The Stellar Population in the Galactic Center
The Closest Look at Young S-Stars near the Black Hole
Within a distance of 0.04 pc from the the super massive black hole (SMBH) in the center of our Galaxy exists a group of young stars. Given how inhospitable the region is for star formation, their presence is more puzzling the younger we estimate their ages (Eisenhauer et al. 2005, Martins et al. 2008). It is highly unlikely that the young S-stars formed at their present location, since the SMBH’s tidal forces are too strong to allow star formation at these distances. With all the observational constraints and theoretical complexities the question of the origin and distribution of young stars in the GC has become one of the most remarkable issues in this field.
By co-adding up to 105 hr of spectra we have obtained high signal-to-noise H- and K-band spectra of eight brightest stars in the field orbiting the SMBH. The deep H- and K-band spectra of S2 (K-band S/N: 480 and H-bandS/N: 280), S4, and also combined spectrum of other fainter S-stars, disclose a clear Stark broadening in the Brackett lines that implies high surface gravity of S-stars. This finding is established by our detailed stellar atmospheric+evolutionary model analysis, which employs line profiles of the complete Brackett series(excellent indicators of gravity)in the H- and K-bands.
We derive an age of 6.6(+4.7- 3.4) Myr for S2. With higher uncertainties, we estimate the age range of the other studied S-stars to be less than 15 Myr. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than a field binary-disruption scenario that occurred over a longer period of time. Although the proposed scenarios so far show that a disk origin for S-stars is possible, it is unclear whether the necessary conditions predicted by different scenarios are fulfilled in the GC.
Spectroscopic Detection of a Cusp of Late-Type Stars around the Central Black Hole in the Milky Way
Theoretical stellar dynamics predicts the formation of a dense stellar cusp of old stars within a dynamically relaxed cluster around a massive black hole. Such a cusp has so far escaped unambiguous observational confirmation specially within the central 0.5 pc (Bartko et al. 2010). Due to the high extinction and extreme stellar crowding, Particularly the increasing density of young stars toward the center, it is observationally challenging to confirm whether or not a stellar cusp exists.
By co-adding spectroscopic observations taken over a decade, we identify new late-type stars within the central 0.2 pc of the Galaxy. The unique advantage of our spectroscopic study is that through an individual age estimation, we can select a stellar population that is old enough to have undergone dynamical relaxation.
The updated star count, based on individual spectral classification, is used to reconstruct the surface density profile of giant stars. Our study, for the first time, finds a cusp in the surface number density of the spectroscopically identified old (>3 Gyr) giants population (mK<17) within 0.02-0.4 pc described by a single power law with an exponent Γ= 0.34 ± 0.04.
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.