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Contact Binary Star Formation

Presentation #400.01 in the session Stellar Dynamics 2: Binaries.

Published onJul 01, 2023
Contact Binary Star Formation

A contact binary system consists of two stars orbiting so closely they share an outer atmosphere. Approximately one in two hundred solar-type stars have a smaller companion in contact. Hence, they are common and long-lived. No complete models of their formation have been published, although outlines of possibly relevant mechanisms (e.g., Eggleton 2012, JASS, 29, 145) and more detailed study of individual steps (e.g., Jiang et al. 2012, MNRAS, 421, 2769) have been. Observational data are available on the detached progenitors of contact systems (stellar initial mass function, distributions of orbital period and mass ratio in young binary stars) as well as on the orbital period and mass ratio distribution of contact systems. These can provide guidance in the construction of formation models and statistical tests of model predictions.

The sequence of steps in a full model of contact binary formation requires consideration of a variety of dynamical and stellar structure processes. 1) The observed distribution of periods and mass ratios for ZAMS solar mass stars in close binaries is reasonably modelled by disk fragmentation and migration while protostars (Tokovinin & Moe 2020, MNRAS, 491, 5158). By contrast, the suggestion of Eggleton (2012) of a sequence of triple star formation via dynamic interactions of young binaries followed by Kozai cycles with tidal friction is not consistent with the inclination distribution of third bodies in close detached systems or contact binaries. 2) We computed magnetic braking times as a function of the two stellar masses to bring the close binaries into contact. This suffices for a narrow range of primary masses in the ~Gyr timescale observed for formation by Hwang & Zakamska (2020, MNRAS, 493, 2271). 3) Initial mass transfer will be overflow of the primary onto the secondary (which does not yet fill the Roche lobe). As found by Jiang et al. (2012), low mass ratio systems are dynamically unstable and merge immediately. 4) Once full contact is reached, mass is transferred back to the primary. This is thermally unstable for high mass ratio systems. These systems can transfer mass on a thermal timescale until they find a stable mass ratio (and do so without overflowing the L2 point). Together these four steps result in a narrow range of system masses and an intermediate range of mass ratios generally consistent with observations. Future work is to combine these initial conditions with the contact binary evolution modeling of Molnar et al. (2022, AAS Mtg #240, 308.01) to generate predictions of population distributions that can be tested in detail against observations.

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