DASCH Discovery of A Possible Nova-like Outburst in A Peculiar Symbiotic Binary

TL;DR

This paper reports the discovery of a peculiar binary star system that experienced a nova-like outburst, likely caused by hydrogen shell-burning on a white dwarf, with detailed photometric and spectroscopic analysis revealing its unique properties.

Contribution

First detailed analysis of a symbiotic binary with a nova-like outburst from archival photographic plates, suggesting a new class of outburst mechanisms.

Findings

Brightened by 1.5 magnitudes in 1942, then faded over years.

Binary system consists of an M0III star and a likely white dwarf.

Outburst likely caused by hydrogen shell-burning on the white dwarf.

Abstract

We present photometric and spectroscopic observations of a peculiar variable (designated DASCH J075731.1+201735 or J0757) discovered from our DASCH project using the digitized Harvard College Observatory archival photographic plates. It brightened by about 1.5 magnitudes in B within a year starting in 1942, and then slowly faded back to its pre-outburst brightness from 1943 to the 1950s. The mean brightness level was stable before and after the outburst, and ellipsoidal variations with a period of $P=119.18\pm0.07$ days are seen, suggesting that the star is tidally distorted. Radial-velocity measurements indicate that the orbit is nearly circular ($e=0.02\pm0.01$) with a spectroscopic period that is the same as the photometric period. The binary consists of a $1.1\pm0.3 M_\odot$ M0III star, and a $0.6\pm0.2 M_\odot$ companion, very likely a white dwarf (WD). Unlike other symbiotic binaries, there is no sign of emission lines or a stellar wind in the spectra. With an outburst timescale of ~10 years and estimated B band peak luminosity M_B~0.7, J0757 is different from any other known classic or symbiotic novae. The most probable explanation of the outburst is Hydrogen shell-burning on the WD, although an accretion-powered flare cannot be ruled out.