------------------------------------------------------------------------
 GCstar.tex ApJ, in press
X-Spam-Status: No


%arXiv:0705.3649

\documentclass[12pt,preprint]{aastex}
\begin{document}
\title{On the origin of kinematic distribution of the sub-parsec
young stars in the Galactic center}
\author{Qingjuan Yu$^{1,}$\footnotemark[3], Youjun Lu$^1$, \&
D.~N.~C. Lin$^{1,2}$}
\affil{$^1$Department of Astronomy and Astrophysics, University of
            California, Santa Cruz, CA 95064, USA \\
        $^2$Kavli Institute of Astronomy \& Astrophysics, Peking
            University, Beijing, China}
\footnotetext[3]{Also a Hubble Fellow at the Department of Astronomy,
                  University of California at Berkeley, Berkeley, CA 
94720.}
\email{yqj, lyj, lin@ucolick.org}

\begin{abstract}

Observations indicate the presence of a massive black hole in the Galactic
center. Within a half-parsec from the Galactic center, there is a population
of coeval young stars which appear to reside in a coherent disk. 
Surrounding this dynamically-cool stellar system, there is a population of 
stars with a similar age and much larger eccentricities and inclinations 
relative to the disk. We propose a hypothesis for the origin of this 
dynamical dichotomy.  Without specifying any specific mechanism, we 
consider the possibility that both stellar populations were formed within 
a disk some 6$\pm$2 Myr ago. But this orderly structure was dynamically 
perturbed outside-in by an intruding object with a mass $\sim 10^4 
M_\odot$, which may be an intermediate-mass black hole or a dark stellar 
cluster hosting an intermediate-mass black hole. We suggest that the 
perturber migrated inward to $\sim0.15-0.3 {\rm pc}$ from the Galactic 
center as a consequence of orbital decay under the action of dynamical 
friction. Along the way, it captured many stars in the outer disk
region into its mean-motion resonance, forced them to migrate with it, 
closely encountered with them, and induced the growth of their 
eccentricity and inclination.  But stars in the inner regions of the disk 
retain their initial coplanar structure. Quantitatively, a perturber on a 
low-inclination or overhead orbit to the disk plane can reproduce the 
observed kinematic structure of these young stars.  But this process is 
unlikely to produce the controversial two-disk structure.  We predict that 
some of the inclined and eccentric stars surrounding the disk may have 
similar Galactocentric semimajor axis. Future precision determination of 
their kinematic distribution of these stars will not only provide a test 
for this hypothesis but also evidences for the presence of an 
intermediate-mass black hole or a dark cluster at the immediate proximity 
of the massive black hole at the Galactic center.
\end{abstract}

\keywords{Black hole physics: Galaxy: center-stars: kinematics and
dynamics: stellar dynamics}

\end{document}