 505 days

We report the results of a coherent study of a new class of halo stars defined on the basis of the chemical compositions of three metal-poor objects (Fe/H] similar or equal to -2) that exhibit unusually low abundances of alpha-element (Mg, Si, Ca) and neutron-capture (Sr, Y, Ba) material. Our analyses confirm and expand on earlier reports of atypical alpha- and neutron-capture abundances in BD + 80degrees 245, G4- 36, and CS 22966-043. We also find that the latter two stars exhibit unusual relative abundance enhancements within the iron peak (Cr, Mn, Ni, Zn), along with what may be large abundances of Ga, an element not previously reported as being observed in any metal-poor star. These results provide further evidence that chemical enrichment and star formation histories varied from region to region within the Milky Way halo. Comparing the chemical abundances of the newly identified stellar population to supernova model yields, we derive supernova ratios of Type Ia versus Type II events in the range of 0.6 less than or similar to (N-Ia/N-II)(New Pop) less than or similar to 1.3. For the Sun, we derive 0.18 +/- 0.01 < (N-Ia/N-II)(.) < 0.25 +/- 0.06, supernova ratios in good agreement with values found in the literature. Given the relatively low metallicity and relatively high (N-Ia/N-II) ratios of the low-alpha stars studied here, these objects may have been born from material produced in the yields of the earliest Type Ia supernova events. We also report the results of a preliminary attempt to employ the observed chemical abundances of low-metallicity stars in the identification, and possible cosmic evolution, of Type Ia supernova progenitors, and we discuss the limitations of current model yields.

We have conducted spectrum analyses of 24 field metal-poor ([Fe/H] < -2) red horizontal-branch (RHB) stars identified in the HK objective-prism survey and 6 such stars in the globular cluster M15, based on high-quality spectra (R similar to 40,000, S/N similar to 100) obtained with the Magellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 m telescope at Las Campanas Observatory. The atmospheric parameters of the RHB stars provide interesting bridges between turnoff stars of similar temperature and red giant branch (RGB) stars of similar gravity, and they permit investigations of abundance trends [X/Fe] versus [Fe/H] in a relatively unexplored region of the temperature- gravity plane. We find that the T-eff, log g, v(t), and [Fe/H] values determined from our spectra are consistent with expectations from literature spectroscopic studies of other evolved metal-poor stellar classes. We show that the RHB stars have abundance distributions that are consistent with typical halo stars of similar metallicities. The photometric and spectroscopic gravities of the M15 stars differ by amounts that grow with declining temperature. We use a regression derived from these differences to calculate photometric gravities for the field RHB stars. Then we use the locations of the field RHB stars among the evolutionary tracks of Cassisi et al. in the log g versus log T-eff plane to estimate their masses and lifetimes as RHB stars. We use these lifetimes to estimate the size of the metal-poor HB population from which they arise. Then, using counts of HB and RGB stars in metal-poor globular clusters, we conclude that the number of metal-poor RGB stars at high latitudes (vertical bar b vertical bar > 30 degrees) brighter than V = 15 exceeds those identified in extant objective-prism surveys by more than an order of magnitude. Finally, we deduce the effective temperature of the fundamental red edge of the metal-poor RR Lyrae instability strip, log T-eff(FRE) = 3.80 +/- 0.01, from the interface between the temperature distributions of metal- poor field RHB stars and the RR Lyrae stars of similar [Fe/H] in five metal- poor globular clusters.

We have conducted a photometric and high- resolution spectroscopic analysis of the high- latitude (l = 1 degrees, b = -55 degrees) metal- poor RR Lyrae star TY Gru (= CS 22881-071). We find this star to have large overabundances of carbon and neutron- capture elements. Mass transfer from an asymptotic giant branch binary companion prior to its RR Lyrae evolutionary state is the simplest explanation of this circumstance. Unfortunately, TY Gru is afflicted by the Blazhko effect, which greatly complicates accurate measurement of the motion of its center of mass. We have not yet detected the small orbital motion that would be produced by a distant degenerate companion of TY Gru.

We combine Spitzer and ground-based observations to measure the microlens parallax of OGLE-2005-SMC-001, the first such space-based determination since S. Refsdal proposed the idea in 1966. The parallax measurement yields a projected velocity (v) over tilde similar to 230 km s(-1), the typical value expected for halo lenses, but an order of magnitude smaller than would be expected for lenses lying in the Small Magellanic Cloud itself. The lens is a weak (i.e., non-caustic-crossing) binary, which complicates the analysis considerably but ultimately contributes additional constraints. Using a test proposed by Assef and coworkers, which makes use only of kinematic information about different populations but does not make any assumptions about their respective mass functions, we find that the likelihood ratio is L-halo/L-SMC = 20. Hence, halo lenses are strongly favored, but Small Magellanic Cloud (SMC) lenses are not definitively ruled out. Similar Spitzer observations of additional lenses toward the Magellanic Clouds would clarify the nature of the lens population. The Space Interferometry Mission could make even more constraining measurements.

CS 29497-030 and CS 31062-050 belong to a sample of C-rich, s-process rich and extremely metal-poor stars (CEMP-s+r). To explain the s-process enrichment, we considered these stars to be extrinsic asymptotic giant branch (AGB) stars, belonging to binary systems where the more massive AGB companion polluted the observed star (of similar to 0.8M center dot) with efficient stellar winds. To explain the r-process enrichment, we assumed that the parental cloud was already enriched in r-process elements.

I report some results of an echelle spectroscopic survey of RR Lyrae stars begun in 2006 that I presented in my Henry Norris Lecture of 2010 January 4. Topics include (1) atmospheric velocity gradients, (2) phase-dependent envelope turbulence as it relates to Peterson's discoveries of axial rotation on the horizontal branch and to Stothers' explanation of the Blazhko effect, (3) the three apparitions of hydrogen emission during a pulsation cycle, (4) the occurrence of He I lines in emission and absorption, (5) detection of He II emission and metallic line doubling in Blazhko stars, and finally (6) speculation about what helium observations of RR Lyrae stars in omega Centauri might tell us about the putative helium populations and the horizontal branch of that strange globular cluster.

We present a detailed chemical abundance study of eight RR Lyrae variable stars of subclass c (RRc). The target RRc stars chosen for study exhibit "Blazhko-effect" period and amplitude modulations to their pulsational cycles. Data for this study were gathered with the echelle spectrograph of the 100 inch du Pont telescope at Las Campanas Observatory. Spectra were obtained throughout each star's pulsation cycle. Atmospheric parameters-effective temperature, surface gravity, microturbulent velocity, and metallicity-were derived at multiple phase points. We found metallicities and element abundance ratios to be constant within observational uncertainties over the pulsational cycles of all stars. Moreover, the alpha-element and Fe-group abundance ratios with respect to iron are consistent with other horizontal-branch members (RRab, blue and red non-variables). Finally, we have used the [Fe/H] values of these eight RRc stars to anchor the metallicity estimates of a large-sample RRc snapshot spectroscopic study being conducted with the same telescope and instrument combination employed here.