The discovery of a new massive O-type close binary: tau CMa (HD 57061), based on HIPPARCOS and Walraven photometry

Abstract

We present an interpretation of Hipparcos Hp and Walraven VBLUW photometry of the brightest star in the open cluster NGC 2362 (although not necessarily associated with this cluster), HD 57061 (30tau CMa, HR 2782, HIP 35415). In this multiple system the central component consist of a visual double star, two O-type stars separated by 0\farcs 151, which is also known to be a 154\fd 9 period single lined spectroscopic binary. It is now shown that this system also contains a massive close binary with a period of 1\fd 282122, most probably as the main component of the spectroscopic binary. This system therefore contains both the longest period spectroscopic binary and the shortest period eclipsing binary known for O-type stars. The shape of the light curve is characteristic for heavily distorted double star components. An estimate has been made for various physical parameters of the system. The system seems to be typical for some binary interaction results obtained in N-body simulations for open clusters. Improved ephemeris are provided for two similar stars that were observed around the same time as HD 57061: HD 57060 and HD 167971. Based on observations made with the ESA Hipparcos satellite, and on observations collected at the European Southern Observatory, La Silla, Chile