Results - Two disks of young stars
The central parsec of the Galaxy contains a cluster of about a hundred massive young stars. The majority are O supergiants and Wolf-Rayet (WR) stars with an estimated age of about 6 million years. About 30% of the Galactic WR stars are found in the nuclear star cluster.
90 WR/O stars in the central 0.5 pc of our Galaxy: Blue circles indicate clockwise orbits and red circles indicate counter-clockwise orbits.
Several research groups inferred that half of the WR/O stars (located at projected distances to SgrA* between 0.8'' and 12'') are compatible with being members of a clockwise rotating system. The rotation axis of this system shows a strong transition from the inner to the outer regions as a function of the projected distance from SgrA*. The distribution of the counter-clockwise WR/O star is not isotropic at the 98% confidence level. It is compatible with a coherent structure, possibly a warped disk in a dissolving state. The existence of these young massive stars indicates that star formation must have recently taken place at or near the Galactic Center within the last few million years. This is surprising, the "paradox of youth", since regular star formation processes are likely to be suppressed by the tidal forces from the massive black hole. Many scenarios have been suggested for the origin of these stars. These include in situ star formation through gravitational fragmentation of gas in disk(s) formed from infalling molecular cloud(s); transport of stars from far out by an infalling young stellar cluster, or through disruption of binary stars on highly elliptical orbits by the massive black hole; and rejuvenation of old stars due to stellar collisions and tidal stripping. The two leading scenarios for the O and WR stars outside the central 0.8'' are the infalling cluster and the in situformation scenarios.
In the infalling cluster scenario, a cluster of young stars formed far from the GC (a few parsecs) and in-spirals to Sgr A*. During the in-spiral the stars are stripped from the young cluster, forming a disk-like structure. In a mass segregated cluster the most massive stars are positioned in the cluster core and are the last to be stripped at the innermost regions of the GC, close to the MBH. The final stripped stars are suggested to be the young O and WR stars observed in the GC. In the in-situ model, a clump or clumps of gas fall into the GC, where they form a disk-like structure. The gaseous disk then fragments to form new low mass stars in a stellar disk, that grow, through accretion, into the more massive stars currently observed in the GC.
Three-dimensional visualization of a warped and tilted clockwise disk together with a counter clockwise system at large inclination. The clockwise disk is shown assuming locally flat circular orbits (a = 1''–10''). The counter-clockwise system is shown as a flat, non-warped ring in the range a= 3.5''–7''.
The recent observation of a warp in the clockwise disk of stars and the steep stellar surface density distribution (Bartko2008) favor in situ star formation in gaseous accretion disks as the origin of the young massive stars. Although the infall of two clusters may possibly explain the two systems of WR/O stars, the stars would have remained in a disk for too short to develop the observed warp. The young stars observed in the inner R~1'' are less massive B-stars (so called the 'S-stars') and are likely to originate from a different scenario then the O and WR stars. We have shown that S2 is really a young massive (M>14Msun) star and not a stripped older (and less massive) star.