Thursday, July 13, 2006

Blasting out black holes from the galactic disk

A recent paper (1) shows that the black hole X-ray binary XTE J1118+480 might be an ejected black hole from the galactic disk. This system consists in a small star orbiting a black hole, it was discovered in the year 2000 and it was since then an interesting object because of its location in the metal poor galactic halo.

Using the 10 m Keck telescope the spectra of the small star has been observed and surprisingly enough it was very rich in metals (remember that in astronomy anything else from hidrogen and helium is considered as metal, this is because this elements are synthetized inside the cores of stars, elements heavier than iron are believed to form in supernova explosions) suggesting it might actually be a double star from the galactic disk (where the metal abundance is much higher than in the halo), when the massive star in the system went supernova it might caused an asymmetrical blast (by the so called "acoustic mechanism") that expelled the system from the galactuc disk (2).

After reading the paper it seems to me that the metallicity is way too high, it's even higher than the Sun, this means that

  • This is a relatively recent event so the star is from a later generation than the Sun, so it formed from a mollecular cloud with a higher metal abundance.
  • As I mentioned before some of this heavy elements are formed in supernova explosions, making the assymetric natal kick not so necesary, after all.

Because the estimated age of the small star is around 5 Gyr, so the first scenario is very likely and certainly (almost) discards the origin of this system inside a globular cluster.

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