The separation distribution and merger rate of double white dwarfs: improved constraints

• Published in: Monthly notices of the Royal Astronomical Society Vol. 476; no. 2; pp. 2584 - 2590
• Main Authors: Maoz, Dan, Hallakoun, Na'ama, Badenes, Carles
• Format: Journal Article
• Language: English
• Published: Oxford University Press 11.05.2018
• Subjects: binaries: close; white dwarfs; binaries: spectroscopic; supernovae: general
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sty339

Abstract 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 studies on radial velocity variation. One of the studies is 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 the other is based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which with its 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 (1σ, 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 ∝ aαda, is α = −1.30 ± 0.15 (1σ) +0.05 (systematic). The Galactic WD merger rate per WD is Rmerge = (9.7 ± 1.1) × 10−12 yr−1. 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 Way's 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.