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.

The odds of detecting WD binaries at short orbital periods are good. If all type Ias are produced by C/O WD mergers, we expect a WD binary to merge within the Milky Way every \(\sim 10^{2}\,{\rm yr}\). The orbital decay time is \(200\,{\rm Myr}\) at an orbital period of 2 hours, therefore we estimate \(\sim 2\) million WDs with orbital periods under two hours within the galaxy. WDs at short orbital periods (under 15 minutes) may be tidally heated to surface temperatures \(T{\rm eff}\gtrsim 20,000\)K (Fuller et al., 2013), brightening them significantly and increasing the odds of detecting these systems in magnitude-limited surveys.

The SWARMS survey (Badenes et al., 2009; Mullally et al., 2009) was started in 2009 to identify pre-merging binary WDs in the large (\(\sim\)15,000 objects) database of WD spectra accumulated by the Sloan Digital Sky Survey. SWARMS uses the \(\sim\)15 minute sub-exposures to look for RV shifts (Figure 1). The typical time lag between sub-exposures (\(\sim\) half an hour to a few hours) is comparable to the orbital periods of pre-merging WDs, and the expected RV shifts (\(\gtrsim\) 100 km s\({}^{−1}\)) can be easily detected at the spectral resolution of SDSS, putting SWARMS in a good position to identify most of the pre-merging systems in this data base (see Badenes et al. 2009 for details). Unfortunately, followup of the SWARMS survey did not provide data of sufficient quality to measure RV curves, mainly due to bad weather and an aging spectrograph on the KPNO 4m. The time is ripe to revisit this program, especially given the large increase in spectroscopic white-dwarf samples.

Methods and Goals:

Our aim is to mine these samples to find and follow-up on candidate short-period WD binaries, ultimately deriving RV curves, component masses, periods, separations, and estimates of merger times. We will make use of multi spectroscopy available for many systems (better for systems with periods longer than typical exposure times, e.g., \(\sim\)15 minutes) to identify candidates with large RV variations. Additionally, single-epoch systems with very large RVs (\(\gtrsim 300\,{\rm km}\,{\rm s}^{-1}\)) are also indicative of binaries because it exceeds galactic orbital velocities. Very short period systems (\(P\lesssim 30\)) minutes will have significant orbital motion within the exposure time of the spectra which will broaden absorption lines. However, the RV shift can still be hundreds of \({\rm km}\,{\rm s}^{-1}\), which is still easily detectable.

The SWARMS sample was based on 19,000 spectroscopically confirmed WDs from SDSS DR7. New datasets have grown to include 14,000 new WDs with spectra from SDSS III BOSS observations in DR10 and DR12 (Kepler 2014, Kepler 2015) and several thousand new WDs from LAMOST (Zhao 2012, Cui 2012). SWARMS identified \(\sim 15\) good candidates, and we expect to more than double this sample with new data. Including systems with only single good epochs will increase the list further, and we expect to have dozens of candidates viable for follow-up observations.

Systems with the highest RV shifts, smallest uncertainties, and multiple epochs with RV shifts will represent our highest priority targets. Additionally, we plan to cross-reference our sample with those with GAIA distances and proper motions as they become available. Systems with large galactocentric velocities are good candidates for containing neutron star/black hole companions due to supernova kicks, and will be prioritized for follow-up.

After ranking the best candidates through these methods, we will utilize spectroscopic follow-up resources on large telescopes (Keck with PI Fuller’s preferred access, SALT with co-I Jha’s preferred access, and potentially Gemini through competitive US NOAO time). We will begin our reconnaissance with two short-exposure spectra to measure RV variations and confirm binarity, and then derive full radial-velocity curves for the confirmed systems.

Our work will contribute significantly to the known sample of massive pre-merging binary WDs, and will be a key ingredient in any updated calculation of the WD merger rate (Badenes 2012).

\label{fig:rvshift}Radial velocity shifts of absorption lines in spectra of SDSS J0926+5718, showing clear variations of \(\sim 500\,{\rm km}\,{\rm s}^{-1}\).

White dwarfs at short orbital periods may be tidally heated to surface temperatures \(T{\rm eff}\gtrsim 20,000\)K (Fuller et al., 2013), brightening them significantly and increasing the odds of detecting these systems in magnitude-limited surveys.

Phased radial velocity curve of the 12 minute binary WD SDSS J0651+2844 (Hermes et al., 2012).

Known compact WD binary systems from (Gianninas 2015).

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