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.

I am greatly honored to be the second Bohdan Paczynski Medal lecturer. Bohdan was a dear friend who left an indelible imprint on my life. He was, as well, an invaluable participant in my early explorations of RR Lyrae spectra, the subject of this lecture. I share the hope of Polskie Towarzystwo Astronomiczne that the Paczynski lecture series will serve to remind present and future astronomers, particularly those who will never have the opportunity to see or hear Bohdan Paczynski in person, of the boisterous enthusiasm, sharp wit, and penetrating insight that he brought to every scientific question, to every conversation, to every social occasion that attracted his attention. To this end I begin with a few memories of Bohdan from the mid 20th century.

We present a detailed spectroscopic analysis of RR Lyrae (RRL) variables in the globular cluster NGC 5139 (omega Cen). We collected optical (4580-5330 angstrom), high-resolution (R similar to 34,000), high signal-to-noise ratio (similar to 200) spectra for 113 RRLs with the multifiber spectrograph M2FS at the Magellan/Clay Telescope at Las Campanas Observatory. We also analyzed high-resolution (R similar to 26,000) spectra for 122 RRLs collected with FLAMES/GIRAFFE at the Very Large Telescope, available in the ESO archive. The current sample doubles the literature abundances of cluster and field RRLs in the Milky Way based on high-resolution spectra. Equivalent-width measurements were used to estimate atmospheric parameters, iron, and abundance ratios for alpha (Mg, Ca, Ti), iron peak (Sc, Cr, Ni, Zn), and s-process (Y) elements. We confirm that omega Cen is a complex cluster, characterized by a large spread in the iron content: -2.58 <= [Fe/H] <= -0.85. We estimated the average cluster abundance as <[Fe/H]> = -1.80 +/- 0.03, with sigma = 0.33 dex. Our findings also suggest that two different RRL populations coexist in the cluster. The former is more metal-poor ([Fe/H] less than or similar to - 1.5), with almost solar abundance of Y. The latter is less numerous, more metal-rich, and yttrium enhanced ([Y/Fe] greater than or similar to 0.4). This peculiar bimodal enrichment only shows up in the s-process element, and it is not observed among lighter elements, whose [X/Fe] ratios are typical for Galactic globular clusters.

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1-2 km s(-1). The new amplitudes were combined with literature data for 23 RRab and 3 RRc stars (total sample of 74 RRLs), which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to recast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multiperiodicity (Blazhko effect). The RV slope (logP-A(V-r)) when compared with the visual slope (logP-A(V)) is shallower, and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of horizontal branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip.