A White Dwarf Companion To SDSS J1257.63+54
The evolution of most modest stars, either in close binaries or isolated, will end with a compact white dwarf (WD) comprised of electron degenerate matter (Chandrasekhar 1934). Close pairs of white dwarfs with combined masses greater than the Chandrasekhar limit of 1.4 have long been discussed as potential progenitors of Type Ia supernovae (see Iben & Tutukov 1984; Webbink 1984).
Disappointingly, despite searches, (Robinson & Shafter 1987; Marsh et al. 1995; Napiwotzki et al. 2003) no secure examples of double white dwarfs both massive enough with short enough periods to merge within a Hubble time have been found to date. Thus putting a potential stumbling block in the SN Type Ia WD-WD merger scenario.
The only positive thus far, is that a meagre 1 in every 1000 WDs are required to be in such systems to match Type Ia rates (Nelemans et al. 2001b). Since only of order 1000 white dwarfs have been searched for binarity to date, the deficit is not yet significant. This implies that it is worth a concerted effort of cataclysmic variable star experts searching for the elusive short period WD-WD system.
Recently, in an effort to identify potential WD-WD mergers below the Hubble time, Marsh et al. (2011) examined the spectra of WDs showing hydrogen absorption only from the Sloan Digital Sky Survey (SDSS) (Eisenstein et al. 2006), with the express intent of looking for objects of discrepant radial velocity.
Marsh et al. (2011) outlined one such candidate: SDSS J1257+5428. On the surface, it was an obvious outlier with a mean radial velocity of −300kms-1 (essentially 300kms-1 away from us here on Earth). This stellar object was the subject of a similar study by Badenes et al. (2009). They found that it was a binary, measured a radial velocity semi-amplitude of 323±6kms-1 on a period of 4.56 hours and fitted their spectra with a white dwarf of temperature, ∼ 9000K, and high mass, ∼ 0.9.
Although these parameters give a minimum mass for the companion of 1.6, suggesting that it is either a neutron star or black-hole, Marsh et al. (2011) now believe these to be possible duds due to the nature of spectral fits conducted by Badenes et al. (2009). Badenes et al. (2009) estimated a distance of 48 parsecs for SDSS J1257+5428, implying that such systems may be rather common, however they failed to fit the narrow cores of the Balmer emission lines. Thus while they favored a neutron star or a black-hole for the companion, they could not entirely eliminate the possibility that it was another WD.
However, Kulkarni & van Kerkwijk (2010) took three high signal-to-noise spectra which showed asymmetries suggesting that their spectra could be fit by a combination of a cool, low mass WD (6250 ± 250K, 0.15 ± 0.05) plus a hotter, massive WD companion (13,000± 800K, 0.92 ± 0.13). Again, opening up the debate of this system as a WD-WD binary and thus a Type Ia supernova progenitor. This finding has been complemented by Marsh et al. (2011) who presented optical spectroscopy and ultraviolet fluxes, which directly reveal the presence of the second WD in the system.
This was supplemented by the agreement of Marsh et al. (2011) with Kulkarni & van Kerwijk (2010) that the SDSS J1257+5428 system could have a total mass greater than the Chandrasekhar limit required for a type Ia supernova explosion. However, Marsh et al. have gone on to state that it may not be as explosive as previously wanted; stating that due to the low mass ratio (where ) the system will is likely to evolve into an accreting double WD AM CVn (named after AM Canum Venaticorum) star. Not quite the explosive end that we wanted!
- Badenes, C. et al. (2009) First Results From The SWARMS Survey. SDSS 1257+5428: A Nearby, Massive White Dwarf Binary With A Likely Neutron Star Or Black Hole Companion. The Astrophysical Journal, 707 (2) pp.971-978.
- Kulkarni, S. R.; van Kerkwijk, M. H. (2010) The (Double) White Dwarf Binary SDSS 1257+5428. The Astrophysical Journal, 719 (2) pp. 1123-1131.
- Marsh, T.M. et al. (2011) Detection Of A White Dwarf Companion To The White Dwarf SDSSJ125733.63+542850.5. The Astrophysical Journal, 736 (2) Article I.D.: 95.
Suggest Further Reading:
- Woosley, S. E.; Weaver, T.A. (1994) Sub-Chandrasekhar Mass Models For Type Ia Supernovae. The Astrophysical Journal, 423 (1) pp. 371-379.