The massive binary CPD - 41° 7742 - II. Optical light curve and X-ray observations

Free access article

Issue		A&A Volume 441, Number 1, October I 2005


Page(s)		213 - 229
Section		Stellar structure and evolution
DOI		http://dx.doi.org/10.1051/0004-6361:20052746

A&A 441, 213-229 (2005)
DOI: 10.1051/0004-6361:20052746

The massive binary CPD - 41° 7742

II. Optical light curve and X-ray observations

H. Sana¹, E. Antokhina², P. Royer³, J. Manfroid¹, E. Gosset¹, G. Rauw¹ and J.-M. Vreux¹

¹ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5c, 4000 Liège, Belgium
    e-mail: [sana;manfroid;gosset;rauw;vreux]@astro.ulg.ac.be
² Sternberg Astronomical Institute, Moscow State University, Universitetskii Pr., 13, 119899 Moscow, Russia
    e-mail: elant@sai.msu.ru
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium
    e-mail: pierre@ster.kuleuven.ac.be

(Received 21 January 2005 / Accepted 11 May 2005)

Abstract
In the first paper of this series, we presented a detailed high-resolution spectroscopic study of CPD - 41° 7742 , deriving for the first time an orbital solution for both components of the system. In this second paper, we focus on the analysis of the optical light curve and on recent XMM-Newton X-ray observations. In the optical, the system presents two eclipses, yielding an inclination $i\sim77\degr$ . Combining the constraints from the photometry with the results of our previous work, we derive the absolute parameters of the system. We confirm that the two components of CPD - 41° 7742 are main sequence stars (O9 V + B1-1.5 V) with masses ( $M_1\sim18$ $M_{\odot}$ and $M_2\sim10$ $M_{\odot}$ ) and respective radii ( $R_1\sim7.5$ $R_{\odot}$ and $R_2\sim5.4$ $R_{\odot}$ ) close to the typical values expected for such stars.

We also report an unprecedented set of X-ray observations that almost uniformly cover the 2.44-day orbital cycle. The X-ray emission from CPD - 41° 7742 is well described by a two-temperature thermal plasma model with energies close to 0.6 and 1.0 keV, thus slightly harder than typical early-type emission. The X-ray light curve shows clear signs of variability. The emission level is higher when the primary is in front of the secondary. During the high emission state, the system shows a drop of its X-ray emission that almost exactly matches the optical eclipse. We interpret the main features of the X-ray light curve as the signature of a wind-photosphere interaction, in which the overwhelming primary O9 star wind crashes into the secondary surface. Alternatively the light curve could result from a wind-wind interaction zone located near the secondary star surface. As a support to our interpretation, we provide a phenomenological geometric model that qualitatively reproduces the observed modulations of the X-ray emission.

Key words: stars: individual: CPD - 41° 7742 -- stars: binaries: close -- stars: binaries: eclipsing -- stars: early-type -- stars: fundamental parameters -- X-rays: stars

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