Sean G. Ryan

A Subaru/High Dispersion Spectrograph Study of Lead (Pb) Abundances in Eight s-Process Element-rich, Metal-poor Stars

We report the abundances of neutron-capture elements in eight carbon-rich, metal-poor (� 2:7 �½ Fe=H ��� 1:9) stars observed with the Subaru Telescope High Dispersion Spectrograph. The derived abundance patterns indicate that the neutron-capture elements in these objects primarily originated from s-process nucleosynthesis, although the [Ba/Eu] abundance ratios in some objects are lower than that of the solar system s-process component. The present analysis has yielded the Pb abundances for seven objects as well as an upper limit for one object from use of the Pb i � 4057 and � 3683 lines. The values of [Pb/ Ba] in these objects cover a wide range, between � 0.3 and 1.2. Theoretical studies of s-process nucleosynthesis at low metallicity are required to explain this large dispersion of the [Pb/Ba] values. Variations in radial velocity have been found for two of the eight objects, suggesting that, at least in these instances, the observed excess of s-process elements is due to the transfer of material across a binary system including an AGB star. Comparisons with predictions of AGB nucleosynthesis models are discussed. Subject headings: nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — stars: carbon — stars: Population II On-line material: machine-readable table

Neutron Capture Elements in s -Process-rich, Very Metal-poor Stars

We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly aUected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the —nal abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z 38) to Pb (Z 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the eUects of hyper—ne splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any speci—c stellar model. The neutron exposure q is estimated to be about 0.7 mbarn~1, signi—cantly larger than that which best —ts solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also —nd that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r D 0.1).

The Chemical Composition of Carbon-rich, Very Metal Poor Stars: A New Class of Mildly Carbon Rich Objects without Excess of Neutron-Capture Elements

We report on an analysis of the chemical composition of —ve carbon-rich, very metal poor stars based on high-resolution spectra. One star, CS 22948-027, exhibits very large overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al. This result can be interpreted as a consequence of mass transfer from a binary companion that previously evolved through the asymptotic giant branch stage. By way of contrast, the other four stars we investigate exhibit no overabundances of barium ([Ba/Fe] 0), while three of them have mildly enhanced carbon and/or nitrogen ([C ] N] D ]1). We have been unable to determine accurate carbon and nitrogen abundances for the remaining star (CS 30312-100). These stars are rather similar to the carbon-rich, neutron-capture

Extremely Metal-poor Stars. IX. CS 22949-037 and the Role of Hypernovae

We have obtained high-resolution spectra of the extremely metal-poor star CS 22949-037 ( ) [Fe/H] p 3.79 in the region of the ultraviolet NH bands at 3360–3370 A u , which confirm our earlier report that it exhibits extreme nitrogen enhancement. We find . The star is also known to have large relative overabundances, [N/Fe] p 2.3 0.4 of order 0.5–1 dex, of carbon and the a-elements. This abundance pattern is not explained by any standard Galactic chemical enrichment model of which we are aware. The large N enhancement is, however, predicted by the zero heavy element, , hypernova models of Woosley & Weaver and Fryer, Woosley, & Heger. M 200 M, If the rotating models of Fryer et al. are relevant to the enrichment of CS 22949-037, one might expect it to possess a large overabundance of oxygen, with a mass fraction similar to or larger than that of nitrogen, and an [O i] l6300.3 line strength of ∼2–8 mA u . Subject headings: nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: Population II

Rapid rotation of ultra-li-depleted halo stars and their association with blue stragglers

Observations of 18 halo main-sequence turnoff stars, four of which are extremely deficient in Li, show that three of the Li-poor ones have substantial line broadening. We attribute this to stellar rotation. Despite the great ages of halo stars, for G202-65, BD +51°1817, and Wolf 550 we infer v sin i = 8.3 ± 0.4, 7.6 ± 0.3, and 5.5 ± 0.6 km s-1, respectively. The stated errors are 3 σ. For CD -31°19466 we derive a 3 σ upper limit v sin i < 2.2 km s-1. The three rotating stars are known spectroscopic binaries. We explain the high rotation velocities in terms of mass and angular momentum transfer onto the surface of the turnoff star from an initially more massive donor. Estimates of the specific angular momentum of accreted material indicate that quite small transfer masses could have been involved, although the unknown subsequent spin-down of the accretor prevents us from assigning definitive values for each star. The accretor is now seen as an ultra-Li-deficient star whose origin makes it a low-mass counterpart of field blue stragglers. The Li could have been destroyed before or during the mass transfer episode. Such objects must be avoided in studies of the primordial Li abundance and in investigations into the way normal single stars process their initial Li.

Ultra-Lithium-deficient Halo Stars and Blue Stragglers: A Common Origin?*

We present data for four ultraLi-de—cient, warm, halo stars. The Li de—ciency of two of these is a new discovery. Three of the four stars have eUective temperatures K, in contrast to pre- T eff D 6300 viously known Li-de—cient halo stars, which spanned the temperature range of the Spite plateau. In this paper we propose that these and previously known ultraLi-de—cient halo stars may have had their surface lithium abundances reduced by the same mechanism as produces halo —eld blue stragglers. Even though these stars have yet to reveal themselves as blue stragglers, they might be regarded as ii blue- stragglers-to-be.ˇˇ In our proposed scenario, the surface abundance of Li in these stars could be destroyed (1) during the normal premain-sequence single-star evolution of their low-mass precursors, (2) during the postmain-sequence evolution of an evolved mass donor, and/or (3) via mixing during a mass- transfer event or stellar merger. The warmest Li-de—cient stars at the turnoU would be regarded as emerging ii canonical ˇˇ blue stragglers, whereas cooler ones represent subturnoU-mass blue-stragglers-to- be. The latter are presently hidden on the main sequence, Li depletion being possibly the clearest signa- ture of their past history and future signi—cance. Eventually, the main-sequence turnoU will reach down to their mass, exposing those Li-depleted stars as canonical blue stragglers when normal stars of that mass evolve away. Arguing against this uni—ed view is the observation that the three Li-depleted stars at K are all binaries, whereas very few of the cooler systems show evidence for binarity; it is T eff ^ 6300 thus possible that two separate mechanisms are responsible for the production of Li-de—cient main- sequence halo stars. Subject headings: binaries: spectroscopicblue stragglersearly universeGalaxy: halo ¨ stars: abundancesstars: Population II

Europium Isotope Ratios in s-Process Element-enhanced Metal-poor Stars: A New Probe of the 151Sm Branching*

We report on the first measurement of the Eu isotope fractions (151Eu and 153Eu) in s-process element-enhanced metal-poor stars. We use these ratios to investigate the 151Sm branching of s-process nucleosynthesis. The measurement was made by detailed study of Eu II lines that are significantly affected by hyperfine splitting and isotope shifts in spectra of the carbon-rich very metal poor stars LP 625-44 and CS 31062-050, observed with the Subaru Telescope High Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu + 153Eu)] derived for LP 625-44 and CS 31062-050 are 0.60 and 0.55, respectively, with uncertainties of about ±0.05. These values are higher than found in solar system material but agree well with the predictions of recent s-process models. We derive new constraints on the temperature and neutron density during the s-process based on calculations of pulsed s-process models for the 151Eu fraction.

Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. I. Observational Data

We have obtained high-resolution (R 50;000 or 90,000), high-quality (S=N k100) spectra of 22 very metal- poor stars ((Fe/HP � 2: 5) with the High Dispersion Spectrograph fabricated for the 8.2 mS ubaru Telescope. The spectra cover the wavelength range from 3500 to 5100 8; equivalent widths are measured for isolated lines of numerous elemental species, including the � -elements, the iron-peak elements, and the light and heavy neutron-capture elements. Errors in the measurements and comparisons with previous studies are discussed. These data will be used to perform detailed abundance analyses in the following papers of this series. Radial velocities are also reported and are compared with previous studies. At least one moderately r-process-enhanced metal-poor star, HD 186478, exhibits evidence of a small-amplitude radial velocity variation, confirming the binary status noted previously. During the course of this initial program, we have discovered a new moderately r-process-enhanced, very metal-poor star, CS 30306� 132 ((Fe/H �¼� 2:4; (Eu/Fe �¼þ 0:85), which is discussed in detail in the companion paper.

Spectroscopic Studies of Extremely Metal-Poor Stars with the SUBARU High Dispersion Spectrograph I. Observational Data

We have obtained high-resolution (R 50;000 or 90,000), high-quality (S=N k100) spectra of 22 very metal- poor stars ((Fe/HP � 2: 5) with the High Dispersion Spectrograph fabricated for the 8.2 mS ubaru Telescope. The spectra cover the wavelength range from 3500 to 5100 8; equivalent widths are measured for isolated lines of numerous elemental species, including the � -elements, the iron-peak elements, and the light and heavy neutron-capture elements. Errors in the measurements and comparisons with previous studies are discussed. These data will be used to perform detailed abundance analyses in the following papers of this series. Radial velocities are also reported and are compared with previous studies. At least one moderately r-process-enhanced metal-poor star, HD 186478, exhibits evidence of a small-amplitude radial velocity variation, confirming the binary status noted previously. During the course of this initial program, we have discovered a new moderately r-process-enhanced, very metal-poor star, CS 30306� 132 ((Fe/H �¼� 2:4; (Eu/Fe �¼þ 0:85), which is discussed in detail in the companion paper.