EDP Sciences
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Volume 421, Number 2, July II 2004
Page(s) 693 - 702
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20040255

A&A 421, 693-702 (2004)
DOI: 10.1051/0004-6361:20040255

Early-type stars in the young open cluster IC 1805

I. The SB2 system BD $\mathsf{+60^{\circ}\,497}$ and the probably single stars BD $\mathsf{+60^{\circ}\,501}$ and BD $\mathsf{+60^{\circ}\,513}$
G. Rauw and M. De Becker

Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, Bât. B5c, 4000 Liège, Belgium

(Received 12 February 2004 / Accepted 30 March 2004)

We investigate the multiplicity of three O-type stars in the very young open cluster IC 1805. All our targets were previously considered as spectroscopic binaries, but no orbital solution was available for any of them. Our results confirm the binarity of BD $+60^{\circ}\,497$ and we provide the very first orbital solution for this double-lined spectroscopic binary. This is only the second O-star binary in IC 1805, and the first SB2 system, for which an orbital solution is now available. BD $+60^{\circ}\,497$ has an orbital period of 3.96 days and consists of an evolved O6.5 V((f)) primary and an O8.5-9.5 V((f)) secondary with minimum masses of $m_1\,\sin^{3}{i} = 13.9~M_{\odot}$ and  $m_2\,\sin^{3}{i} = 10.9~M_{\odot}$. The observed primary/secondary mass ratio (1.28) appears lower than expected from a comparison with single star evolutionary models (1.60-1.74). For the other two stars, BD $+60^{\circ}\,501$ and BD $+60^{\circ}\,513$, we find no significant radial velocity variations, suggesting that they are most probably single. Although a fraction of binaries among the early-type stars of IC 1805 as high as 80% has been advocated in the literature, our results suggest that this number might be overestimated.

Key words: stars: binaries: spectroscopic -- stars: early-type -- stars: fundamental parameters -- stars: individual: BD $+60^{\circ}\,497$ -- stars: individual: BD $+60^{\circ}\,501$ -- stars: individual: BD $+60^{\circ}\,513$

Offprint request: G. Rauw, rauw@astro.ulg.ac.be

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