10948(  1) - 04/03/06 11:05  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 10948

Version: 1    Check-in Time: 03-Apr-2006 15:05:02

Title
Exploring the Center of M31
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Type             Cycle     Parallel Pointing Tolerance
AR               15        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. Rosanne Di Stefano            Smithsonian Institution Astrophysical
                                          Observatory
   CoI: Dr. Jochen Greiner                Max-Planck-Institut fur extraterrestrische    N
                                          Physik
   CoI: Dr. Marina Orio                   University of Wisconsin - Madison             N
   CoI: Dr. Albert Kong                   Massachusetts Institute of Technology         N
   CoI: Dr. Pauline Barmby                Smithsonian Institution Astrophysical         N
                                          Observatory
   CoI: Mr. Jifeng Liu                    Smithsonian Institution Astrophysical         N
                                          Observatory
   CoI: Mr. Thomas Nelson                 University of Wisconsin - Madison             N

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Abstract

Supermassive black holes (BHs) in the centers of galaxies interact with nearby
stars, e.g., stripping giants of their gaseous envelopes. High nuclear stellar
densities can foster interactions among stars. These processes lead to the creation
of X-ray sources (XRSs), such as the hot cores of stripped giants, or X-ray
binaries. Chandra has identified more than 100 XRSs in the central 8' x 8' of M31.
This region has also been the subject of numerous HST observations. We propose to
use archival HST data to study correlations between the XRSs and both nearby stars
and ionization features. We will find counterparts or to place significant limits on
the flux of possible counterparts for a significant fraction (1/3-1/2) of the XRSs
in these fields. We will estimate the ages of the X-ray active systems, search for
differences between them and stars in their immediate neighborhood, study the
results as a function of distance from the nucleus, and compare the bulge and disk
populations. This detailed work, possible only in M31, will shed light on the
interactions that occur in the centers of galaxies. XRSs created near the center
emit copious amounts of X-ray and UV radiation, with a reasonably steady average
greater than 10^{39}-10^{40} erg s^{-1}, independent of direct emission from the BH.
This is a form of feedback that has not yet been well studied, and which can be
better understood through the work we propose. A second important focus of the
archival work is to illuminate the nature of the still poorly understood classes of
supersoft sources (SSSs) and quasisoft sources (QSSs). Some SSSs may be progenitors
of Type Ia supernovae; some may be accreting intermediate-mass BHs. Appropriate
models for QSSs have not yet been developed. The bulge of M31 has a high density of
these sources. Work with the HST archive will provide unique insight into the
possible natures of these enigmatic and potentially important sources.
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