The Stellar Population in the Galactic Center
The Closest Look at Young S-Stars near the Black Hole
Within a distance of 0.04 parsecs from the supermassive black hole (SMBH) in the center of our Galaxy, a group of young stars resides. Given how inhospitable the region is for star formation, their presence is the more puzzling the younger they appear to be (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 Galactic Center has become one of the most remarkable issues in this field.
By co-adding up to 105 hours of spectroscopy, we have obtained high signal-to-noise H- and K-band spectra of the 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 the combined spectrum of other fainter S-stars, disclose a clear broadening in the Brackett lines, implying high surface gravity of the S-stars. This finding is established by our detailed stellar atmospheric and evolutionary model analysis, which employs line profiles of the complete Brackett series (excellent indicators of gravity) in the H- and K-bands. These stars are bona fide main-sequence stars.
We derive an age of 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 Galactic Center.
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, especially within the central 0.5 parsecs (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 made over a decade, we can identify new late-type stars within the central 0.2 parsecs 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 counts, based on individual spectral classification, are used to reconstruct the surface density profile of giant stars. For the first time, our study has found a cusp in the surface number density of the spectroscopically identified old (> 3 Gyr) giant population (mK < 17) within 0.02 to 0.4 parsecs described by a single power law with an exponent Γ = 0.34 ± 0.04.