------------------------------------------------------------------------
 jlu_yngstars.tex ApJ, accepted
Date: Fri, 29 Aug 2008 10:14:31 -0700
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%astro-ph/0808.3818

\documentclass[12pt,preprint]{aastex}

\begin{document}

\title{A Disk of Young Stars at the Galactic Center as Determined by
Individual Stellar Orbits}

\author{J. R. Lu\altaffilmark{1}, A. M. Ghez\altaffilmark{1,2}, S. D.  
Hornstein\altaffilmark{1,3}, M. R. Morris\altaffilmark{1}, E. E.  

Los Angeles, CA 90095-1562}
\altaffiltext{2}{UCLA Institute of Geophysics and Planetary Physics,
Los Angeles, CA 90095-1565}
\altaffiltext{3}{Center for Astrophysics \& Space Astronomy,
Department of Astrophysical and Planetary Sciences,
University of Colorado, Boulder, CO 80309}
\altaffiltext{4}{Caltech Optical Observatories, California Institute of
Technology, MS 320-47, Pasadena, CA 91125}

\begin{abstract}
We present new proper motions from the 10 m Keck telescopes
for a puzzling population of massive, young stars located within  
3\farcs5
(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
$\gtrsim$ 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 3D 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  
7 stars.
The detected disk contains 50\% of the young stars, is inclined
by $\sim115^\circ$ from the plane of the sky, and is oriented at
a position angle of $\sim100^\circ$ East of North.
Additionally, the on-disk and off-disk populations have similar K-band
luminosity functions and radial distributions that decrease at larger
projected radii as $\propto r^{-2}$.
The disk has an out-of-the-disk velocity dispersion
of 28 $\pm$ 6 km s$^{-1}$, which corresponds to a half-opening angle of
$7^\circ \pm 2^\circ$, and several candidate disk members have
eccentricities greater than 0.2.
Our findings suggest that the young stars may have formed {\it in situ}
but in a more complex geometry than a simple, thin circular disk.
\end{abstract}

\end{document}