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Black Holes provide a breeding ground for stars


The extreme environment surrounding the black hole at the centre of our galaxy is birthplace for new stars, according to a scientist from the University of Leicester.

Dr Sergei Nayakshin and his co-author, Rashid Sunyaev of the Max Plank Institute for Physics in Germany, used Chandra X-ray Observatory images to study the region around Sgr A*, the supermassive black hole at the centre of the Milky Way. Their results challenge the traditional theories of star formation, as they show that stars have formed close in to the black hole and contain a much smaller percentage of low mass stars than predicted.

This is the first solid observational evidence for star formation in an accretion disc around a black hole, and it implies that the inner parsecs of galaxies are even more exotic and interesting places than we thought so far. The results are to be published in the Monthly Notices of the Royal Astronomical Society.

"Massive black holes are usually known for violence and destruction, so it’s remarkable that this black hole helped create new stars, not just destroy them", said Dr Nayakshin.

Until now, scientists have proposed two models for how these stars might have formed: the “migration” model, in which a massive star cluster formed far out from the black hole and then its orbit decayed inwards due to dynamical friction with background stars, and the “in-situ” model, in which the gravitational forces in a dense disc of gas balances out the violent tidal forces around the black hole and allows stars to form.

To find out which model was correct, scientists needed to find out more about the types of stars orbiting the black hole. Gas and dust near the black hole block our view of small stars in infra-red and visible wavelengths. However, low-mass stars are strong X-ray emitters and would produce a glow around Sgr A* that would be detectable in the Chandra observations.

Nayakshin and Sunyaev found that if the migration theory were correct, then there should be as many as a million low-mass stars near Sgr A*, which strongly disagreed with the observed X-ray emission. In contrast, Chandra observations did not rule out the in-situ model, but required that massive stars were unusually abundant, dominating the mass budget of the star cluster.

These findings indicate that stars formed in the accretion disc play a key role in deciding how much gas the super-massive black hole gets for its dinner. Massive stars are also expected to “pollute” these discs with metals that stars produce in their cores, and leave behind stellar-mass black holes orbiting the super-massive one within the inner region of the galactic centre. Further research on these unusual high-mass stars may shed light on the still poorly understood mode for creation of such stars in galaxies in general.

The findings may also explain the mysterious ring of stars recently discovered in the nucleus of our neighbouring galaxy, Andromeda, by astronomers using the Hubble Space Telescope.

Anita Heward | alfa
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