EDP Sciences
Free access
Volume 396, Number 3, December IV 2002
Page(s) 877 - 884
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021273

A&A 396, 877-884 (2002)
DOI: 10.1051/0004-6361:20021273

A 2.4-12  $\mu$m spectrophotometric study with ISO of Cygnus X-3 in quiescence

L. Koch-Miramond1, P. Ábrahám2, 3, Y. Fuchs1, 4, J.-M. Bonnet-Bidaud1 and A. Claret1

1  DAPNIA/Service d'Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
2  Konkoly Observatory, PO Box 67, 1525 Budapest, Hungary
3  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4  Université Paris VII, France

(Received 5 June 2002 / Accepted 28 June 2002)

We present mid-infrared spectrophotometric results obtained with the ISO on the peculiar X-ray binary Cygnus X-3 in quiescence, at orbital phases 0.83 to 1.04. The 2.4-12  $\mu$m continuum radiation observed with ISOPHOT-S can be explained by thermal free-free emission in an expanding wind with, above 6.5 $\mu$m, a possible additional black-body component with temperature $T \sim 250$ K and radius $R \sim 5000$ $R_{\odot}$ at 10 kpc, likely due to thermal emission by circumstellar dust. The observed brightness and continuum spectrum closely match that of the Wolf-Rayet star WR 147, a WN8+B0.5 binary system, when rescaled at the same 10 kpc distance as Cygnus X-3. A rough mass loss estimate assuming a WN wind gives ~ $1.2\,\times\, 10^{-4}$ ${M}_\odot$ yr -1. A line at ~4.3  $\mu$m with a more than 4.3 $\sigma$ detection level, and with a dereddened flux of 126 mJy, is interpreted as the expected He I 3p-3s line at 4.295 $\mu$m, a prominent line in the WR 147 spectrum. These results are consistent with a Wolf-Rayet-like companion to the compact object in Cyg X-3 of WN8 type, a later type than suggested by earlier works.

Key words: stars: binaries: close -- stars: individual: Cyg X-3 -- stars: Wolf-Rayet -- stars: mass-loss -- infrared: stars

Offprint request: L. Koch-Miramond, lkoch@discovery.saclay.cea.fr

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