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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 8, No. 13                                              Jul 2, 1998


Recently submitted papers:
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Email    : peale@io.physics.ucsb.edu
Title    : Microlensing Event Rates and Optical Depth toward the   
           Galactic Center
Author(s): S.J. Peale
Institute: Dept. of Physics, University of California, Santa 
           Barbara, CA 93106, peale@io.ucsb.edu
Paper    : to appear in ApJ
EPrint   : astro-ph/9806368

Abstract:
The dependence of microlensing time scale frequency distributions and
optical depth toward the galactic center on galactic model parameters
is explored in detail for a distribution of stars consisting of the
Zhao (1996) bar and nucleus and the Bahcall and Soneira (1980) double
exponential disk.  The high sensitivity of these two microlensing
measures to the circular velocity model, velocity dispersions, bulge
mass, direction of the line of sight, bar axis orientation, star
spatial distribution and the stellar mass function means no single
galaxy property can be constrained very well without constraining most
of the others. However, this same sensitivity will make microlensing a
powerful member of the suite of observational techniques that will
eventually define the galaxy properties.  The model time scale
frequency distributions are compared with that determined empirically
by the MACHO group throughout. Although the MACHO empirical data are
matched quite well with a nominal velocity model and with a mass
function only of hydrogen burning stars that varies as m^-2.2 to
m^-2.5 in the M star region, uncertainties in galactic structure,
kinematics and content together with the paucity of published
microlensing data preclude any claim of the model representing the
real world. A variation of the mass function ~ m^-1 in the M star
region obtained from recent star counts, both local and in the
galactic bulge, fails to yield a sufficient number of short time scale
events compared to the MACHO data.  The high sensitivity of the
microlensing measures to the direction of the line of sight may mean
that sufficient microlensing data is already in hand to constrain the
bar distribution of stars.  The procedure developed here for
determining the time scale frequency distribution is particularly
convenient for rapidly incorporating model changes as data from all
sources continues to accumulate.
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			       Edited by
Angela Cotera                                Heino Falcke
(cotera@ipac.caltech.edu)                    (hfalcke@mpifr-bonn.mpg.de)
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