Publications (9)9.49 Total impact
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Article: Boron in the Small Magellanic Cloud: A Novel Test of Light Element Production
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ABSTRACT: Hubble Space Telescope STIS observations of the B III resonance line at 2066A have been obtained and analyzed for two Small Magellanic Cloud (SMC) B-type stars. While boron is not detected in either star, upper limits to the boron abundance are set, with 12+log(B/H) le 1.6 for both AV 304 and NGC 346-637. The upper limits are consistent with the relationship between boron and oxygen previously reported for Galactic disk stars. The SMC upper limits are discussed in light of that galaxy's star formation history, present oxygen abundance, and its present cosmic ray flux. The UV spectrum has also been used to determine the iron-group abundances in the SMC stars. For AV 304, [Fe/H]=-0.6 pm 0.2, from both an absolute and a differential analysis (with respect to the Galactic B-type star HD 36591). This is consistent with results from A-F supergiants in the SMC. A lower iron abundance is found for NGC 346-637, [Fe/H]=-1.0 pm 0.3, but this is in good agreement with the supergiant iron abundances in NGC 330,another young SMC cluster. We propose NGC 346-637 may be an unrecognized binary though, which complicates its spectral analysis. Comment: 13 pages, 7 figures. Accepted to ApJ04/2002; -
Article: Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
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ABSTRACT: Boron abundances have been derived for seven main sequence B-type stars from HST STIS spectra around the B III 2066 A line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but it is clearly depleted. In the other four stars, boron is undetectable implying depletions of 1 to 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. Also, 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 analysed the halo B-type star, PG0832+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-AGB star. Comment: 31 pages, 14 figures. accepted to ApJ08/2001; -
Article: First Stellar Abundances in NGC 6822 from VLT-UVES and Keck-HIRES Spectroscopy
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ABSTRACT: We have obtained the first high resolution spectra of individual stars in the dwarf irregular galaxy, NGC 6822. The spectra of the two A-type supergiants were obtained at the VLT and Keck Observatories, using UVES and HIRES, respectively. A detailed model atmospheres analysis has been used to determine their atmospheric parameters and elemental abundances. The mean iron abundance from these two stars is $<$[Fe/H]$>=-$0.49 $\pm$0.22 ({\it $\pm$0.21}, with Cr yielding a similar underabundance, $<$[Cr/H]$>=-$0.50 $\pm$0.20 ({\it $\pm$0.16}). This confirms that NGC 6822 has a metallicity that is slightly higher than that of the SMC, and is the first determination of the present-day iron-group abundances in NGC 6822. The mean stellar oxygen abundance, 12+log(O/H)=8.36 $\pm$0.19 ({\it $\pm$0.21}),is in good agreement with the nebular oxygen results. Oxygen has the same underabundance as iron, $<$[O/Fe]$>$=+0.02 $\pm$0.20 ({\it $\pm$0.21}). This O/Fe ratio is very similar to that seen in the Magellanic Clouds, which supports the picture that chemical evolution occurs more slowly in these lower mass galaxies, although the O/Fe ratio is also consistent with that observed in comparatively metal-poor stars in the Galactic disk. Combining all of the available abundance observations for NGC 6822 shows that there is no trend in abundance with galactocentric distance. However, a subset of the highest quality data are consistent with a radial abundance gradient. Comment: Accepted by ApJ (Sept 13, 2000) 13 pages, 7 tables, 8 figures09/2000; -
Article: Analysis of Four A-F Supergiants in M31 from Keck HIRES Spectroscopy
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ABSTRACT: The first stellar abundances in M31 are presented, based on Keck I HIRES spectroscopy and model atmospheres analyses of three A-F supergiants, 41-2368, 41-3712, and A-207. We also present the preliminary analysis of a fourth star, 41-3654. We find that the stellar oxygen abundances are in good agreement with those determined from nebular studies, even though the stars do {\it not} show a clear radial gradient in oxygen. The uncertainties in the stellar abundances are smaller than the range in the nebular results, making these stars ideal objects for further studies of the distribution of oxygen in M31. We show that the stars can be used to study the abundance distributions of other elements as well, including iron-group and heavier elements. The A-F supergiants also provide direct information on the metallicity and reddening of nearby Cepheid stars. We have examined the metallicity and reddening assumptions used for Cepheids within 1' of our targets and noted the differences from values used in the literature. Comment: Accepted for publication in the ApJ, October 2000 (23 pages, 4 tables, 11 figures)06/2000; -
Article: The interesting problem of the NLTE nitrogen abundance in Vega.
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ABSTRACT: We investigate the NLTE formation of nitrogen lines in Vega and present a new estimate for the nitrogen abundance based on detailed spectrum synthesis. The NLTE problem is very complex such that our final estimate of logɛ(N)=7.6 is very uncertain. The statistical equilibrium of nitrogen is almost completely controlled by the UV flux around 1200A, which is sensitive to both the carbon abundance and the shape of Lyman α. We found that Ly α must be included when computing bound-bound rates, a fact not realized before. The sensitivity of the NLTE effect to the global metallicity found by Takeda (1992a) is entirely due to the corresponding change of the carbon abundance. The inclusion of Ly α reduces this sensitivity considerably. Most accessible Ni lines are in the wings of broad Paschen lines which cause the NLTE abundance corrections to be different for lines of the same multiplet that occur at different distances from the Paschen line center. The nitrogen statistical equilibrium is also slightly sensitive to the collisional excitation cross-sections. For our best estimate, the NLTE corrections are considerable, typically {DELTA}[N/H]=-0.4. An LTE sulphur abundance of logɛ(S)=6.93 (or [S/H]=-0.3) has also been determined. Considering all of the available abundances for Vega, and their likely uncertainties, we find that Vega continues to look like a mild Lambda Bootis star.Astronomy and Astrophysics 04/1996; 309:558-569. · 4.59 Impact Factor -
Article: Boron Abundances in Early Type Stars
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ABSTRACT: Boron is a trace element in hot stars which can be observed from the B II resonance line at 1362 Angstroms. We have used IUE archival spectra to examine the B abundances in early type stars. The abundances of boron in A and B-type stars were originally calculated by Boesgaard & Heacox (1978 ApJ 226, 888) from Copernicus spectra as a method of testing the predictions of galactic cosmic ray spallation reactions. We are primarily interested in the effects of stellar evolution on B. Boesgaard & Heacox do not report any relationship between evolutionary state and the B abundance, however there are very few evolved stars in their sample and the few studied have very large uncertainties in B. Boron is destroyed by protons at temperatures above about 4 million K, thus the stellar surface B abundance is a sensitive monitor of evolutionary processes, such as mass loss and mixing, which would lead to large losses in the surface B. Gies & Lambert (1992 ApJ 387, 673) found evidence of CNO-cycled gas in some slightly evolved B-type stars, suggesting that some mixing may occur in massive stars near the main-sequence. We examine the B abundances in these stars for evidence of a boron-nitrogen relationship.04/1994; 26:910. -
Article: The Chemical Composition of Algol Systems - Part Five - Confirmation of Carbon Deficiencies in the Primaries of Eight Systems
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ABSTRACT: We use low-noise CCD spectra of 4267-Å C II line in the spectra of eight Algol-type eclipsing binaries to measure the C abundances of their primaries. A differential LTE analysis of the Algol binaries with respect to single standard stars of the same spectral type shows marginal to moderate deficiencies of C in the primaries of the binaries. We find [C/H] abundances of -0.62 for U CrB, -0.34 for u Her, ≤+0.24 for δ Lib, -0.22 for β Per, -0.18 for U Sge, -0.48 for λ Tau, -0.35 for TX UMa and -0.16 for RS Vul with respect to the average C abundance of single B-type standard stars. These results confirm those of Cugier and Hardorp, who have used IUE observations of the 1300-Å C II lines to determine C abundances in the same stars. The C deficiencies are a consequence of a previous episode of mass transfer in which the secondary, which was initially the more massive star, has transferred C-deficient, CNO-cycle processed material to the primary. A comparison of observed and theoretical abundances suggests that the mass transfer cannot have been extremely non-conservative, although it may have been mildly so.Monthly Notices of the Royal Astronomical Society 11/1993; 265:581. · 4.90 Impact Factor -
Article: Boron abundance of the Orion B-star HD 35299 : B III as a test of non-LTE
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Article: Boron abundances in main sequence B-type stars: A test of rotational depletion during main sequence evolution
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1994
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University of Texas at Austin
Port Aransas, TX, USA
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