A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits

Abstract

We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 3. 5 (0.14 pc) of the supermassive black hole at the Galactic center. Our proper motion measurements have uncertainties of only 0.07 mas yr −1 (3 km s −1 ), which is 7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr −2 (7 km s −1 yr −1 ). Using these measurements, line-of-sight velocities from the literature, and three-dimensional velocities for additional young stars in the central parsec, we constrain the true orbit of each individual star and directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one of the previously proposed disks of young stars using a method that is capable of detecting disks containing at least seven stars. The detected disk contains 50% of the young stars, is inclined by ∼115 • from the plane of the sky, and is oriented at a position angle of ∼100 • east of north. Additionally, the on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as ∝ r −2 . The disk has an out-of-the-disk velocity dispersion of 28 ± 6 km s −1 , which corresponds to a half-opening angle of 7 • ± 2 • , and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple, thin circular disk.

Young stars in the Galactic center

“…Current observations favor in situ formation based on the steep fall off in the number of young disk stars at larger radii. The observed radial density profile is consistent with ρ(r) ∝ r −2 as predicted for in situ gas disks and is much steeper than the ρ(r) ∝ r −0.75 predicted for in-falling cluster scenarios (Lin & Pringle 1987;Berukoff & Hansen 2006;Levin 2007;Lu et al 2009;Bartko et al 2009). The simplest in situ formation scenarios initially produce circular orbits since gas that flows in at low or moderate rates circularizes prior to the onset of star formation (Nayakshin & Cuadra 2005;Alexander et al 2007;Löckmann et al 2009).…”

“…The observed eccentricity distribution peaks at e=0.3 Yelda et al 2013). Scenarios invoking more rapid gas inflow, such as the infall of a single molecular cloud or the collision of two molecular clouds, could produce disk stars with large initial eccentricities and have the added bonus of producing a large population of young stars off the disk (Bonnell & Rice 2008;Wardle & Yusef-Zadeh 2008;Hobbs & Nayakshin 2009;Mapelli et al 2012), similar to what has been observed (Paumard et al 2006;Bartko et al 2009;Lu et al 2009;Yelda et al 2013). However, the evolution of an initially circular disk over 4-6 Myr depends on the initial mass function (IMF) of the stars and it is possible to evolve to today's observed distribution for moderately top-heavy IMFs .…”

“…These surveys are deep enough to identify B main-sequence stars; however, they have very small fields of view (∼2"), which has led to incomplete spatial coverage (Figure 1). Proper motions and radial velocities of the young stars have revealed coherent dynamical structures, including one well-defined inner disk (Genzel et al 2000;Levin & Beloborodov 2003;Paumard et al 2006), and there is continued discussion on the possibility of a disk warp or a second disk (Genzel et al 2003;Bartko et al 2009;Lu et al 2009;Yelda et al 2013).…”

SGR A* and Its Environment: Low-Mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

“…As displayed in Figures 4(b), (c), RS5 and RS6 have near-IR counterparts. Although there is an early-type star, S1-22, that may be associated with RS6, RS5 is not identified as a near-IR star in various catalogs (e.g., Lu et al 2009Lu et al , 2013. There is no evidence for a known young and massive star associated with RS5.…”

“…To see if these stars have radio continuum counterparts, their positions at the epoch of our 34 GHz observation on 2014 March 9 (2014.19) have been calculated based on proper motions and orbital accelerations have been derived from near-IR observations (Lu et al 2009;Gillessen et al 2009;Yelda et al 2014). Tables 2 and 3 give the positions of the S-cluster and their corresponding positional uncertainties at the epoch of 2014.19 and from two different catalogs (Gillessen et al 2006;Lu et al 2009). Table 4 Table 5 lists Gaussian-fitted positions of 8 radio sources (RS1-8) embedded within the diffuse extended emission associated with the Sgr A * ridge and the plume-like structure at 34 GHz (see Figures 1 and 2).…”