We present the results of an abundance analysis for a sample of stars with −4 <[Fe/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 [Fe/H]< −2.6, including nine with [Fe/H]< −3.0, and one with [Fe/H]= −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 [Fe/H]∼ −3.5, including a normal C+N abundance. We also find that it has very low but measurable Sr and Ba, indicating some neutron-capture activity even at this low of a metallicity. We explore this issue further by examining other very neutron-capture-deficient 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-process-enhanced star, CS 31078-018. This star has [Fe/H]= −2.85, [Eu/Fe]= 1.23, and [Ba/Eu]= −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 . We find that supernovae with progenitor masses between 10 and 20 M ⊙ provide the best matches to our abundances.

There is a consensus that type Ia supernovae (SNe Ia) arise from the thermonuclear explosion of white dwarf stars that accrete matter from a binary companion. However, direct observation of SN Ia progenitors is lacking, and the precise nature of the binary companion remains uncertain. A temporal series of high-resolution optical spectra of the SN Ia PTF 11kx reveals a complex circumstellar environment that provides an unprecedentedly detailed view of the progenitor system. Multiple shells of circumstellar material are detected, and the SN ejecta are seen to interact with circumstellar material starting 59 days after the explosion. These features are best described by a symbiotic nova progenitor, similar to RS Ophiuchi.

We present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a satellite of the Milky Way. This is the largest sample of individual α element (Mg, Si, Ca, Ti) abundance measurements in any single dSph. The measurements are made from Keck/DEIMOS medium-resolution spectra (6400-9000Å, R ∼ 6500). Based on comparisons to published high-resolution (R 20000) spectroscopic measurements, our measurements have uncertainties of σ[Fe/H] = 0.14 and σ[α/Fe] = 0.13. The Sculptor [Fe/H] distribution has a mean [Fe/H] = −1.58 and is asymmetric with a long, metal-poor tail, indicative of a history of extended star formation. Sculptor has a larger fraction of stars with [Fe/H] < −2 than the Milky Way halo. We have discovered one star with [Fe/H] = −3.80±0.28, which is the most metalpoor star known anywhere except the Milky Way halo, but high-resolution spectroscopy is needed to measure this star's detailed abundances. As has been previously reported based on high-resolution spectroscopy, [α/Fe] in Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] ∼ −1.5) have lower [α/Fe] than Galactic halo field stars of comparable metallicity. This indicates that star formation proceeded more gradually in Sculptor than in the Galactic halo. We also observe radial abundance gradients of −0.030 ± 0.003 dex per arcmin in [Fe/H] and +0.013 ± 0.003 dex per arcmin in [α/Fe] out to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly other surviving dSphs as representative of the dwarf galaxies from which the metal-poor tail of the Galactic halo formed.

We present a photometric and spectroscopic study of the white dwarf (WD) population of the populous, intermediateage open cluster M35 (NGC 2168); this study expands upon our previous study of the WDs in this cluster. We spectroscopically confirm 14 WDs in the field of the cluster: 12 DAs, 1 hot DQ, and 1 DB star. For each DA, we determine the WD mass and cooling age, from which we derive each star's progenitor mass. These data are then added to the empirical initial-final mass relation (IFMR), where the M35 WDs contribute significantly to the high-mass end of the relation. The resulting points are consistent with previously published linear fits to the IFMR, modulo moderate systematics introduced by the uncertainty in the star cluster age. Based on this cluster alone, the observational lower limit on the maximum mass of WD progenitors is found to be ∼5.1 M − 5.2 M at the 95% confidence level; including data from other young open clusters raises this limit to as high as 7.1 M , depending on the cluster membership of three massive WDs and the core composition of the most massive WDs. We find that the apparent distance modulus and extinction derived solely from the cluster WDs ((m − M) V = 10.45 ± 0.08 and E(B −V ) = 0.185 ± 0.010, respectively) is fully consistent with that derived from main-sequence fitting techniques. Four M35 WDs may be massive enough to have oxygen-neon cores; the assumed core composition does not significantly affect the empirical IFMR. Finally, the two non-DA WDs in M35 are photometrically consistent with cluster membership; further analysis is required to determine their memberships.

We present new BV I photometry for the halo globular cluster M5 (NGC 5904 = C1516+022), and examine the B-and I-band luminosity functions (LFs), based on over 20,000 stars -one of the largest samples ever gathered for a cluster luminosity function. Extensive artificial star tests have been conducted to quantify incompleteness as a function of magnitude and cluster radius. We do not see evidence in the LF of a "subgiant excess" or of a discrepancy in the relative numbers of stars on the red-giant branch and main sequence, both of which have been claimed in more metal-poor clusters.Enhancements of α-element have been taken into account in our analysis. This improves the agreement between the observed and predicted positions of the "red-giant bump". Depending on the average α-element enhancement among globular clusters and the distance calibration, the observed discrepancy between the theoretical and observed position for a large number of clusters ) can be almost completely removed.The helium abundance of M5, as determined by the population ratio R, is found to be Y = 0.19 ± 0.02. However, there is no other indication that the helium abundance is different from other clusters of similar metallicity, and values calculated for other helium indicators are consistent with Y ≈ 0.23.The relative ages of M5, Palomar 5, M4, NGC 288, NGC 362, NGC 1261, NGC 1851 and NGC 2808 are derived via the ∆V HB T O method using M5's horizontal branch (HB) as a bridge to compare clusters with very different HB morphology. We conclude that at the level of ∼ 1.5 Gyr these clusters of comparable metallicity are the same age with the possible exception of NGC 288 (older by 3.5 ± 1.5 if the reddest NGC 288 HB stars are on the zero-age horizontal branch) and Palomar 5 (which may be marginally younger). Even with NGC 288 set aside, there is a large range in HB morphology between the remaining clusters which appears to eliminate age as the sole second parameter determining HB morphology in the case of constant mass loss between RGB and HB (although a Reimers' mass-loss relation weakens this statement considerably).We are unable to chose between the two competing values for M5's (absolute) metallicity: [Fe/H] = −1.40 (Zinn & West 1984) and −1.17 (Sneden et al. 1992) based on recent high-dispersion spectroscopy. This level of discrepancy has a signifcant effect on the derivation of the distance modulus and absolute age of M5. From subdwarf fitting to the main sequence of the cluster, we find an apparent distance modulus (m − M ) V = 14.41 ± 0.07 for [Fe/H] M5 = −1.40, and 14.50 ± 0.07 if [Fe/H] M5 = −1.17. From comparisons with theoretical isochrones and luminosity functions, we find an absolute age for M5 of 13.5 ± 1 Gyr (internal error, assuming perfect models and no [M /H] error) for the Zinn & West abundance scale and 11 ± 1 Gyr for the higher abundance value.