Transients from the Cataclysmic Deaths of Cataclysmic Variables

TL;DR

This paper investigates the observable outcomes of white dwarf and low-mass star mergers, revealing transient emissions, long-term stellar remnants, and potential identification of past Galactic events as CV mergers.

Contribution

It provides a detailed analysis of the merger process, outflow dynamics, transient emissions, and long-term evolution, offering new insights into the observational signatures of CV mergers.

Findings

Outflows unbind >90% of secondary within days

Transient optical emission lasts about a month with luminosity >10^38 erg/s

Long-term remnants evolve into giants with specific luminosity and temperature

Abstract

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.