A New Dynamical Class of High Period Derivative Contact Binaries
Abstract
We present a new dynamical class of contact binary stars with relatively long orbital periods (>0.8 d) and very large, negative orbital period derivatives (dP/dt < -1.6x10-8). We used the survey of Pietrukowicz et al. (2017), which found 108 systems with large |dP/dt| in a sample of 22,462 binaries. We recomputed dP/dt using the methods of Molnar et al. (2017). All were in or near contact. Our new class consists of 7 of the 8 most extreme values of dP/dt and the only 7 systems with P > 0.8 d (a period range containing <5% of contact binaries). The remaining systems have |dP/dt| < 1.4x10-8, with values distributed symmetrically about zero. Their period changes are likely caused by third bodies. We propose that period change in the new class may be caused by the Darwin secular tidal instability. Nuclear evolution of the primary star is thought to gradually drive contact binaries to longer orbital periods and more extreme mass ratios (e.g., Webbink 1976). When a critical mass ratio is reached (Rasio 1995), tidal instability will drive the systems rapidly towards shorter periods. Tylenda et al. (2011) suggested this as a possible mechanism for the period changes seen in V1309 Sco before its 2008 red nova outburst. However, |dP/dt| in that system was too great for this to apply (Pejcha 2014). Nonetheless a tidal instability stage may have driven V1309 Sco into a more rapid final stage of L2 mass loss (Pejcha et al. 2018). Using moments of inertia from MESA stellar models, we find critical mass ratios in the range 0.12-0.15. Fits to the light curve shapes of all 7 class members are consistent with this range. We have proposed spectroscopic observations that could be used for more stringent tests of our proposal. If successful, this result would constitute observational validation that the critical mass ratio is the relevant initial condition for the final stage of mass loss. This work was supported by NSF grant 1716622. Molnar et al. 2017, ApJ, 840, 1. Pietrukowicz et al. 2017, Acta Astronomica, 67, 115. Pejcha 2014, ApJ, 788, 22. Pejcha et al. 2018, ApJ, 850, 59. Rasio 1995, ApJL, 444, L41. Tylenda et al. 2011, A&A, 528, A114. Webbink 1976, ApJ, 209, 829.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23344805M