Browsing by Subject "ii"
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Item The Bulge Radial Velocity Assay (BRAVA). II. Complete Sample And Data Release(2012-03) Kunder, Andrea; Koch, Andreas; Rich, R. Michael; de Propris, Roberto; Howard, Christian D.; Stubbs, Scott A.; Johnson, Christian I.; Shen, Juntai T.; Wang, Yougang G.; Robin, Annie C.; Kormendy, John; Soto, Mario; Frinchaboy, Peter; Reitzel, David B.; Zhao, HongSheng; Origlia, Livia; Kormendy, JohnWe present new radial velocity measurements from the Bulge Radial Velocity Assay, a large-scale spectroscopic survey of M-type giants in the Galactic bulge/bar region. The sample of similar to 4500 new radial velocities, mostly in the region -10 degrees < l < +10 degrees and b approximate to -6 degrees, more than doubles the existent published data set. Our new data extend our rotation curve and velocity dispersion profile to +20 degrees, which is similar to 2.8 kpc from the Galactic center. The new data confirm the cylindrical rotation observed at -6 degrees and -8 degrees and are an excellent fit to the Shen et al. N-body bar model. We measure the strength of the TiO epsilon molecular band as a first step toward a metallicity ranking of the stellar sample, from which we confirm the presence of a vertical abundance gradient. Our survey finds no strong evidence of previously unknown kinematic streams. We also publish our complete catalog of radial velocities, photometry, TiO band strengths, and spectra, which is available at the Infrared Science Archive as well as at UCLA.Item The Chemical Imprint Of Silicate Dust On The Most Metal-Poor Stars(2014-02) Alexander, P.; Frebel, Anna; Bromm, Volker; Bromm, VolkerWe investigate the impact of dust-induced gas fragmentation on the formation of the first low-mass, metal-poor stars (<1 M-circle dot) in the early universe. Previous work has shown the existence of a critical dust-to-gas ratio, below which dust thermal cooling cannot cause gas fragmentation. Assuming that the first dust is silicon-based, we compute critical dust-to-gas ratios and associated critical silicon abundances ([Si/H](crit)). At the density and temperature associated with protostellar disks, we find that a standard Milky Way grain size distribution gives [Si/H](crit) = -4.5 +/- 0.1, while smaller grain sizes created in a supernova reverse shock give [Si/H](crit) = -5.3 +/- 0.1. Other environments are not dense enough to be influenced by dust cooling. We test the silicate dust cooling theory by comparing to silicon abundances observed in the most iron-poor stars ([Fe/H] < -4.0). Several stars have silicon abundances low enough to rule out dust-induced gas fragmentation with a standard grain size distribution. Moreover, two of these stars have such low silicon abundances that even dust with a shocked grain size distribution cannot explain their formation. Adding small amounts of carbon dust does not significantly change these conclusions. Additionally, we find that these stars exhibit either high carbon with low silicon abundances or the reverse. A silicate dust scenario thus suggests that the earliest low-mass star formation in the most metal-poor regime may have proceeded through two distinct cooling pathways: fine-structure line cooling and dust cooling. This naturally explains both the carbon-rich and carbon-normal stars at extremely low [Fe/H].Item Detection of the Second r-Process Peak Element Tellurium in Metal-Poor Stars(2012-03) Roederer, Ian U.; Lawler, James E.; Cowan, John J.; Beers, Timothy C.; Frebel, Anna; Ivans,, Inese I.; Schatz, Hendrik; Sobeck, Jennifer S.; Sneden, Christopher; Sneden, ChristopherUsing near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD +17 3248, HD 108317, and HD 128279. Tellurium (Te, Z = 52) is found at the second r-process peak (A approximate to 130) associated with the N = 82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.Item An Upper Limit On The Sulphur Abundance In HE 1327-2326(2012-08) Bonifacio, P.; Caffau, E.; Venn, K. A.; Lambert, D. L.; Lambert, D. L.Context. Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] similar to -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims. We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods. We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S i lines of Multiplet 3 at 1045 nm. Results. We do not detect the S i line. A 3 sigma upper limit on the equivalent width (EW) of any line in our spectrum is EW < 0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H] < -2.6. Conclusions. This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e. g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive >fall-back> are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.