David K. Lai

Detailed Abundances for 28 Metal-poor Stars: Stellar Relics in the Milky Way

We present the results of an abundance analysis for a sample of stars with � 4 < ½Fe/H� < � 2. The data were obtained with the HIRES spectrograph at Keck Observatory. The set includes 28 stars, with effective temperature ranging from 4800 to 6600 K. For 13 stars with ½Fe/H� < � 2:6, including nine with ½Fe/H� < � 3:0 and one with ½Fe/H �¼ � 4:0, these are the first reported detailed abundances. For the most metal-poor star in our sample, CS 30336� 049, we measure an abundance pattern that is very similar to stars in the range ½Fe/H ��� 3:5, including a normal C þ N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutroncapture activity even at this low of a metallicity. We explore this issue further by examining other very neutron captureYdeficient stars and find that, at the lowest levels, [Ba/Sr] exhibits the ratio of the main r-process. We also report on a new r-processYenhanced star, CS 31078� 018. This star has ½Fe/H �¼ � 2:85, ½Eu/Fe �¼ 1:23, and ½Ba/Eu �¼ � 0:51. CS 31078� 018 exhibits an ‘‘actinide boost,’’ i.e., much higher [Th/Eu] than expected and at a similar level to CS 31082� 001. Our spectra allow us to further constrain the abundance scatter at low metallicities, which we then use to fit to the zero-metallicity Type II supernova yields of Heger & Woosley (2008). We find that

THE SEGUE STELLAR PARAMETER PIPELINE. III. COMPARISON WITH HIGH-RESOLUTION SPECTROSCOPY OF SDSS/SEGUE FIELD STARS*

The authors report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which they compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R {approx} 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), they empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s{sup -1}, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. They estimate random errors for lower S/N spectra based on numerical simulations.

The [Fe/H], [C/Fe], and [α/Fe] Distributions of the Boötes I Dwarf Spheroidal Galaxy

We present the results of a low-resolution spectral abundance study of 25?stars in the Bo?tes I dwarf spheroidal (dSph) galaxy. The data were obtained with the low resolution imaging spectrometer instrument at Keck Observatory and allow us to measure [Fe/H], [C/Fe], and [?/Fe] for each star. We find both a large spread in metallicity (2.1?dex in [Fe/H]) as well as a low average metallicity in this system, [Fe/H] = ?2.59, matching previous estimates. This sample includes a newly discovered extremely metal-poor star, with [Fe/H] = ?3.8, that is one of the most metal-poor stars yet found in a dSph. We compare the metallicity distribution function of Bo?tes I to analytic chemical evolution models. While the metallicity distribution function of Bo?tes I is best fit by an Extra Gas chemical evolution model, leaky-box models also provide reasonable fits. We also find that the [?/Fe] distribution and the carbon-enhanced metal-poor fraction of our sample (12%) are reasonable matches to Galactic halo star samples in the same metallicity range, indicating that at these low metallicities, systems like the Bo?tes I ultra-faint dSph could have been contributors to the Galactic halo.

A UNIQUE STAR IN THE OUTER HALO OF THE MILKY WAY

As part of a program to measure abundance ratios in stars beyond 15 kpc from the Galactic center, we have discovered a metal-poor star in the outer halo with a unique chemical signature. We originally identified it in the Sloan Extension for Galactic Understanding and Exploration survey as a distant metal-poor star. We obtained a follow-up spectrum using the Echelle Spectrometer and Imager at the Keck 2 telescope, and measure [Fe/H] =–3.17, [Mg/Fe] = –0.10, and [Ca/Fe] = +1.11. This is one of the largest over-abundances of Ca measured in any star to date; the extremely low value of [Mg/Ca] = –1.21 is entirely unique. To have found such an unusual star in our small sample of 27 targets suggests that there may be previously unobserved classes of stars yet to be found in situ in the Galactic halo.

Carbon and Strontium Abundances of Metal-poor Stars*

We present carbon and strontium abundances for 100 metal-poor stars measured from R ~ 7000 spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory. Using spectral synthesis of the G-band region, we have derived carbon abundances for stars ranging from [Fe/H] = -1.3 to [Fe/H] = -3.8. The formal errors are ~0.2 dex in [C/Fe]. The strontium abundance in these stars was measured using spectral synthesis of the resonance line at 4215 A. Using these two abundance measurements along with the barium abundances from our previous study of these stars, we show that it is possible to identify neutron-capture-rich stars with our spectra. We find, as in other studies, a large scatter in [C/Fe] below [Fe/H] = -2. Of the stars with [Fe/H] < -2, 9% ± 4% can be classified as carbon-rich metal-poor stars. The Sr and Ba abundances show that three of the carbon-rich stars are neutron-capture-rich, while two have normal barium and strontium. This fraction of carbon enhanced stars is consistent with other studies that include this metallicity range.

CHEMICAL ABUNDANCES FOR EVOLVED STARS IN M5: LITHIUM THROUGH THORIUM*

We present analysis of high-resolution spectra of a sample of stars in the globular cluster M5 (NGC 5904). The sample includes stars from the red giant branch (RGB; seven stars), the red horizontal branch (two stars), and the asymptotic giant branch (AGB; eight stars), with effective temperatures ranging from 4000 K to 6100 K. Spectra were obtained with the HIRES spectrometer on the Keck I telescope, with a wavelength coverage from 3700 A to 7950 A for the HB and AGB sample, and 5300 A to 7600 A for the majority of the RGB sample. We find offsets of some abundance ratios between the AGB and the RGB branches. However, these discrepancies appear to be due to analysis effects, and indicate that caution must be exerted when directly comparing abundance ratios between different evolutionary branches. We find the expected signatures of pollution from material enriched in the products of the hot hydrogen burning cycles such as the CNO, Ne-Na, and Mg-Al cycles, but no significant differences within these signatures among the three stellar evolutionary branches especially when considering the analysis offsets. We are also able to measure an assortment of neutron-capture element abundances, from Sr to Th, in the cluster. We find that the neutron-capture signature for all stars is the same, and shows a predominately r-process origin. However, we also see evidence of a small but consistent extra s-process signature that is not tied to the light-element variations, pointing to a pre-enrichment of this material in the protocluster gas.

Abundances of Extremely Metal-poor Star Candidates*

We present chemical abundances for 110 stars identified in objective-prism surveys as candidates for being very metal-poor. The abundances are derived from high-S/N, intermediate-resolution spectra obtained with the Keck Observatory Echellette Spectrograph and Imager (ESI). An additional 25 stars with well-determined abundances ranging from [Fe/H] = -1.5 to -3.2 were observed and the results used to help calibrate our analysis and determine the accuracy of our abundance determinations. Abundances for the program stars were measured for Fe, Mg, Ca, Ti, Cr, and Ba with an accuracy of approximately 0.3 dex. Fifty-three of the stars in our sample have [Fe/H] ≤ -2, 22 have [Fe/H] ≤ -2.5, and 13 have [Fe/H] ≤ -2.9. Surprisingly, approximately one-third of the sample is relatively metal-rich, with [Fe/H] > -1.5. In addition to identifying a number of extremely metal-poor stars, this study also shows that moderate-resolution spectra obtained with the Keck ESI yield relatively accurate abundances for stars as faint as V = 14 with modest exposure time (~20 minutes). This capability will prove useful if the so-far elusive stars at [Fe/H] < -4 turn out to be mostly fainter than V = 15.