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Register a new userThe Secondary Stars of Cataclysmic Variables
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Christian Knigge
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I review what we know about the donor stars in cataclysmic variables (CVs), focusing particularly on the close link between these binary components and the overall secular evolution of CVs. I begin with a brief overview of the standard model of CV evolution and explain why the key observables this model is designed to explain - the period gap and the period minimum -- are intimately connected to the properties of the secondary stars in these systems. CV donors are expected to be slightly inflated relative to isolated, equal-mass main-sequence (MS) stars, and this donor bloating has now been confirmed observationally. The empirical donor mass-radius relationship also shows a discontinuity at M_2 = 0.2 M_sun which neatly separates long- and short-period CVs. This is strong confirmation of the basic disrupted magnetic braking scenario for CV evolution. The empirical M_2-R_2 relation can be combined with stellar models to construct a complete, semi-empirical donor sequence for CVs. This sequence provides all physical and photometric properties of normal CV secondaries along the standard CV evolution track. The observed donor properties can also be used to reconstruct the complete evolution track followed by CVs, i.e. the mass-transfer rate and angular-momentum-loss rate as a function of orbital period. Such a reconstruction suggests that angular momentum loss rates below the period gap are too high to be driven solely by gravitational radiation.
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The last few years have seen tremendous progress in our understanding of cataclysmic variable stars. As a result, we are finally developing a much clearer picture of their evolution as binary systems, the physics of the accretion processes powering them, and their relation to other compact accreting objects. In this review, I will highlight some of the most exciting recent breakthroughs. Several of these have opened up completely new avenues of research that will probably lead to additional major advances over the next decade.
Every massive globular cluster (GC) is expected to harbour a significant population of cataclysmic variables (CVs). In this review, I first explain why GC CVs matter astrophysically, how many and what types are theoretically predicted to exist and what observational tools we can use to discover, confirm and study them. I then take a look at how theoretical predictions and observed samples actually stack up to date. In the process, I also reconsider the evidence for two widely held ideas about CVs in GCs: (i) that there must be many fewer dwarf novae than expected; (ii) that the incidence of magnetic CVs is much higher in GCs than in the Galactic field.
We explore the observational appearance of the merger of a low-mass star with a white dwarf (WD) binary companion. We are motivated by Schreiber et al. (2016), who found that multiple tensions between the observed properties of cataclysmic variables (CVs) and standard evolution models are resolved if a large fraction of CV binaries merge as a result of unstable mass transfer. Tidal disruption of the secondary forms a geometrically thick disk around the WD, which subsequently accretes at highly super-Eddington rates. Analytic estimates and numerical hydrodynamical simulations reveal that outflows from the accretion flow unbind a large fraction >~ 90% of the secondary at velocities ~500-1000 km/s within days of the merger. Hydrogen recombination in the expanding ejecta powers optical transient emission lasting about a month with a luminosity > 1e38 erg/s, similar to slow classical novae and luminous red novae from ordinary stellar mergers. Over longer timescales the mass accreted by the WD undergoes hydrogen shell burning, inflating the remnant into a giant of luminosity ~300-5000 L_sun, effective temperature T_eff ~ 3000 K and lifetime ~1e4-1e5 yr. We predict that ~1e3-1e4 Milky Way giants are CV merger products, potentially distinguishable by atypical surface abundances. We explore whether any Galactic historical slow classical novae are masquerading CV mergers by identifying four such post-nova systems with potential giant counterparts for which a CV merger origin cannot be ruled out. We address whether the historical transient CK Vul and its gaseous/dusty nebula resulted from a CV merger.
I review our current understanding of the evolution of cataclysmic variables (CVs). I first provide a brief introductory CV primer, in which I describe the physical structure of CVs, as well as their astrophysical significance. The main part of the review is divided into three parts. The first part outlines the theoretical principles of CV evolution, focusing specifically on the standard disrupted magnetic braking model. The second part describes how some of the most fundamental predictions this model are at last being test observationally. Finally, the third part describes recent efforts to actually reconstruct the evolution path of CVs empirically. Some of these efforts suggest that angular momentum loss below the period gap must be enhanced relative to the purely gravitational-radiation-driven losses assumed in the standard model.
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