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Register a new userTransients from the Cataclysmic Deaths of Cataclysmic Variables
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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.
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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.
We report on XMM-Newton and optical results for 6 cataclysmic variables that were selected from Sloan Digital Sky Survey spectra because they showed strong HeII emission lines, indicative of being candidates for containing white dwarfs with strong magnetic fields. While high X-ray background rates prevented optimum results, we are able to confirm SDSSJ233325.92+152222.1 as an intermediate polar from its strong pulse signature at 21 min and its obscured hard X-ray spectrum. Ground-based circular polarization and photometric observations were also able to confirm SDSSJ142256.31-022108.1 as a polar with a period near 4 hr. Photometry of SDSSJ083751.00+383012.5 and SDSSJ093214.82+495054.7 solidifies the orbital period of the former as 3.18 hrs and confirms the latter as a high inclination system with deep eclipses.
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.
We present the results of MeerKAT radio observations of eleven nearby novalike cataclysmic variables. We have detected radio emission from IM Eri, RW Sex, V3885 Sgr and V603 Aql. While RW Sex, V3885 Sgr and V603 Aql had been previously detected, this is the first reported radio detection of IM Eri. Our observations have doubled the sample of non-magnetic CVs with sensitive radio data. We observe that at our radio detection limits, a specific optical luminosity $gtrsim 2.2times 10^{18}$ erg/s/Hz (corresponding to $M_V lesssim 6.0$) is required to produce a radio detection. We also observe that the X-ray and radio luminosities of our detected novalikes are on an extension of the $L_{X} propto L_{R}^{sim 0.7}$ power law originally proposed for non-pulsating neutron star low-mass X-ray binaries. We find no other correlations between the radio emission and emission in other wavebands or any other system parameters for the existing sample of radio-detected non-magnetic CVs. We measure in-band (0.9-1.7 GHz) radio spectral indices that are consistent with reports from earlier work. Finally, we constructed broad spectral energy distributions for our sample from published multi-wavelength data, and use them to place constraints on the mass transfer rates of these systems.
The space density of the various classes of cataclysmic variables (CVs) could only be weakly constrained in the past. Reasons were the small number of objects in complete X-ray flux-limited samples and the difficulty to derive precise distances to CVs. The former limitation still exists. Here the impact of Gaia parallaxes and implied distances on the space density of X-ray selected complete, flux-limited samples is studied. The samples are described in the literature, those of non-magnetic CVs are based on ROSAT (RBS - ROSAT Bright Survey & NEP -- North Ecliptic Pole), that of the Intermediate Polars stems from Swift/BAT. All CVs appear to be rarer than previously thought, although the new values are all within the errors of past studies. Upper limits at 90% confidence for the space densities of non-magnetic CVs are $rho_{rm RBS} < 1.1 times 10^{-6}$ pc$^{-3}$, and $rho_{rm RBS+NEP} < 5.1 times 10^{-6}$ p$^{-3}$, for an assumed scale height of $h=260$ pc and $rho_{rm IPs} < 1.3 times 10^{-7}$ p$^{-3}$ for the long-period Intermediate Polars at a scale height of 120 pc. Most of the distances to the IPs were under-estimated in the past. The upper limits to the space densities are only valid in the case where CVs do not have lower X-ray luminosities than the lowest-luminosity member of the sample. These results need consolidation by larger sample sizes, soon to be established through sensitive X-ray all-sky surveys to be performed with eROSITA on the Spektrum-X-Gamma mission.
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