M. J. Sarna

The 12C/13C Ratio as a Tracer of the Evolution of Post Common Envelope Systems and Cataclysmic Variables

To provide a direct test of common envelope (CE) evolution which can be easily confirmed by observations, we (Sarna et al. 1995) recently modelled the change in the abundance ratio of 12C/13C on the surface of the lower mass star of a binary during the CE phase. The model is based on the fact that it is probable that the dwarf star accretes material during the CE phase. Since, during the CE phase, the dwarf secondary effectively exists within the atmosphere/envelope of the giant or supergiant primary, the accreted material has the abundances/composition of a giant/supergiant star. The 12C/13C ratio is known to decrease from approximately 90 in dwarf stars (in which the 13CO band at 2.3448 microns is barely visible) to approximately 10 in giants (in which the 13CO band at 2.3448 microns is fairly prominent). Hence, by measuring the 12C/13C ratio in post common envelope binaries (PCEBs) and comparing it to our models we would be able not only to confirm the CE theory but also to determine the amount of mass accreted during the CE phase and hence the initial mass of the dwarf component prior to the CE phase. We also propose an evolutionary scenario in which PCEBs with secondary component mass near 1.0 M⊙ start semi-detached evolution almost immediately after the CE phase. The progenitor system is a wide binary consisting of a 3M⊙ primary with a 1.0 M⊙ secondary star.

The Detached Binary RE1629+781: A Low-Mass Magnetic Secondary Star?

We present low resolution spectra (3400–8900 A) of the close detached white dwarfred dwarf binary RE1629+781, discovered by ROSAT. The spectra clearly show a composite nature and resemble those of pre-cataclys-mic binaries. They show H-Balmer absorption features from the white dwarf and TiO and NaI in absorption from the red dwarf secondary. Narrow Balmer and CaII in emission, most likely originating from the cool star, are also present. Using a pure hydrogen model, fits to the H β , H γ and H δ absorption yield a temperature of 41 800 ± 2000 K and surface gravity of log g = 8.0 ±0.5 for the white dwarf. Using the TiO bands to type the secondary star, we estimate the red dwarf to be a dM4. A main-sequence assumption for the secondary results in a mass range 0.16–0.24 M⊙ and a radius 0.2–0.3 R⊙. Earlier observations by Sion, Holberg & Barstow (1994, PASP, submitted) discuss the active nature of the low-mass star in this close binary is confirmed, then not only would it be the first detection of a magnetically active M dwarf in a close binary, but it also suggests the likelihood that magnetic braking could still be an efficient mechanism of angular momentum loss, which has far reaching evolutionary implications on the formation and evolution of cataclysmic binaries.

Chemical Evolution of Algol-Type Stars

The evolution of the chemical abundances at the surface of both components of Algol-type binaries is examined. First, we have examined some mixing processes which can affect element distribution in components of Algol-type stars. Second, the C abundance determinations from theoretical models are compared with values known from observational analysis. We conclude that there is no significant difference in chemical composition in components between case AB and early case B mass transfer. We have found a real difference between systems which evolve from initial mass ratios of 10/4 and 10/9.