Tidal tail ejection as a signature of Type Ia supernovae from white DWARF mergers

Abstract

The merger of two white dwarfs may be preceded by the ejection of some mass in "tidal tails," creating a circumstellar medium around the system. We consider the variety of observational signatures from this material, which depend on the lag time between the start of the merger and the ultimate explosion (assuming one occurs) of the system in a Type Ia supernova (SN Ia). If the time lag is fairly short, then the interaction of the supernova ejecta with the tails could lead to detectable shock emission at radio, optical, and/or X-ray wavelengths. At somewhat later times, the tails produce relatively broad NaID absorption lines with velocity widths of the order of the white dwarf escape speed (∼1000 km s-1). That none of these signatures have been detected in normal SNe Ia constrains the lag time to be either very short (≲ 100 s) or fairly long (≳ 100 yr). If the tails have expanded and cooled over timescales ∼104 yr, then they could be observable through narrow NaID and Ca II H&K absorption lines in the spectra, which are seen in some fraction of SNe Ia. Using a combination of three-dimensional and one-dimensional hydrodynamical codes, we model the mass loss from tidal interactions in binary systems, and the subsequent interactions with the interstellar medium, which produce a slow-moving, dense shell of gas. We synthesize NaID line profiles by ray casting through this shell, and show that in some circumstances tidal tails could be responsible for narrow absorptions similar to those observed. © 2013. The American Astronomical Society. All rights reserved..