Recently, Phd student ZHANG Xudong, Prof. QIAN Shengbang and Dr. LIAO Wenping, from Yunnan Observatories of Chinese Academy of Sciences, revealed the different evolutionary pathways for A- and W-subtype contact binaries. Monthly Notices of the Royal Astronomical Society published their paper online.

Secondary components of W UMa-type contact binaries have two special properties, excess in radius and luminosity. Based on these two special properties, the authors find the different evolutionary pathways between A- and W-subtype contact binaries. Overluminosity in A-subtype is because the secondary components are evolved from initial more massive stars, while in W-subtype it is due to energy transfer.

A list of 117 contact binaries (48 A-subtype samples and 69 W-subtype samples) are compiled in this paper. Their spectroscopic and photometric parameters are derived by different researchers. By comparing the radial density distribution between secondary components of these samples and main-sequence stars, the authors find A-subtype contact binaries have some evolutionary relationship with main-sequence stars whose masses are higher than 1.8 Msun. In other words, A-subtype contact binaries are evolved from stars which have initial masses higher than 1.8 Msun and experience mass ratio reverse. However, the different Mass-Luminosity relation between two subtypes indicates another possibility for the W-subtype secondaries.

For W-subtype contact binaries, luminosity increase of secondaries and luminosity decrease of primaries shows linear relation. The linear relation indicates that energy transfer leads to their excess in radius and luminosity. And this is also in agreement with thermal relaxation oscillation (TRO) theory. The W-subtype contact binaries may have experienced one or several TRO cycles.

This study sheds new light on the formation and evolution of A- and W-subtype contact binaries.

Contact:
QIAN Shengbang, YNAO, CAS
qsb@ynao.ac.cn