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From: Lowell  lowell@mpe.mpg.de 
X-Sender: lowell@ibm-s-1.mpe-garching.MPG.DE
To: Galactic Center Newsletter <gcnews@aoc.nrao.edu>
cc: Reinhard Genzel <genzel@mpe.mpg.de>
Subject:astro-ph/0001428 - Stellar Dynamics in the Galactic centre: Proper Motions and Anisotropy
MIME-Version: 1.0

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%astro-ph/0001428
\begin{document}

\title{Stellar Dynamics in the Galactic centre: 
Proper Motions and Anisotropy }
\author{R. Genzel$^{1}$, C.Pichon$^{2,3}$, A.Eckart$^{1}$, O.E.Gerhard$^{2}$ and
T.Ott$^{1}$}
\institute{
$^{1}$ Max-Planck Institut f\"{u}r extraterrestrische Physik, Garching,
Germany \\
$^{2}$ Astronomisches Institut, Universit\"{a}t Basel, Switzerland \\
$^{3}$ Observatoire de Strasbourg, 11 rue de l'Observatoire, Strasbourg,
France}
\date{\today}
\maketitle

\begin{abstract}
We report a new analysis of the stellar dynamics in the Galactic centre,
based on improved sky and line-of-sight velocities for more than one hundred
stars in the central few arcseconds from the black hole candidate SgrA*. The
main results are:

$\bullet$ Overall the stellar motions do not deviate strongly from isotropy.
For those 32 stars with a determination of all three velocity components the
absolute, line of sight and sky velocities are in good agreement, consistent
with a spherical star cluster. Likewise the sky-projected radial and
tangential velocities of all 104 proper motion stars in our sample are also
consistent with overall isotropy.

$\bullet$ However, the sky-projected velocity components of the young,
early type stars in our sample indicate significant deviations from
isotropy, with a strong radial dependence. Most of the bright HeI
emission line stars at separations from 1" to 10'' from SgrA* are on
tangential orbits.  This tangential anisotropy of the HeI stars and
most of the brighter members of the IRS16 complex is largely caused by
a clockwise (on the sky) and counter-rotating (line of sight, compared
to the Galaxy), coherent rotation pattern. The overall rotation of
the young star cluster probably is a remnant of the original angular
momentum pattern in the interstellar cloud from which these stars were
formed.

$\bullet$ The fainter, fast moving stars within
$\approx{}1^{\prime\prime}$ from SgrA* appear to be largely moving on
radial or very elliptical orbits. We have so far not detected
deviations from linear motion (i.e. acceleration) for any of
them. Most of the SgrA* cluster members also are on clockwise
orbits. Spectroscopy indicates that they are early type stars. We
propose that the SgrA* cluster stars are those members of the early
type cluster that happen to have small angular momentum and thus can
plunge to the immediate vicinity of SgrA*.

$\bullet$ We derive  an anisotropy-independent estimate of  the Sun-Galactic
centre   distance  between 7.8   and  8.2 kpc,  with   a  formal statistical
uncertainty of $\pm 0.9 {\ {\rm kpc}}$.

     
$\bullet$    We explicitly include  velocity  anisotropy   in estimating the
central mass distribution. We show how  Leonard-Merritt and Bahcall-Tremaine
mass estimates  give systematic offsets in  the inferred mass of  the central
object  when  applied to  finite concentric  rings  for  power law clusters.
Corrected Leonard-Merritt  projected  mass  estimators  and  Jeans  equation
modelling confirm previous conclusions (from isotropic models) that a compact
central mass concentration (central density $\geq$10$^{{\ 12.6 }}$M$_{{\sun}
} $ pc$^{{\ -3}}$) is present and dominates the potential between 0.01 and 1
pc.    Depending on the modelling method used the derived central mass ranges 
between  2.6 and $3.3 \times     10^{{\ 6}}M_{{\sun}   }  $  for
R$_{{\sun}}=8.0 {\ {\rm kpc}}$.
\end{abstract}
\end{document}



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