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Insights on the Evolution of B Stars in Close Binary Systems Revealed by the r-q Diagram

Presentation #205.08 in the session Binary Stellar System - iPoster Session.

Published onJun 29, 2022
Insights on the Evolution of B Stars in Close Binary Systems Revealed by the r-q Diagram

The r-q diagram for interacting binaries is a plot of the fractional radius of the mass gainer (Rp/a) versus the mass ratio (Mdonor/Mgainer). Interpreted through the hydrodynamical computations of gas stream motion by Lubow & Shu (1975, ApJ,198, 383) the r-q diagram can reveal whether the gas stream in an individual system has a direct or tangential impact, or misses the mass gainer completely to build an accretion disk. The current consensus is that at least half of the OB stars are formed in binary or multiple star systems. The evolution of OB stars is greatly influenced by whether the stars begin as close binaries and the evolution of the binary systems depends on whether the mass transfer is conservative or nonconservative. Since the orbital separation of the stars and the system’s mass ratio evolve with time, it is possible to identify a post mass reversal epoch during which most of the systemic mass and angular momentum loss occurs. This epoch is when the impact angle of the gas stream is nearly tangential (Region 2 in the r-q diagram). FUV observations, mostly with IUE, have confirmed the latter. In this poster we show the results of calculations of trajectories through the r-q diagram for fully conservative mass transfer, conservation of mass but not angular momentum, and conservation of angular momentum but not mass loss. We identify systems that currently fit into each of these categories and estimate their rate of systemic mass loss from IUE HIRES spectra. We revisit the decades old hypothesis that all Be stars were formed as close binaries in view of the increasing number of sdO companions that are being discovered (Wang, L., et al., 2021, AJ, 248) and consideration of angular momentum transfer. We also discuss how the proposed Polstar mission will reveal critical information on the systemic mass and angular momentum loss in OB binaries. The authors appreciate support from NASA grants 80NSSC18K0919 and HST-GO-15659.002.

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