Stellar Models of Evolved Secondaries in Cataclysmic Variables

Abstract

In this paper we study the impact of chemically evolved secondaries on CV evolution. We find that when evolved secondaries are included a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. We argue that in order to explain this spread a considerable fraction of all CVs should have evolved stars as the secondaries. The evolved stars become fully convective at lower orbital periods. Therefore, even if there was an abrupt decrease in magnetic braking for fully convective stars (contrary to open cluster data) it would not be expected to produce a sharp break in the period distribution for CVs. We also explore recent proposed revisions to the angular momentum loss rate for single stars, and find that only modest increases over the saturated prescription are consistent with the overall observed spindown pattern. We compare predictions of our models with diagnostics of the mass accretion rate in WDs and find results intermediate between the saturated and the older braking prescription. Taken together these suggest that the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period, but is still significantly lower than in the traditional model. Alternative explanations for the CV period gap are discussed.

An3He-DRIVEN INSTABILITY NEAR THE FULLY CONVECTIVE BOUNDARY

“…These three effects conspire to make the radius glitches most visible in relatively unevolved, low mass models with low mass-loss rates. We note a small radius glitch occurs even in models with low mass-loss rates but insufficient 3 He to induce the instability, which occurs as 3 He is homogenized throughout the star as the model crosses the fully convective boundary; we believe this is the same feature noted in D' Antona & Mazzitelli (1982) and Andronov & Pinsonneault (2004).…”

“…We have negelected to include rotational mixing in our models, which could be non-negligible in the case of such close binary systems in which the secondary may be relatively rapidly rotating due to tidal locking, which could help to drive meridional flows (Eddington 1925;Sweet 1950;Zahn 1992) and result in mixing. Furthermore, mixing may also be driven by tidal distortions of the potential due to its close companion (Tassoul & Tassoul 1982;Andronov & Pinsonneault 2004). If the timescale for mixing is shorter than the timescale in between convective kissing episodes, the mixing of material could serve to eliminate episodic mixing events.…”

“…If the timescale for mixing is shorter than the timescale in between convective kissing episodes, the mixing of material could serve to eliminate episodic mixing events. With the inclusion of tidal deformation, the estimates of the minimum timescale for mixing are of order the timescale between convective episodes for systems at 3 hrs (see Andronov & Pinsonneault 2004, Fig. 9 for timescales).…”

ULTRACAM observations of SDSS J170213.26 + 322954.1 ��� an eclipsing cataclysmic variable in the period gap

“…Several independent theoretical studies predict the existence of a population of CVs with substantially evolved secondary stars (e.g Baraffe & Kolb 2000; Schenker et al 2002; Podsiadlowski, Han & Rappaport 2003; Andronov & Pinsonneault 2004). Under the disrupted magnetic braking model of CV evolution, the period gap is caused by a cessation of magnetic braking which occurs when the secondary star becomes fully convective.…”

Population Synthesis of Cataclysmic Variables. I. Inclusion of Detailed Nuclear Evolution

“…We also refer the reader to the work ofAndronov and Pinsonneault (2004) who examined how the evolution of chemically evolved CVs is affected by various descriptions of magnetic stellar wind braking.…”