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Register a new userThe separation distribution and merger rate of double white dwarfs: improved constraints
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We obtain new and precise information on the double white dwarf (DWD) population and on its gravitational-wave-driven merger rate, by combining the constraints on the DWD population from two previous radial-velocity-variation studies: One based on a sample of white dwarfs (WDs) from the Sloan Digital Sky Survey (SDSS, which with its low spectral resolution probes systems at separations a<0.05 au), and one based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which, with high spectral resolution, is sensitive to a<4 au). From a joint likelihood analysis, the DWD fraction among WDs is fbin=0.095+/-0.020 (1-sigma, random) +0.010 (systematic) in the separation range ~<4 au. The index of a power-law distribution of initial WD separations (at the start of solely gravitational-wave-driven binary evolution), N(a)da ~ a^alpha da, is alpha=-1.30+/-0.15 (1-sigma) +0.05 (systematic). The Galactic WD merger rate per WD is R_merge=(9.7+/-1.1)e-12 /yr. Integrated over the Galaxy lifetime, this implies that 8.5-11 per cent of all WDs ever formed have merged with another WD. If most DWD mergers end as more-massive WDs, then some 10 per cent of WDs are DWD-merger products, consistent with the observed fraction of WDs in a high-mass bump in the WD mass function. The DWD merger rate is 4.5-7 times the Milky Ways specific Type-Ia supernova (SN Ia) rate. If most SN Ia explosions stem from the mergers of some DWDs (say, those with massive-enough binary components) then ~15 per cent of all WD mergers must lead to a SN Ia.
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From a sample of spectra of 439 white dwarfs (WDs) from the ESO-VLT Supernova-Ia Progenitor surveY (SPY), we measure the maximal changes in radial-velocity (DRVmax) between epochs (generally two epochs, separated by up to 470d), and model the observed DRVmax statistics via Monte-Carlo simulations, to constrain the population characteristics of double WDs (DWDs). The DWD fraction among WDs is fbin=0.100+/-0.020 (1-sigma, random) +0.02 (systematic), in the separation range ~<4AU within which the data are sensitive to binarity. Assuming the distribution of binary separation, a, is a power-law, dN/da ~ a^alpha, at the end of the last common-envelope phase and the start of solely gravitational-wave-driven binary evolution, the constraint by the data is alpha=-1.3+/-0.2 (1-sigma) +/-0.2 (systematic). If these parameters extend to small separations, the implied Galactic WD merger rate per unit stellar mass is R_merge=(1-80)e-13 /yr/Msun (2-sigma), with a likelihood-weighted mean of R_merge=(7+/-2)e-13 /yr/Msun (1-sigma). The Milky Ways specific Type-Ia supernova (SN Ia) rate is likely R_Ia~1.1e-13 /yr/Msun and therefore, in terms of rates, a possibly small fraction of all merging DWDs (e.g. those with massive-enough primary WDs) could suffice to produce most or all SNe Ia.
Double white dwarf (double-WD) binaries may merge within a Hubble time and produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger products have higher velocity dispersion because they are older. With the power of Gaia data, we show strong evidence for double-WD merger products among high-mass WDs by analyzing the transverse-velocity distribution of more than a thousand high-mass WDs (0.8--1.3 $M_odot$). We estimate that the fraction of double-WD merger products in our sample is about 20 %. We also obtain a precise double-WD merger rate and its mass dependence. Our merger rate estimates are close to binary population synthesis results and support the idea that double-WD mergers may contribute to a significant fraction of type Ia supernovae.
We use multi-epoch spectroscopy of about 4000 white dwarfs in the Sloan Digital Sky Survey to constrain the properties of the Galactic population of binary white dwarf systems and calculate their merger rate. With a Monte Carlo code, we model the distribution of DRVmax, the maximum radial velocity shift between exposures of the same star, as a function of the binary fraction within 0.05 AU, fbin, and the power-law index in the separation distribution at the end of the common envelope phase, alpha. Although there is some degeneracy between fbin and alpha, the the fifteen high DRVmax systems that we find constrain the combination of these parameters, which determines a white dwarf merger rate per unit stellar mass of 1.4(+3.4,-1.0)e-13 /yr/Msun (1-sigma limits). This is remarkably similar to the measured rate of Type Ia supernovae per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0(+1.6,-0.6)e-14 /yr/Msun. We conclude that there are not enough close binary white dwarf systems to reproduce the observed Type Ia SN rate in the classic double degenerate super-Chandrasekhar scenario. On the other hand, if sub-Chandrasekhar mergers can lead to Type Ia SNe, as recently suggested by some studies, they could make a major contribution to the overall Type Ia SN rate. Although unlikely, we cannot rule out contamination of our sample by M-dwarf binaries or non-Gaussian errors. These issues will be clarified in the near future by completing the follow-up of all 15 high DRVmax systems.
We present Chandra and Swift X-ray observations of four extremely low-mass (ELM) white dwarfs with massive companions. We place stringent limits on X-ray emission from all four systems, indicating that neutron star companions are extremely unlikely and that the companions are almost certainly white dwarfs. Given the observed orbital periods and radial velocity amplitudes, the total masses of these binaries are greater than 1.02 to 1.39 Msun. The extreme mass ratios between the two components make it unlikely that these binary white dwarfs will merge and explode as Type Ia or underluminous supernovae. Instead, they will likely go through stable mass transfer through an accretion disk and turn into interacting AM CVn. Along with three previously known systems, we identify two of our targets, J0811 and J2132, as systems that will definitely undergo stable mass transfer. In addition, we use the binary white dwarf sample from the ELM Survey to constrain the inspiral rate of systems with extreme mass ratios. This rate, 0.00017/year, is consistent with the AM CVn space density estimated from the Sloan Digital Sky Survey. Hence, stable mass transfer double white dwarf progenitors can account for the entire AM CVn population in the Galaxy.
Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly-rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two white dwarfs complete a final merger on a dynamical timescale, and are tidally disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot corona and a thick, differentially-rotating disk. The disk is strongly susceptible to the magnetorotational instability (MRI), and we demonstrate that this leads to the rapid growth of an initially dynamically weak magnetic field in the disk, the spin-down of the white dwarf merger, and to the subsequent central ignition of the white dwarf merger. Additionally, these magnetized models exhibit new features not present in prior hydrodynamic studies of white dwarf mergers, including the development of MRI turbulence in the hot disk, magnetized outflows carrying a significant fraction of the disk mass, and the magnetization of the white dwarf merger to field strengths $sim 2 times 10^8$ G. We discuss the impact of our findings on the origins, circumstellar media, and observed properties of SNe Ia and magnetized white dwarfs.
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