The Evolution of Massive Close Binaries with Mass Loss and Overshooting. An Application to V729 Cyg (=BD+40 4220)

The evolution of massive close binaries is altered if the effects of mass loss through a stellar wind and overshooting from the convective core are taken into account. The occurrence of mass transfer as well as the extent of the mass transfer itself differs from the classical case (Doom and De Greve 1983, Doom 1984). The main-sequence widening due to the enlargement of the convective core results in an enhancement of the frequency of case A of mass transfer. It occurs for initial periods much longer than in the classical case. A cosiderable number of early-type systems will therefore undergo this type of mass transfer (Sybesma, 1985b). Systems with primary masses larger than 35 M will most likely not undergo mass transfer at all as these stars do not form red supergiants if overshooting is coupled to the effects of mass loss through stellar wind. These systems will only be able to undergo case A of mass transfer, and then only for initial periods below 2–3 days.

A Review of Early-Type Close Binary Systems

This is a review of close binary systems with very early spectral types (B, O, Of, and WR stars). We limit our selection to systems with Roche model photometric solution only. There are 10 contact systems known at present. Three of them (V701 Sco, BH Cen, and RZ Pyx) are located in the vicinity of the zero-age contact (ZC) according to a Spectral Type - Period diagram. Only the first two systems have absolute dimensions available. Both of these fall into the ZC zone in the logP - logm diagram and the logm - logR diagram. The system TU Mus was thought to be a ZC system is located in the evolved contact zone (EC) in the above diagrams. Both V701 Sco and BH Cen were thought to be having mass ratios about unity. With the much improved light curves of BH Cen and new analysis the mass ratio has been revised to 0.84! This result may favor Shu's model for early-type zero-age contact systems. The evolved systems might be undergone Case A mass exchange except for two systems (V729 Cyg and RY Set) which might be from Case B. The systems V367 Cyg was classified by Plavec as a W Ser system may be a evolved contact system.It is interesting to note that V729 Cyg (07f + Ofla), UW CMa(07f + O) and CQ Cep(WN7 + O) are all highly evolved contact systems. The Max II of UW CMa and CQ Cep are displaced to 0.78 and 0.80 phase, respectively. The displacement for V729 Cyg is not known due to poor coverage at this phase. The most distinct feature going from Of to Wolf-Rayet stars is the increasing domination of emission lines. It is suspected that this phaseshifts reflect the increasing activity of mass-flow in the common envelope.There are 8 semidetached systems with reliable absolute dimensions. It is believed that 6 of them are derived from Case A while the remaining 2 are from Case B mass exchange.

Current Observational Development in Early-Type Contact Binary Systems

This is a summary of current observational results of early-type contact binary systems. The classical approaches to derive a photometric solution of a very close binary are found to be incapable of distinguishing whether a system is detached, semidetached, or in contact. A survey of contact systems based on the more modern approach of Wilson and Devinney (1971) and others are summarized in the following. Contact systems with spectral type 0 are: V382 Cyg (Devinney 1976 private communication, Bloomer et al. 1979), V729 Cyg (Leung and Schneider 1978c), UW CMa (Leung and Schneider 1978b), AO Cas (Schneider and Leung 1978); with spectral type B are: SV Cen (Wilson and Starr 1976, Rucinski 1976), V701 Sco (Wilson and Leung 1977, Anderson et al. 1979), BH Cen (Leung and Schneider 1977), RZ Pyx (Devinney 1976 private communication); with spectral type A are: AU Pup (Leung and Schneider 1978a), V1073 Cyg (Leung and Schneider 1978a), V535 Ara (Leung and Schneider 1978a), BV 845 (Leung and Darland 1978), V1010 Oph (Leung and Wilson 1977).

Contact binaries of spectral type O

The eclipsing binaries UW CMa, AO Cas, and V729 Cyg have been systems of great interest for over fifty years. The light curves are complex and suffer significant changes on a time scale of months, but the primary attraction of these systems is that both components have O-type spectra; thus they present us with some of the few possibilities for direct measurement of absolute dimensions of very massive stars. Much effort has been expended on these systems, but no really consistent model has emerged.