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Astronomy & Astrophysics

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, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
2 Sternberg Astronomical Institute, Moscow State University, Universitetskii Pr., 13, 119899 Moscow, Russia
3 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium

* Research Fellow FNRS, Belgium
** Research Director FNRS, Belgium
*** Research Associate FNRS, Belgium

+ Based on observations collected at the European Southern Observatory (La Silla, Chile) and with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA).


     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-Newto X-ray observations. In the optical, the system presents two eclipses, yielding an inclination i ~ 77°. 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 (M1 ~ 18 Msol and M2 ~ 10 Msol) and respective radii (R1 ~ 7.5 Rsol and R2 ~ 5.4 Rsol) 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 - 410 7742 -- stars: binaries: close -- stars: binaries: eclipsing -- stars: early-type --
stars: fundamental parameters -- X-rays: stars

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