Tuesday, March 5, 2013


Like other galaxies, the Milky Way has at its centre a black hole, known as Sagittarius A* (SgrA*). This black hole has the mass of four million solar masses. A cloud of ionised gas and dust was spotted earlier this year falling towards SgrA*. At the time, it was suggested that this cloud formed when gas streaming from two nearby stars collided. However new research suggests this cloud might be the visible trail of a planet-forming disk which surrounds a young, low-mass star; this disc will be devoured by the black hole before it can evolve into a solar system.

Modelling work by Ruth Murray-Clay and Avi Loeb of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts suggests that planets can form within the powerful gravitational field of a giant black hole. They investigated whether the cloud’s mixture of gas and ionised dust might come from a planet-forming disk surrounding a single young star. Murray-Clay and Loeb’s model showed that gravitational interactions dislodged a young, low-mass star orbiting near the ring’s inner edge. This star, which is now heading for Sagittarius A* in an elliptical orbit, is too faint to be detected. The proto-planetary dust cloud is being disrupted and the researchers are able to detect this debris. There is a ring of young stars that orbits at about 0.03 parsecs (one-tenth of a light year) from Sagittarius A*. Young stars have been observed throughout the Galaxy to often have planet-forming disks.

The main theoretical problem with the model is that Murray-Clay and Loeb’s calculations show that there is only a 0.1% chance a recently dislodged star would have the same orbit as the gas cloud. The model predicts that the gas cloud should have a dense core, which would show by an increase in brightness when the cloud gets closer to the black hole. If the model is shown to be correct, then it would suggest that young low mass stars that remain in the ring have disks that are strong enough to create planets.

Even though material could start falling onto the accretion disk surrounding the black hole by the end of 2013, it will take 20-40 years for all of it to be swallowed. The type of activity shown by the material falling in – whether as bursts, steady brightening, or a jet of hot gas - will give researchers insights into why Sagittarius A* is so inactive compared with other supermassive black holes. Although the planetary cloud is headed for destruction, the star is likely to survive, as the tidal forces from the black hole are strong enough to strip gas away from the star but not to pull the star itself apart.

The image is an artist's impression of the protoplanetary disc being pulled into SgrA*.

http://www.scientificamerican.com/article.cfm?id=gas-cloud-hurtling-toward-milky-ways-black-hole-may-harbor-young-star; http://www.bbc.co.uk/news/science-environment-19558443

Image: ESO/MPE/M.Schartmann http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef017d3bfc17c2970c-pi


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