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The Possible Future Merger Of The WD-WD Binary SDSS J010657.30-100003.3

01/05/2012

Isolated bodies similar to our solitary, lonely Sun are something of a rarity in the Universe. Stars often form inside huge stellar nurseries (Hayashi 1961; Wolfire & Cassinelli 1987; Prialnik 2000) out of the primordial dense regions of hydrogen gas clouds e.g. N11 or the Orion Nebula, forming in groups of two, but sometimes three or more, locked in a mutual gravitationally bound system. These systems often exhibit more exotic and chaotic behaviour than solitary stars: and as such have light profiles which vary on short (e.g. [~8hrs: W Ursae Majoris]; [19.63hrs: SZ Herculis]) to long timescales (e.g. [2.867 days: β Persei/Algol]; [12.94 days: β Lyrae]) as determined by the interacting period of the system. However, there are some binary systems which undergo an altogether more complex and interesting evolution, dependent on a slight difference in their initial masses (Gänsicke et al. 2001).

Once a main sequence star approaches the end of its nuclear burning phase it will begin to cool and expand, forming a red giant and eventual expelling it’s outer envelope leaving behind a white dwarf. However, extremely low mass white dwarfs (ELM WDs) are not thought to be the result of isolated stellar evolution due to the very slow nuclear fusion rates in very low mass main sequence stars (Brown et al. 2011). Hence, the formation of ELMs needs some sort of new driving mechanism, a way to evolve low mass main sequence stars faster.

The Universe is not old enough to produce ELM WDs through single star evolution. Therefore, the system that is the subject of this post, SDSS J010657.30-100003.3, begins its life as two ordinary main sequence stars with similar or slightly less mass than that of our Sun (Kilic et al. 2010) that will undergo significant mass loss during their formation in binary systems. The majority of ELM WDs have been identified as companions to milli-second pulsars. However, not all ELM WDs have such companions (van Leeuwen et al. 2007; Agüeros et al. 2009a). Radial velocity, radio and X-ray observations of the lowest gravity WD found in the Sloan Digital Sky Survey (SDSS), (e.g. SDSS J0917+4638), show that the companion is almost certainly another WD (Kilic et al. 2007a,b; Agüeros et al. 2009b).

Mergers of binary white dwarfs (WDs) have been proposed to explain supernovae (SNe) Ia events, extreme helium stars including R Coronae Borealis (RCrB), single sub-dwarf B and sub-dwarf O stars (Iben & Tutukov 1984; Webbink 1984; Heber 2009). However, radial velocity surveys of WDs prior to the work conducted on  SDSS J010657.30-100003.3, have not revealed a large binary population that will merge within an approximate Hubble time,  (Marsh 1995; Maxted et al. 2000; Napiwotzki et al. 2001, 2002; Nelemans et al. 2005). Kilic et al. (2010) were particularly interested in the evolution of this system from the present day. They have theorised that this system will eventually merge in 37 Myr (37,000,000 years), a comparably short time scale compared to the Hubble time/age of the Universe. After this merger, instead of undergoing the catastrophic Type 1a explosion, they will form a new star: releasing huge amounts of gravitational wave energy, essentially sending out harmonic ripples in the fabric of space-time (Lorén–Aguilar et al. 2005).

The reason for this eventually rather than a Type Ia SNe is the extremely low masses of the two white dwarfs. Based on the work provided by Panei et al. (2007) for ELM WDs, the SDSS J010657.30-100003.3 system is likely to contain a 0.17 WD with a 0.37 WD companion at a separation of 0.32. Hence, when this system merges their combined mass will not make the Chandrasekhar limit (1.4) required to initiate a Type Ia supernova detonation. Instead, this new 0.54 WD creation will be gravitationally stable.

Hence, as we do not have a Type Ia SNe as the resulting fate of this dual WD system, an altogether different scenario may be possible. High-field magnetic white dwarfs, with magnetic fields in excess of 10 G and up to 10 G (Schmidt et al. 2003), have been long suspected to be the result of stellar mergers. García-Berro et al. (2012) have theorised that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores can produce magnetic fields of the required strength that do not decay for long timescales. Hence, the rebirth of this system may be as a highly-magnetic white dwarf system or a magnetar (Wickramasinghe & Ferrario 2000; King, Pringle & Wickramasinghe 2001).

Hence, this end fate for this dual WD system is believed to be the first of its kind; although others will more than likely be discovered following the techniques employed by Kilic et al. (2010). Yet this discovery will allow our ancestors to witness on of the most the amazing spectacles: the rebirth of a two white dwarfs as one, although estimated from current period derivative of SDSS J010657.30-100003.3 to be sometime 37 million years from now!

Journal Reference:

  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2011) The Shortest Period Detached Binary White Dwarf SystemMonthly Notices of the Royal Astronomical Society: Letters, 413 (1): pp. L54-L60.

Suggested Further Reading:

  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2011) The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars & Underluminous SupernovaeMonthly Notices of the Royal Astronomical Society: Letters, 411 (1): pp. L31-L35.
  • Kilic, M.; Brown, W.R.; Kenyon, S. J. et al.  (2010) The ELM Survey. I. A Complete Sample of Extremely Low-Mass White DwarfsThe Astrophysical Journal, 723 (2): pp. 1072-1081.
  • Kilic, M.; Brown, W.R.; Allende-Prieto, C.; Kenyon, S.J.; Panei, J.A. (2010) The Discovery of Merging Binary White Dwarfs Within 500 MyrThe Astrophysical Journal, 716 (1): pp. 122-130.
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