One million years until graceland: finding galactic Ia progenitors

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Motivation

The in-spiral and coalescence of double white dwarf (WD) binaries due to gravitational wave emission has several possible outcomes, all of them of the highest astronomical interest. When the combined mass exceeds the Chandrasekhar limit (\(M_{\rm Ch}\)), the merger could lead to a Type Ia Supernova (SN) (Iben et al., 1984; Webbink, 1984), although accretion induced collapse to a neutron star is also a possibility (Saio et al., 1998; Schwab et al., 2016). Recent work shows that the delay time distribution of SN Ia behaves like 1/t for t \(\gtrsim\) 1 Gyr, exactly as predicted by a population of binaries that merge through gravitational wave emission (Maoz et al., 2014). Sub-Chandrasekhar WD mergers have been linked to a new class of fast, faint transients with low ejected mass and unusual nucleosynthetic signatures (Bildsten 2007, Shen 2010), like SN2002bj (Poznanski 2009) and SN2005E (Perets 2010) or even possibly normal SN Ia (Kerkwijk 2010, Sim 2010). Although several binary WD systems have been discovered in the last years (e.g., Gianninas et al., 2015), the vast majority of the known pre-mergers have low mass He WD primaries, and combined masses well below \(M_{\rm Ch}\). Additionally, current empirical constraints on the merger rate are highly uncertain due to the very small number of known short period systems (Brown 2016).

Basic Idea

Compact binary WDs have large orbital velocities, with \(v_{\rm orb}\simeq 500\,{\rm km\,s}^{-1}(P_{\rm orb}/15\,{\rm min})^{-1/3}\) for an equal-mass \(0.6\,M_{\odot}\) WD binary. Such binaries should exhibit large radial velocities (RVs) that can be measured even in low resolution wide-field spectroscopic surveys. WD binaries with orbital periods of minutes to hours can thus be found with follow-up observations of targets with the largest RV variations.