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M33 X-7: Evolution Of Unusual Black Hole Binary System


In a distant galaxy hidden away in one of the furthest corners of our Universe, lives an extraordinarily massive black hole orbiting around its sister binary star in a highly unusual tight orbit: named M33 X-7. M33 X-7 has been observed as one of the most interesting astrophysical objects discovered, with a multitude of unusual properties: the tight orbit, the large masses of the star and black hole, the X-ray luminosity of the black hole and why its companion star is less luminous than one would expect, given its mass. This provides a gap, a gap for a model which explains all of these observations (Abubekerov et al. 2009).

M33 X-7 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning 15.65 black hole orbiting an underluminous 70 main sequence (MS) companion in a slightly eccentric 3.45 day orbit (Pietsch et al. 2006; Orosz et al. 2007).

M33 X-7 is thereofre an evolutionary challenge, given the massive components and its tight orbit relative to the large H-rich black hole (BH) companion. However, Valsecchi et al. (2010) have produced an extremely plausible account for the evolution of this system, which improves on previous understanding of how the most massive stars evolve and interact with their host environment.

The progenitor comprises a primary of 97 (BH progenitor) and a secondary of 32 (BH-companion progenitor) in an orbit of 2.9 days. During the first 1.8Myr the evolution is driven by mass loss via stellar winds, causing a decrease of the gravitational attraction between the components and expansion of the orbit to 3.25 days. As is expected, the more massive primary will evolve faster than its lower mass companion, with a subsequent mass transfer from the secondary to the primary.  The primary eventually evolves to become a Wolf-Rayet star. During the first 0.1Myr of mass transfer the 32 secondary swells into a 69 O-spectral type star, with the primary becoming a 51 Wolf-Rayet with a strong stellar wind. This wind will eventually blow away the outer envelope, exposing the 25 He core.

At the same time, the now more massive secondary is losing mass via its own O-star wind, albeit at a lower rate. At this time the orbit of the binary is circular and the spin period of each star is expected to be synchronized with the orbital period. The synchronization is due to exchange of angular momentum between the stars and their orbit caused by tidal interaction.

The final stages of the primary’s life during and beyond carbon burning are too short (60yr for an initially 25 He star) to significantly change the stellar and orbital parameters. At the end of the primary’s life, after 3.7Myr, M33 X-7 comprises a 16 evolved Wolf-Rayet star with an Fe-Ni core, and a 64.5 O-star companion in a 3.5 day orbit. Unable to support itself through further nuclear fusion, this massive He-rich star collapses into a BH.

This model represents a massive step closer to understand the progenitors and evolution of compact stellar binaries, whilst simultaneously providing mechanisms for various orbital, spectral and mass transfer properties.

Journal References:

  • Valsecchi, F. et al. (2009) The Eclipsing Black Hole X-ray Binary M33 X-7: Understanding The Current PropertiesThe Astrophysical Journal, 701 (1) pp.668-690.
  • Belczynski, K. et al. (2010) On The Maximum Mass Of Stellar Black Holes. The Astrophysical Journal714 (1) pp.1217-1226.
  • Valsecchi, F.; Kalogera,V. et al. (2010) Formation Of The Black-Hole Binary M33 X-7 Through Mass Exchange In A Tight Massive System. Nature, 467 pp.756-757.

Suggested Further Reading:

  • McClintock, J.E et al. (2008) Precise Measurement Of The Spin Parameter Of The Stellar-Mass Black Hole M33 X-7The Astrophysical Journal, 679 (1) pp.37-40.
  • Abubekerov, M.K et al. (2009) The Mass Of The Black Hole In The X-ray Binary M33 X-7 & The Evolutionary Status Of M33 X-7. Astronomy Reports, 53 pp.232-242.
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