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.
This picture shows combined spectra (2004 to 2016) of eight young S-stars (K < 15.5 mag) in the K-band.
This picture shows combined spectra (2004 to 2016) of eight young S-stars (K < 15.5 mag) in the K-band.
This image illustrates combined spectra (2004 to 2016) of eight young S-stars (K < 15.5 mag) in the H-band. The most prominent absorption features are He I(1.7002μm) and Brackett lines (Br10–Br15).
This image illustrates combined spectra (2004 to 2016) of eight young S-stars (K < 15.5 mag) in the H-band. The most prominent absorption features are He I(1.7002μm) and Brackett lines (Br10–Br15).
This HR diagram shows young bright S-stars in the Teff−log(L) plane. The black crosses mark the fitted stellar parameters for the S-stars on the HR diagram in the Teff−log(L) plane. The stellar models for the Milky Way (Brott et al. 2011) and solar metallicity, rotating, Geneva isochrones (Ekstrom et al. 2012) of 3 to 40 Myr are shown in solid blue and dashed orange lines, respectively.
This HR diagram shows young bright S-stars in the Teff−log(L) plane. The black crosses mark the fitted stellar parameters for the S-stars on the HR diagram in the Teff−log(L) plane. The stellar models for the Milky Way (Brott et al. 2011) and solar metallicity, rotating, Geneva isochrones (Ekstrom et al. 2012) of 3 to 40 Myr are shown in solid blue and dashed orange lines, respectively.
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.
This image shows an example of combined spectra of giant stars within the central 0.2 parsecs. By combining multiepoch AO-assisted spectroscopic observations, we can study stars in the vicinity of Sgr A* down to 17.5 mag. We have detected the first five warm giants of the type G2-G8III in the region. High S/N spectra enable us to assign temperatures from the spectral CO band head features of these stars.
This image shows an example of combined spectra of giant stars within the central 0.2 parsecs. By combining multiepoch AO-assisted spectroscopic observations, we can study stars in the vicinity of Sgr A* down to 17.5 mag. We have detected the first five warm giants of the type G2-G8III in the region. High S/N spectra enable us to assign temperatures from the spectral CO band head features of these stars.
This image shows surface density profiles of late-type stars (mK < 17) out to distances of 10 arcseconds (0.4 parsecs). We find a cusp structure in the surface number density of the old giant population within 0.02 to 0.4 parsecs described by a single power law with an exponent Γ = 0.34 ± 0.04 (blue dashed line). For the central 1 arcsecond (red star), we use our star counts with 100% photometric and spectral classification completeness. For larger radii (2 to 10 arcseconds), we apply the data from Bartko et al. 2010, corrected for the small fraction of young stars. The gray points illustrate the distribution of the bright and faint giants from Gallego-Cano et al. 2018.
This image shows surface density profiles of late-type stars (mK < 17) out to distances of 10 arcseconds (0.4 parsecs). We find a cusp structure in the surface number density of the old giant population within 0.02 to 0.4 parsecs described by a single power law with an exponent Γ = 0.34 ± 0.04 (blue dashed line). For the central 1 arcsecond (red star), we use our star counts with 100% photometric and spectral classification completeness. For larger radii (2 to 10 arcseconds), we apply the data from Bartko et al. 2010, corrected for the small fraction of young stars. The gray points illustrate the distribution of the bright and faint giants from Gallego-Cano et al. 2018.
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.
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.
