The young stars in the Galactic Center
The best-measured spectrum of the S-stars we have obtained from S2. A co-added H- and K-band spectrum obtained over several years is shown in the following figure:
The presence of the Br-gamma and He-I lines is indicative for B-type stars. Most of the S-stars show similar spectra, and hence they are mostly massive, main sequence stars.
This finding was a surprise, nick-named the paradox of youth (Eisenhauer et al. 2005, Martins et al. 2008): The life-time of these stars is only around 100 million years, much smaller than the estimated relaxation time in the Galactic Center, roughly 3 billion years. Yet, the stars are on orbits, where the tidal forces would prevent their formation. How have they come there?
The current best idea is the Hills mechanism. In that picture the stars originally were binaries, which happened to be scattered towards the massive black hole. In the three-body interaction with the massive partner, one of the components of the binary stars gets ejected at high velocity, taking away orbital energy from its partner, which in turn is stuck on a tight orbit around the black hole.
At larger distances (r > 1"), we find another population of stars: Even more massive, even younger O- or WR-type stars. A large fraction of these stars move coherently, the most prominent feature being a warped disk formed by the clock-wise moving stars (Paumard et al. 2006, Bartko et al. 2009). Except for eight stars, we cannot detect the weak accelerations anymore at these radii. With 2D positions, proper motion and radial velocity we can measures only five dynamic quantities, where six would fully determine the orbit. However, taking an ensemble of stars, we can show in a statistical sense that a large fraction of them moves in a disk. The following plot is the same projection as for the S-stars, and shows the probability distribution of the angular momentum vector as constrained by the dynamic data of the young, massive stars between 1" < r < 3.5" and with mK < 14.5. One can see the (significant) overdensity defining the clockwise stellar disk.
Further, we have shown that the IMF of these massive young stars is very top-heavy (Bartko et al. 2010)
The stars in the clockwise disk are even younger than the S-stars. Yet, their formation history appears to be solved: They have formed roughly 6 million years ago from a massive (around 100000 solar masses) molecular cloud falling into the Galactic Center. Simulations show that the gas will circularize, and form stars in a very unusual mode. The simulations show that such event results in the observed disk structure, the top-heavy mass function and the observed density profile.