WD 1242-105: a new merging white dwarf binary in the solar neighborhood

Six by six arcminute field around the white dwarf WD 1242-105, courtesy SDSS DR9. The WD is the bluish object at centre. This nearby (39 pc) white dwarf has been found to be a binary object which will merge within three quarters of a billion years, but not catastrophically, i.e. it is likely not a Type Ia supernova progenitor. The paper is J.H. Debes et al., 2015, to appear in the Astronomical Journal (preprint pdf). From the abstract:

Characterizing the local space density of double degenerate binary systems is a complementary approach to broad sky surveys of double degenerates to determine the expected rates of white dwarf binary mergers, in particular those that may evolve into other observable phenomena such as extreme helium stars, Am CVn systems, and supernovae Ia. However, there have been few such systems detected in local space. We report here the discovery that WD 1242−105, a nearby bright WD, is a double-line spectroscopic binary consisting of two degenerate DA white dwarfs of similar mass and temperature, despite it previously having been spectroscopically characterized as a single degenerate. Follow-up photometry, spectroscopy, and trigonometric parallax have been obtained in an effort to determine the fundamental parameters of each component of this system. The binary has a mass ratio of 0.7 and a trigonometric parallax of 25.5 mas, placing it at a distance of 39 pc. The system’s total mass is 0.95 M⊙ and has an orbital period of 2.85 hours, making it the strongest known gravitational wave source (logh=−20.78) in the mHz regime. Because of its orbital period and total mass, WD 1242−105 is predicted to merge via gravitational radiation on a timescale of 740 Myr, which will most likely not result in a catastrophic explosion.

It is interesting to note the existence of other similar objects relatively close to the Sun, and their eventual fates. The paper authors write (some references omitted):

There are other double degenerate systems that are likely to be closer than WD 1242−105, but lack measured parallaxes or will not merge within a Hubble time. Another merging WD system with a period roughly twice as long as WD 1242−105, NLTT 53177, may be closer by a few parsecs, given the inferred spectroscopic distance of its two components. WD 1242−105 is a near twin of the compact component to the WD 1704+481 system, which consists of three white dwarfs, two of which are in an orbit with a period of 0.145 d. The mass ratio of this pair is also 0.7, with a similar difference in their gravitational redshifts. The spectroscopic distance of the distant third component is 40 pc, which is similar to WD 1242−105’s parallax. Finally, there are other double degenerate systems within 25 pc of the Sun, but those have gravitational wave merger times longer than a Hubble time. We can also investigate the eventual fate of the system. [Figure 5 shows] WD 1242−105 compared to other DDs and relative to the stability criteria of Marsh et al. (2004), which dictates whether objects merge violently with the possibility of detonation or stably through Roche lobe overflow mass transfer. All of the massive merger systems in the ELM Survey are found due to the ≈ 0.2M⊙ ELM white dwarfs. Hence, these tend to have extreme mass ratios (q ≈ 0.2), which should lead to stable mass transfer AM CVn objects. On the other hand, WD 1242−105 has q = 0.7, which will lead to unstable mass transfer and a merger.



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