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On the Orbital Period and Models of V Sge

Smak J.

Acta Astronomica

2022

Vol. 72, No. 1

21--29

The orbital period of V Sge is decreasing at a rate which increased from dP/dt=-(4.11±0.33)×10-10 in 1962 to -(5.44±0.61)× 10-10 in 2022. This implies that the mass transfer from the secondary component is accelerating. From the evidence based on the orbital period variations, combined with estimates of the mass loss from the system based on radio observations, it follows that (1) the mass transfer rate from the secondary component is larger than M2=-5×10-6 M☉/yr, possibly as large as M2=-2.5×10-5 M☉/yr, and (2) the mass loss rate from the primary component is M1=-4×10-7 M☉/yr or larger. Close similarity of V Sge to binary Wolf-Rayet stars supports the model with primary component being a hot, evolved star loosing its mass. Several arguments are presented which exclude the alternative model with primary component being a white dwarf with an accretion disk.

The Low-Mass Limit for Total Mass of W UMa-type Binaries

Stępień K.

Acta Astronomica

2006

Vol. 56, No. 4

347--364

The observations of W UMa type stars show a well-defined short-period limit of 0.22 d, which is equivalent to a lower mass limit of approximately 1 Msolar for the total binary mass. It is currently believed that cool contact binaries are formed from detached binaries losing angular momentum (AM) via a magnetized wind. Orbital evolution of detached binaries with various component masses was followed until the primary component reached the critical Roche surface and the Roche lobe overflow (RLOF) began. It was assumed that the minimum initial, i.e., ZAMS, orbital period of such binaries is equal to 2 d and that the components lose AM just as single stars. According to the mass-dependent formula for AM loss rate of single stars, derived in this paper, the AM loss time scale increases substantially with decreasing stellar mass. The formula was applied to binaries with the initial primary component masses between 1.0 Msolar and 0.6 Msolar and two values of mass ratio q=1 and 0.5. Detailed calculations show that the time needed to reach RLOF by a 1 Msolar primary is of the order of 7.5 Gyr, but it increases to more than 13 Gyr for a binary with an initial primary mass of 0.7 Msolar Binaries with less massive primaries have not yet had time to reach RLOF even within the age of the Universe. This sets a lower mass limit for the presently existing contact binaries at about 1.0 Msolar-1.2 Msolar, in a good agreement with observations.