Issue |
A&A
Volume 398, Number 2, February I 2003
|
|
---|---|---|
Page(s) | 455 - 466 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20021194 | |
Published online | 21 January 2003 |
More nitrogen rich B-type stars in the SMC cluster, NGC 330
1
Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain
2
The Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, N. Ireland e-mail: P.Dufton@Queens-Belfast.ac.uk
Corresponding author: D. J. Lennon, djl@ing.iac.es
Received:
12
June
2002
Accepted:
9
August
2002
High resolution spectra of seven early B-type giant/supergiant stars
in the SMC cluster NGC 330 are analysed to
obtain their chemical compositions relative to SMC field and
Galactic B-type stars. It is found that all seven stars
are nitrogen rich with an abundance approximately 1.3 dex
higher than an SMC main-sequence field B-type star, AV304.
They also display evidence for deficiencies in carbon,
but other metals have abundances typical of the SMC.
Given the number of B-type stars with low projected rotational
velocities in NGC 330 (all our targets have v sin km s-1),
we suggest that it is unlikely that the stars in our sample
are seen almost pole-on, but rather that they are intrinsically
slow rotators. Furthermore, none of our objects displays any evidence of
significant Balmer emission excluding the possibility that these
are Be stars observed pole-on. Comparing these results with
the predictions of stellar evolution models including the effects
of rotationally induced mixing, we conclude that
while the abundance patterns may indeed be
reproduced by these models, serious discrepancies
exist. Most importantly, models including the effects of initially
large rotational velocities do not reproduce the
observed range of effective temperatures of our sample, nor the
currently observed rotational velocities.
Binary models may be able to produce stars in the observed
temperature range but again may be incapable
of producing suitable analogues with low rotational velocities.
We also discuss the clear need for stellar evolution calculations
employing the correct chemical mix of carbon, nitrogen and oxygen for
the SMC.
Key words: stars: early-type / supergiants / rotation / evolution
© ESO, 2003
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