The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics, and Formation

Abstract

We report the definite spectroscopic identification of ≃ 40 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ≃ 80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick ( |h|/R ≃ 0.14) rotating disks. These stellar disks have fairly sharp inner edges (R ≃ 1 ′′ ) and surface density profiles that scale as R −2 . We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the 'counter-clockwise' disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (ρ core 3 × 10 8 M ⊙ pc −3 ), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ≃ 6 ± 2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ≃ 1.5 × 10 4 M ⊙ . Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.

Orbits and origins of the young stars in the central parsec of the galaxy

“…Previous measurements of the radial distribution of young stars yield a steep radial profile consistent with in situ formation [3]. Also, the eccentricities of the stars have previously been estimated from observations by assuming that the stars orbit in a disk; however, there are conflicting results claiming that the stars in the clockwise-rotating disk are on nearly circular orbits [3] or on eccentric orbits [20]. Determining the radial profile and stellar eccentricities of stars in a disk may provide observational constraints on the origin of the young stars.…”

“…On the other hand, an inspiraling star cluster would dissolve into a disk of stars with a flatter radial profile (Σ ∝ r −0.75 ; [14]) whose orbital eccentricities would reflect the eccentricity of the cluster's orbit, which could be either circular or eccentric [15,16,17,18,19,14]. Previous measurements of the radial distribution of young stars yield a steep radial profile consistent with in situ formation [3]. Also, the eccentricities of the stars have previously been estimated from observations by assuming that the stars orbit in a disk; however, there are conflicting results claiming that the stars in the clockwise-rotating disk are on nearly circular orbits [3] or on eccentric orbits [20].…”

“…that are candidate disk members and have existing radial velocity measurements [3]. We define a primary sample of 32 young stars found in our astrometric data sets that have projected radii between 0.…”

“…[2,3]). The origin of such young stars has been difficult to explain since the gas densities observed today in the Galactic Center are orders of magnitude too low for a gas clump to overcome the extreme tidal forces and collapse to form stars [4,5].…”

“…Analyses of the statistical properties of the three-dimensional velocity vectors for these stars suggest that they lie in a clockwise-rotating disk [6] and perhaps also a counter-clockwise-rotating disk [9] that is nearly perpendicular to the first, with the disks extending from ∼0. 8 to at least 7 [3]. Both in situ gas disk and in-spiraling star cluster formation scenarios have been used to explain the kinematics of this young star population and to predict that the stars should lie in a common orbital plane.…”