Astron. Astrophys. 321, 254-267 (1997)

Abundances of the ³ He star 3 Cen A (B5 IIIp) from ultraviolet IUE spectra ^*

F. Castelli ¹, M. Parthasarathy ² and M. Hack <sup>3

1</sup> CNR-Gruppo Nazionale Astronomia and Osservatorio Astronomico, Via G.Tiepolo 11, I-34131 Trieste, Italy
² Indian Institute of Astrophysics, Bangalore 560034, India
³ Dipartimento di Astronomia, Università di Trieste, Via G.Tiepolo 11, I-34131 Trieste, Italy

Received 9 July 1996 / Accepted 18 October 1996

Abstract

High resolution (0.010-0.025 nm) IUE ultraviolet spectra of the peculiar ³ He star 3 Cen A were analyzed in the whole 120-320 nm region. Abundances were derived from the comparison of the observed and computed spectra. For computing spectra a Kurucz ATLAS9 model with parameters =17500 K, , and microturbulent velocity =0 km  s^-1   was adopted. A rotational velocity vsini =0 km  s^-1   was assumed. Colour indices, flux distribution, ultraviolet Si lines, and Balmer profiles were used to fix the model parameters. The observed spectra are reproduced at best with the following abundances: He [-0.55], B [-1.0], C [-0.75], N [-0.5], Mg [-0.5], Al [-1.5] underabundant, P [+1.6], Sc [+1.0], Mn [+1.4], Cu [+2.0] overabundant, Ga [+3.25], Kr [+3.3], Hg [+4.5] extremely overabundant.

The comparison of the ultraviolet spectrum and of the abundances of 3 Cen A with those of the normal B-type star  Her and of the Pop II peculiar B-type star Feige 86 has shown that the same elements contribute to the line spectrum of  Her and 3 Cen A even if with different abundances, so that the spectra can be compared. In the case of Feige 86, the overabundance of heavy elements which are not observed in 3 Cen A (i.e. Pt II, Pt III, Au II, Au III, probably Mo III), the lower overabundance of Ga [+1.0], and the larger underabundance of the light elements C [-2.5] and N [-2.0] make the ultraviolet line spectra of Feige 86 somewhat different from those of both  Her and 3 Cen A. Also the He I lines have different profiles in the two peculiar stars, although He is underabundant in both.

With the specific abundances of 3 Cen A we computed an opacity sampling ATLAS12 model having the same parameters as the ATLAS9 model. We found that the energy distribution from the ATLAS9 model and solar abundances is not very different from that derived from the ATLAS12 model, the synthetic spectrum approach, and the specific abundances of 3 Cen A.

Key words: stars: abundances stars: chemically peculiar stars: individuals (3 Cen A, Feige 86,  Her) ultraviolet: stars

* Based on observations by the International Ultraviolet Explorer (IUE)

Send offprint requests to: F. Castelli

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Contents

• 1. Introduction
• 2. The data
• 3. The atmospheric parameters of 3 Cen A
  • 3.1. and from Strömgren photometery
  • 3.2. from spectrophotometery
  • 3.3. from the ultraviolet Si lines
  • 3.4. from Balmer profiles
  • 3.5. The microturbulent velocity
  • 3.6. Adopted model parameters
• 4. The atmospheric parameters of the comparison stars  Her and Feige 86
  • 4.1.  Her
  • 4.2. Feige 86
• 5. The analysis
• 6. Abundances of 3 Cen A
  • 6.1. Helium
  • 6.2. Abundances from the ultraviolet region
• 7. The abundances of the comparison stars Feige 86 and  Her
  • 7.1. Feige 86
  • 7.2.  Her
• 8. Comparison of abundances
• 9. The strong ultraviolet lines
• 10. The opacity-sampled model
• 11. Conclusions
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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