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  • Space & astronomy

Published online: 26 September 2007 | Corrected online: 15 October 2007 | doi:10.1038/nchina.2007.198

Overcontact binaries: Two of a kind

Xiaoling Zhang

Introduction

So far, V382 Cyg and TU Mus are the only two known O-type overcontact binary stars — binary components that share a common atmosphere — with periods shorter than two days. Despite decades of observations, however, scientists still do not know how their orbital periods change with time. Over the past few years, an international team of researchers led by Shengbang Qian, Eduardo Fernandez Lajus and Jerzy Kreiner¹ have been observing both systems with telescopes at Yunnan Observatory in China and La Plata Observatory in Argentina. With the addition of new lightcurves, they deduced a satisfactory orbital period pattern, which provides details of the two systems.

The researchers found that the orbital periods of both systems spiral. The periods are 1.8855 days increasing by an average of 3.80 seconds every century and 1.3873 days increasing by an average of 3.46 seconds every century, for V382 Cyg and TU Mus respectively. The periods were also found to oscillate around the centroid; V382 Cyg oscillated by 0.0148 days every 47.7 years and TU Mus by 0.01 days every 47.73 years.

The researchers suggest that the period increases because of slow mass transfer from the secondary to the primary star; the periodic change is caused by a nearby invisible star, which may also be responsible for the formation of the overcontact binaries by reducing the angular momentum at an earlier evolutionary stage. The two binaries are known to each have a remote companion star. With the addition of this member, each binary forms a quadruple star system.

The authors of this work are from:
National Astronomical Observatories, Yunnan Observatory, Chinese Academy of Sciences, Kunming, China; United Laboratory of Optical Astronomy, Chinese Academy of Sciences (ULOAC), Beijing, China; Graduate University of the Chinese Academy of Sciences, Beijing, China; Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Buenos Aires, Argentina; Mount Suhora Observatory, Pedagogical University of Cracow, Krakow, Poland.

Reference

1. Qian, S. B.et al. Evolutionary states of the two shortest period O-type overcontact binaries V382 Cyg and TU Mus. Mon. Not. R. Astron. Soc. doi: 10.1111/j.1365-2966.2007.12184.x (2007). | Article |