Astronomy and Astrophysics, volume 385, 143-151 (2002/4-1)
Pristine CNO abundances from Magellanic Cloud B stars. I. The LMC cluster NGC 2004 with UVES.
KORN A.J., KELLER S.C., KAUFER A., LANGER N., PRZYBILLA N., STAHL O. and WOLF B.
Abstract (from CDS):
We present chemical abundances for four main sequence B stars in the young cluster NGC 2004 in the Large Magellanic Cloud (LMC). Apart from HII regions, unevolved OB-type stars are currently the only accessible source of present-day CNO abundances for the MCs not altered by stellar evolution. Using UVES on the VLT, we obtained spectra of sufficient resolution (R=20000) and signal-to-noise (S/N≥100) to derive abundances for a variety of elements (He, C, N, O, Mg and Si) with NLTE line formation. This study doubles the number of main sequence B stars in the LMC with detailed chemical abundances. More importantly and in contrast to previous studies, we find no CNO abundance anomalies brought on by e.g. binary interaction or rotational mixing. Thus, this is the first time that abundances from HII regions in the LMC can sensibly be cross-checked against those from B stars by excluding evolutionary effects. We confirm the HII-region CNO abundances to within the errors, in particular the extraordinarily low nitrogen abundance of ε(N)≃7.0. Taken at face value, the nebular carbon abundance is 0.16dex below the B-star value which could be interpreted in terms of interstellar dust depletion. Oxygen abundances from the two sources agree to within 0.03dex. In comparison with the Galactic thin disk at MC metallicities, the Magellanic Clouds are clearly nitrogen-poor environments.
stars: abundances - stars: atmospheres - stars: early type - galaxies: Magellanic Clouds - galaxies: clusters: individual: NGC 2004
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