Dasch Discovery of a Possible Nova-like Outburst in a Peculiar Symbiotic Binary

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 mag in B within a year starting in 1942, and then slowly faded back to its pre-outburst brightness from 1943 to 1950s. The mean brightness level was stable before and after the outburst, and ellipsoidal variations with a period of P = 119.18 +/- 0.07 days are seen, suggesting that the star is tidally distorted. Radial-velocity measurements indicate that the orbit is nearly circular (e = 0.02 +/- 0.01) with a spectroscopic period that is the same as the photometric period. The binary consists of a 1.1 +/- 0.3 M-circle dot M0III star, and a 0.6 +/- 0.2 M-circle dot 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 similar to 10 years and estimated B-band peak luminosity M-B similar to 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.