Browsing by Subject "giants"

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Boron Abundances In B-Type Stars: A Test Of Rotational Depletion During Main-Sequence Evolution
(2002-01) Venn, Kim A.; Brooks, A. M.; Lambert, David L.; Lemke, M.; Langer, N.; Lennon, D. J.; Keenan, F. P.; Lambert, David L.
Boron abundances have been derived for seven main-sequence B-type stars from Hubble Space Telescope STIS spectra around the B III lambda2066 line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but is clearly depleted. In the other four stars, boron is undetectable, implying depletions of 1-2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. In addition, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near-solar iron group abundances, as expected for young stars in the solar neighborhood. We have also analyzed the halo B-type star PG 0832 + 676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H] < -2.5. These and other published abundances are used to infer the star's evolutionary status as a post-asymptotic giant branch star.
Chemical Abundances Of The Leo II Dwarf Galaxy
(2009-01) Shetrone, Matthew D.; Siegel, Michael H.; Cook, David O.; Bosler, Tammy; Shetrone, Matthew D.; Siegel, Michael H.
We use previously published moderate-resolution spectra in combination with stellar atmosphere models to derive the first measured chemical abundance ratios in the Leo II dwarf Spheroidal (dSph) galaxy. We find that for spectra with signal-to-noise ratio greater than 24, we are able to measure abundances from weak Ti, Fe, and Mg lines located near the calcium infrared triplet (CaT). We also quantify and discuss discrepancies between the metallicities measured from Fe I lines and those estimated from the CaT features. We find that while the most metal-poor ([Fe/H] < - 2.0]) Leo II stars have Ca and Ti abundance ratios similar to those of Galactic globular clusters, the more metal-rich stars show a gradual decline of Ti, Mg, and Ca abundance ratio with increasing metallicity. Finding these trends in this distant and apparently dynamically stable dSph galaxy supports the hypothesis that the slow chemical enrichment histories of the dSph galaxies is universal, independent of any interaction with the Milky Way. Combining our spectroscopic abundances with published broadband photometry and updated isochrones, we are able to approximate stellar ages for our bright red giant branch stars to a relative precision of 2-3 Gyr. While the derived age-metallicity relationship of Leo II hints at some amount of slow enrichment, the data are still statistically consistent with no enrichment over the history of Leo II.
Improved Ti II Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
(2013-10) Wood, M. P.; Lawler, James E.; Sneden, Christopher; Cowan, John J.; Sneden, Christopher
Atomic transition probability measurements for 364 lines of Ti II in the UV through near-IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer (FTS) and a new echelle spectrometer are combined with published radiative lifetimes to determine these transition probabilities. The new results are in generally good agreement with previously reported FTS measurements. Use of the new echelle spectrometer, independent radiometric calibration methods, and independent data analysis routines enables a reduction of systematic errors and overall improvement in transition probability accuracy over previous measurements. The new Ti II data are applied to high-resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ti abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. The Ti abundances derived using Ti II for these two stars match those derived using Ti I and support the relative Ti/Fe abundance ratio versus metallicity seen in previous studies.
Nucleosynthesis Predictions For Intermediate-Mass Asymptotic Giant Branch Stars: Comparison To Observations Of Type I Planetary Nebulae
(2009-01) Karakas, Amanda I.; van Raai, Mark A.; Lugaro, Maria; Sterling, N. C.; Dinerstein, Harriet L.; Dinerstein, Harriet L.
Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of similar to 3-8 M(circle dot). These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of the convective envelope, in addition to the slow neutron capture process operating in the He-shell (the s-process). We compare the predicted abundances of elements up to Sr from models of intermediate-mass asymptotic giant branch (AGB) stars to measured abundances in Type I PNe. In particular, we compare predictions and observations for the light trans-iron elements Se and Kr, in order to constrain convective mixing and the s-process in these stars. A partial mixing zone is included in selected models to explore the effect of a (13)C pocket on the s-process yields. The solar-metallicity models produce enrichments of [(Se, Kr)/Fe] less than or similar to 0.6, consistent with Galactic Type I PNe where the observed enhancements are typically less than or similar to 0.3 dex, while lower metallicity models predict larger enrichments of C, N, Se, and Kr. O destruction occurs in the most massive models but it is not efficient enough to account for the less than or similar to 0.3 dex O depletions observed in some Type I PNe. It is not possible to reach firm conclusions regarding the neutron source operating in massive AGB stars from Se and Kr abundances in Type I PNe; abundances for more s-process elements may help to distinguish between the two neutron sources. We predict that only the most massive (M greater than or similar to 5 M(circle dot)) models would evolve into Type I PNe, indicating that extra-mixing processes are active in lower-mass stars (3-4 M(circle dot)), if these stars are to evolve into Type I PNe.
Our Milky Way As A Pure-Disk Galaxy-A Challenge for Galaxy Formation
(2010-09) Shen, Juntai T.; Rich, R. Michael; Kormendy, John; Howard, Christian D.; De Propris, Roberto; Kunder, Andrea; Shen, Juntai T.; Kormendy, John
Bulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here, we model the stellar kinematics of the Bulge Radial Velocity Assay ( BRAVA) and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best-fit model has a half-length of similar to 4 kpc and extends 20 degrees from the Sun-Galactic center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than similar to 8% of the disk mass. Thus, the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy Formation is dominated by hierarchical clustering and galaxy mergers.
Planetary Transit Candidates In The Corot LRa01 Field
(2012-02) Carone, L.; Gandolfi, D.; Cabrera, J.; Hatzes, A. P.; Deeg, H. J.; Csizmadia, S.; Patzold, M.; Weingrill, J.; Aigrain, S.; Alonso, R.; Alapini, A.; Almenara, J. M.; Auvergne, M.; Baglin, A.; Barge, P.; Bonomo, A. S.; Borde, P.; Bouchy, F.; Bruntt, H.; Carpano, S.; Cochran, W. D.; Deleuil, M.; Diaz, R. F.; Dreizler, S.; Dvorak, R.; Eisloffel, J.; Eigmuller, P.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Fridlund, M.; Gazzano, J. C.; Gibson, N.; Gillon, M.; Gondoin, P.; Grziwa, S.; Gunther, E. W.; Guillot, T.; Hartmann, M.; Havel, M.; Hebrard, G.; Jorda, L.; Kabath, P.; Leger, A.; Llebaria, A.; Lammer, H.; Lovis, C.; MacQueen, P. J.; Mayor, M.; Mazeh, T.; Moutou, C.; Nortmann, L.; Ofir, A.; Ollivier, M.; Parviainen, H.; Pepe, F.; Pont, F.; Queloz, D.; Rabus, M.; Rauer, H.; Regulo, C.; Renner, S.; de la Reza, R.; Rouan, D.; Santerne, A.; Samuel, B.; Schneider, J.; Shporer, A.; Stecklum, B.; Tal-Or, L.; Tingley, B.; Udry, S.; Wuchterl, G.; Cochran, W. D.; Endl, M.; MacQueen, P.J.
Context. CoRoT is a pioneering space mission whose primary goals are stellar seismology and extrasolar planets search. Its surveys of large stellar fields generate numerous planetary candidates whose lightcurves have transit-like features. An extensive analytical and observational follow-up effort is undertaken to classify these candidates. Aims. We present the list of planetary transit candidates from the CoRoT LRa01 star field in the Monoceros constellation toward the Galactic anti-center direction. The CoRoT observations of LRa01 lasted from 24 October 2007 to 3 March 2008. Methods. We acquired and analyzed 7470 chromatic and 3938 monochromatic lightcurves. Instrumental noise and stellar variability were treated with several filtering tools by different teams from the CoRoT community. Different transit search algorithms were applied to the lightcurves. Results. Fifty-one stars were classified as planetary transit candidates in LRa01. Thirty-seven (i.e., 73% of all candidates) are >good> planetary candidates based on photometric analysis only. Thirty-two (i.e., 87% of the >good> candidates) have been followed-up. At the time of writing twenty-two cases were solved and five planets were discovered: three transiting hot-Jupiters (CoRoT-5b, CoRoT-12b, and CoRoT-21b), the first terrestrial transiting planet (CoRoT-7b), and another planet in the same system (CoRoT-7c, detected by radial velocity survey only). Evidence of another non-transiting planet in the CoRoT-7 system, namely CoRoT-7d, was recently found as well.