* Astronomy

Title: DASCH J075731.1+201735: Discovery of A Peculiar Slow Nova in A Peculiar Symbiotic Binary
Authors: Sumin Tang, Jonathan Grindlay, Maxwell Moe, Jerome Orosz, Robert Kurucz, Samuel Quinn, Matthew Servillat

We present photometric and spectroscopic observations of a peculiar slow nova (designated DASCH J075731.1+201735 or J0757) discovered using digitised scans from our DASCH project of the Harvard College Observatory archival photographic plates. The source 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 ~1950s. Its current spectral type is M0III. 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, indicating that the red giant is at least close to filling its Roche lobe. 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. Unlike other symbiotic stars, there is no sign of emission lines or a stellar wind in the spectra. With an outburst timescale of ~10 yr and estimated B band peak luminosity M_B~1.4, J0757 is different from any other known classic novae or symbiotic novae which are powered by thermonuclear runaways, and is different from dwarf novae which are powered by gravitational energy release. The most probable explanation of the outburst is steady nuclear burning of infalling material on the surface of a white dwarf, triggered by increased accretion rate in the accretion disk due to disk instabilities. The absence of emission lines and an infrared excess indicates that the nuclear burning during the outburst is quasi-steady, with no significant mass loss. We suggest that J0757 could be a promising channel of type Ia Supernovae (SNe) progenitors.

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