An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553

We provide a high dispersion line-by-line abundance analysis of five red HB stars in the extremely metal rich galactic globular cluster NGC 6553. These red HB stars are significantly hotter than the very cool stars near the tip of the giant branch in such a metal rich globular cluster and hence their spectra are much more amenable to an abundance analysis than would be the case for red giants. We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and considerably higher than that obtained from an analysis of two red giants in this cluster by Barbuy etal (1999). The relative abundance for the best determined alpha process element (Ca) indicates an excess of alpha process elements of about a factor of two. The metallicity of NGC 6553 reaches the average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap

The Chemical Compositions of the SRd Variable Stars-- II. WY Andromedae, VW Eridani, and UW Librae

Chemical compositions are derived from high-resolution spectra for three stars classed as SRd variables in the General Catalogue of Variable Stars. These stars are shown to be metal-poor supergiants: WY And with [Fe/H] = -1.0, VW Eri with [Fe/H] = -1.8, and UW Lib with [Fe/H] = -1.2. Their compositions are identical to within the measurement errors with the compositions of subdwarfs, subgiants, and less evolved giants of the same FeH. The stars are at the tip of the first giant branch or in the early stages of evolution along the asymptotic giant branch (AGB). There is no convincing evidence that these SRd variables are experiencing thermal pulsing and the third dredge-up on the AGB. The SRds appear to be the cool limit of the sequence of RV Tauri variables.Comment: 14 pages, 1 figure, 4 table

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the iron-peak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range $-4 \le$ [Fe/H] $\le -2.5$. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such a behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected $^{56}$Ni determined from modeling the early light curves of individual supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova which is larger for larger explosion energy and the larger Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical Journal, more discussion on the Galactic chemical evolutio

The dynamics of liquid slugs forced by a syringe pump

Microfluidic processes for chemical synthesis have become popular in recent years. The small scale of the chemical reactions promise greater control over reaction conditions and more timely creation of products. The small scale of microfluidics poses its own set of problems, however. At the microscale, the dominant fluid forces are viscous resistance and surface tension. The effects of viscosity and scale reduce the Reynolds number and make mixing difficult. Much work has been done to control mixing at the microscale. This problem is concerned with a different microfluidic problem: delivering reactants to the site of reaction. A common setup is to attach syringes full of reactant to a reaction chamber by narrow hydrophobic tubing. Using a stepper motor, a controlled dose of liquid may be injected into the tube. The hydrophobosity causes the dose to curve outward on the sides, becoming a "slug" of reactant with air in front and behind. The syringe at the rear is then switched for one full of air, and air pressure is used to drive the slug to the reaction site. If too much pressure is applied, the slug will arrive with a significant back pressure that will be relieved through bubbling in the reaction site. This causes the formation of a foam and is highly undesirable. We present a simple model based on Boyle’s law for the motion of a slug through a tube. We then extend this model for trains of slugs separated by air bubbles. Last, we consider the case of a flooded reaction site, where the forward air bubble must be pushed through the flooding liquid. In conclusion, we have determined the dynamics of a single slug moving towards an empty reaction chamber giving the final equilibrium position of the slug. A phase-plane analysis then determined a condition on the size of the slug needed to ensure that it comes to rest without oscillating about the equilibrium position. The effect of a flooded reaction chamber was then considered. In this case it is impossible to avoid bubbling due to the design of the device. We found that it is possible, however, to reduce the bubbling by minimising the back pressure behind the slug. Finally, the dynamics of multiple slugs with or without a flooded reaction chamber has been investigated

Keck-Nirspec Infrared OH Lines: Oxygen Abundances in Metal-Poor Stars Down to [Fe/H] = -2.9

Infrared OH lines at 1.5 - 1.7 um in the H band were obtained with the NIRSPEC high-resolution spectrograph at the 10m Keck Telescope for a sample of seven metal-poor stars. Detailed analyses have been carried out, based on optical high-resolution data obtained with the FEROS spectrograph at ESO. Stellar parameters were derived by adopting infrared flux method effective temperatures, trigonometric and/or evolutionary gravities and metallicities from FeII lines. We obtain that the sample stars with metallicities [Fe/H] < -2.2 show a mean oxygen abundance [O/Fe] ~ 0.54, for a solar oxygen abundance of epsilon(O) = 8.87, or [O/Fe] ~ 0.64 if epsilon(O) = 8.77 is assumed.Comment: To be published in ApJ 575 (August 10

Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants

We present chemical abundances in K and M red-giant members of the Galactic bulge derived from high-resolution infrared spectra obtained with the Phoenix spectrograph on Gemini-South. The elements studied are carbon, nitrogen, oxygen, sodium, titanium, and iron. The evolution of C and N abundances in the studied red-giants show that their oxygen abundances represent the original values with which the stars were born. Oxygen is a superior element for probing the timescale of bulge chemical enrichment via [O/Fe] versus [Fe/H]. The [O/Fe]-[Fe/H] relation in the bulge does not follow the disk relation, with [O/Fe] values falling above those of the disk. Titanium also behaves similarly to oxygen with respect to iron. Based on these elevated values of [O/Fe] and [Ti/Fe] extending to large Fe abundances, it is suggested that the bulge underwent a more rapid chemical enrichment than the halo. In addition, there are declines in both [O/Fe] and [Ti/Fe] in those bulge targets with the largest Fe abundances, signifying another source affecting chemical evolution: perhaps Supernovae of Type Ia. Sodium abundances increase dramatically in the bulge with increasing metallicity, possibly reflecting the metallicity dependant yields from supernovae of Type II, although Na contamination from H-burning in intermediate mass stars cannot be ruled out.Comment: ApJ in pres

An abundance analysis for four Red Horizontal Branch Stars in the extremely metal rich globular cluster NGC 6528

We present the results of the first analysis of high dispersion spectra of four red HB stars in the metal rich globular cluster NGC 6528, located in Baade's Window. We find that the mean [Fe/H] for NGC 6528 is +0.07+-0.01 dex (error of the mean), with a star-to-star scatter of sigma = 0.02 dex (4 stars), although the total error is likely to be larger (~0.1 dex) due to systematic errors related to the effective temperature scale and to model atmospheres. This metallicity is somewhat larger than both the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and that found in our previous paper for NGC 6553. However, we find that the spectra of clump stars in NGC 6528 and NGC 6553 are very similar each other, the slightly different metal abundances found being possibly due to the different atmospheric parameters adopted in the two analyses. For NGC 6528 we find excesses for the alpha-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn is found to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O; however this is somewhat uncertain due to the dependence of the O abundance on the adopted atmospheric parameters and to coupling between C and O abundances in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~ +0.4) in all stars examined. Since the present analysis is based on higher quality material, we propose to revise our previous published metal abundance for NGC 6553 to [Fe/H]=$0.06+-0.15.Comment: accepted for publication in the September 2001 issue of The Astronomical Journal; 3 new figures and updated results and calibration

Tracing the Galactic thick disk to Solar metallicities

We show that the Galactic thick disk reaches at least solar metallicities, and that it experienced strong chemical enrichment during a period of ~3 Gyr, ending around 8-9 Gyr ago. This finding puts further constraints on the relation and interface between the thin and thick disks, and their formation processes. Our results are based on a detailed elemental abundance analysis of 261 kinematically selected F and G dwarf stars in the solar neighborhood: 194 likely members of the thick disk and 67 likely members of the thin disk, in the range -1.3<[Fe/H]<+0.4.Comment: Accepted for publication in ApJ Letter

New Lithium Measurements in Metal-Poor Stars

We provide *lambda*6708 Li 1 measurements in 37 metal-poor stars, most of which are poorly-studied or have no previous measurements, from high-resolution and high-S/N spectroscopy obtained with the McDonald Observatory 2.1m and 2.7m telescopes. The typical line strength and abundance uncertainties, confirmed by the thinness of the Spite plateau manifested by our data and by comparison with previous measurements, are <=4 mAng and <=0.07-0.10 dex respectively. Two rare moderately metal-poor solar-Teff dwarfs, HIP 36491 and 40613, with significantly depleted but still detectable Li are identified; future light element determinations in the more heavily depeleted HIP 40613 may provide constraints on the Li depletion mechanism acting in this star. We note two moderately metal-poor and slightly evolved stars, HIP 105888 and G265-39, that appear to be analogs of the low-Li moderately metal-poor subgiant HD 201889. Preliminary abundance analysis of G 265-39 finds no abnormalities that suggest the low Li content is associated with AGB mass-transfer or deep mixing and p-capture. We also detect line doubling in HIP 4754, heretofore classified as SB1.Comment: Accepted for publication in PASP, volume 912 (Feb 2012) 15 pages, 3 figures, 2 table

Lambda-Cold Dark Matter, Stellar Feedback, and the Galactic Halo Abundance Pattern

(Abridged) The hierarchical formation scenario for the stellar halo requires the accretion and disruption of dwarf galaxies, yet low-metallicity halo stars are enriched in alpha-elements compared to similar, low-metallicity stars in dwarf spheroidal (dSph) galaxies. We address this primary challenge for the hierarchical formation scenario for the stellar halo by combining chemical evolution modelling with cosmologically-motivated mass accretion histories for the Milky Way dark halo and its satellites. We demonstrate that stellar halo and dwarf galaxy abundance patterns can be explained naturally within the LCDM framework. Our solution relies fundamentally on the LCDM model prediction that the majority of the stars in the stellar halo were formed within a few relatively massive, ~5 x 10^10 Msun, dwarf irregular (dIrr)-size dark matter halos, which were accreted and destroyed ~10 Gyr in the past. These systems necessarily have short-lived, rapid star formation histories, are enriched primarily by Type II supernovae, and host stars with enhanced [a/Fe] abundances. In contrast, dwarf spheroidal galaxies exist within low-mass dark matter hosts of ~10^9 Msun, where supernovae winds are important in setting the intermediate [a/Fe] ratios observed. Our model includes enrichment from Type Ia and Type II supernovae as well as stellar winds, and includes a physically-motivated supernovae feedback prescription calibrated to reproduce the local dwarf galaxy stellar mass - metallicity relation. We use representative examples of the type of dark matter halos we expect to host a destroyed ``stellar halo progenitor'' dwarf, a surviving dIrr, and a surviving dSph galaxy, and show that their derived abundance patterns, stellar masses, and gas masses are consistent with those observed for each type of system.Comment: 10 pages, 3 figures, version accepted by Ap