Michael Lemke

The carbon-to-oxygen ratio in halo dwarfs

We have used four C I lines at 9100 A and the O I triplet at 7770 A to determine C and O abundances in a sample of 34 field halo dwarfs, which span the metallicity range −3.0 ≤ [Fe/H] ≤ −0.8. The C I line enable us to obtain, for the first time, C abundances for these metal-deficient stars that are based on atomic, rather than molecular, lines. Additional observations of Fe I and Fe II lines at 4500 and 5200 A have been used to help determine atmosphere parameters of the stars

First stellar abundances in NGC 6822 from VLT-UVES and Keck-Hires spectroscopy

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 Very Large Telescope and Keck Observatories, using the Ultraviolet-Visual Echelle Spectrograph and the High Resolution Echelle Spectrometer, 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 = -0.49 ± 0.22 (±0.21), with Cr yielding a similar underabundance, = -0.50 ± 0.20 (±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 ± 0.19 (±0.21), is in good agreement with the nebular oxygen results. Oxygen has the same underabundance as iron, = +0.02 ± 0.20 (±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 is consistent with a radial abundance gradient. More high-quality stellar and nebular observations are needed to confirm this intriguing possibility.

Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution*

Boron abundances have been derived for seven main-sequence B-type stars from Hubble Space Telescope STIS spectra around the B III j2066 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 identi—ed from the literature. In addition, several boron-depleted nitrogen-rich stars are identi—ed, 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 —nd [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

The boron abundance of procyon

The B I 2496.8 A resonance line and HST/GHRS echelle spectra are used with model atmospheres and synthetic spectra to derive the B abundance of the F dwarfs Procyon (alpha CMi), Theta UMa, and Iota Peg. The B abundance (log epsilonB~/=2.3) of Theta UMa and Iota Peg is similar to that derived by Boesgaard and Heacox from the B II resonance line in spectra of A and B-type stars. These two dwarfs show normal abundances of Li, Be, and B. Procyon, which is highly depleted in Li and Be, is depleted in B by a factor of at least 3. Comparison of the spectra of Procyon and the halo dwarf HD 140283 shows that the B abundance assigned by Duncan, Lambert, and Lemke (1992) to three halo dwarfs is not greatly overestimated as a result of contamination of the B I line by an unidentified line.

Boron in the Small Magellanic Cloud: A Novel Test of Light-Element Production*

Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations of the B III resonance line at λ2066 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) ≤ 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 galaxys star formation history, present oxygen abundance, and 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 ± 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 ± 0.3, but this is in good agreement with the supergiant iron abundances in NGC 330, another young SMC cluster. We propose that NGC 346-637 might be an unrecognized binary, however, which complicates its spectral analysis.