Star formation in the vicinity of the supermassive black hole at the Galactic Centre
Dr. Mark Wardle, Macquarie University
The disruptive tidal field near supermassive black holes overcomes the self-gravity of objects that are less dense than the Roche density. This was once expected to suppress star formation within several parsecs of Sgr A*, the four million solar mass black hole at the centre of the Galaxy. It has since become apparent that things are not this simple: Sgr A* is surrounded by a sub-parsec-scale orbiting disk of massive stars, indicating a star formation event occurred a few million years ago. And on parsec scales, star formation seems to be happening now: there are proplyd candidates and protostellar outflow candidates, as well as methanol and water masers that in the galactic disk would be regarded as sure-fire signatures of star formation. In this talk, I shall consider how star formation can occur so close to Sgr A*.
The stellar disk may be created through the partial capture of a molecular cloud as it swept through the inner few parsecs of the galaxy and temporarily engulfed Sgr A*. This rather naturally creates a disk of gas with the steep surface density profile of the present stellar disk. The inner 0.04 pc is so optically thick that it cannot fragment, instead accreting onto Sgr A* in a few million years; meanwhile the outer disk fragments and creates the observed stellar disk. The isolated young stellar objects found at larger distances, on the other hand, can be explained by stabilisation of clouds or cloud cores by the high external pressure that permeates the inner Galaxy. A virial analysis shows that clouds are indeed tidally disrupted within 0.5 pc of Sgr A*, but outside this the external pressure allows self-gravitating clouds to survive, providing the raw material for ongoing star formation.
Our Current Understanding of Classical Be Stars
Dr. Thomas Rivinius, Chile, ESO Paranal
I will introduce Be stars as B-type stars with gaseous disks in Keplerian rotation. These disks form by mass ejection from the star itself and their evolution is then governed by viscosity. The observables and their formation in the disk will be discussed, as well as what we know about the central stars: they are the most rapidly rotating non-degenerate stars, they are non-radial pulsators, and they do not show magnetic fields. The pulsation is clearly (phenomenologically) linked to the mass ejection, but the physical mechanism responsible for the ejection and disk formation is not known. Finally, I will discuss several open questions of broader interest, including the (possibly absent) chemical mixing of very rapid rotators and the unexpectedly large viscosity of Be star disks.