Presentation #308.01 in the session Binaries at Different Evolutionary Stages.
A contact binary system consists of two stars orbiting so closely they share an outer atmosphere. Molnar et al (2019, AAS Meeting #233, id.448.05) presented an outline of the evolutionary stages of the formation, life, and death of these systems, with a focus on stage 6 (rapid inspiral due to the Darwin tidal instability). They also presented very approximate computations of the main sequence and post-main sequence stages (denoted stages 4 and 5) using MESA computations of single star evolution for a sequence of masses to approximate the interior structure of the primary star along with the assumptions of conservation of total system mass and angular momentum. In the present work we compute fully the evolution from first contact until the onset of the Darwin instability. We use time variable mass transfer rates in MESA determined by the conservation requirements. Stellar rotation is also included assuming synchronous rotation at all times. Computations are done for a full grid of initial primary star masses and initial mass ratios. It is found that steady state evolution is possible at nearly all times. The timescale for evolution is set by the gradually increasing radius of the primary star due to nuclear evolution and mass transfer.
The results of these computations can be compared with the properties of ensembles of contact binary stars in a wide variety of ways. 1) Since the time dimension is fully computed, theoretical period distributions and H-R diagram distributions can be computed and compared with survey samples of known age distribution. 2) With correct internal structure of the primary, a more realistic minimum mass ratio as a function of system mass and orbital period is computed. 3) A new instability is found for initial mass ratios >0.7. Newly formed systems in the range quickly transfer large quantities of mass until they are in the stable range. Consequently, we predict the high mass ratio range will be rare among contact binaries although it is not rare for the detached systems that are their progenitors. 4) We also identify the possibility of merger at first contact for systems with sufficiently low initial mass ratio. This work was supported by NSF grant 1716622.