**AAS 195th Meeting, January 2000**

*Session 51. ISM: Theory and Modelling*

Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall
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## [51.01] Maximum Entropy Reconstruction of the Interstellar Medium

*J. S. Arabadjis, J. N. Bregman (University of Michigan Astronomy Department)*

We have developed a technique to map the three-dimensional
structure of the local interstellar medium ({\sc ism}) using
a maximum entropy reconstruction ({\sc mer}) technique. A
set of column densities **N** to stars of known distance
can in principle be used to recover a three-dimensional
density field **n**, since the two quantities are
related by simple geometry through the equation **N** =
{\sf C} \cdot **n**, where {\sf C} is a tensor
characterizing the stellar spatial distribution. In
practice, however, there is an infinte number of solutions
to this equation. We use a {\sc mer} algorithm to find the
density field containing the least information which is
consistent with the observations. The solution obtained with
this technique is, in some sense, the {\sc ism} model
containing the minimum structure. We apply the algorithm to
several simulated data sets to demonstrate its feasibility
and success at recovering ``real'' density contrasts.

One application of this technique is to soft X-ray
absorption columns. A large set of columns can be obtained
by modelling the several hundred stellar spectra in the *
ROSAT* data archives at the {\sc heasarc}. The absorption of
soft X-rays by the {\sc ism} is independent of the molecular
state of the absorbers (monatomic or molecular gas, or dust
grains), as well as their (modest) ionization state, making
it an ideal tracer of the total amount of (baryonic)
material. By applying this method to column density sets
derived from different tracers -- for example, dust columns
derived from stellar color excesses -- one can determine the
structures in, and relative fractions of, the various phases
of the {\sc ism}.

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