\documentstyle{preprint}
\begin{document}
\title{EVIDENCE FOR THE LARGE-SCALE DISSOCIATION OF MOLECULAR GAS IN THE
INNER SPIRAL ARMS OF M81}
\author{R.~J.\ Allen\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
(rjallen@stsci.edu)\\
\and
J.~H.\ Knapen\\
\\
Department of Physical Sciences\\
University of Hertfordshire\\
College Lane, Hatfield\\
Herts AL10 9AB, UK\\
(knapen@star.herts.ac.uk)\\
\and
R.\ Bohlin\\
\\
Space Telescope Science Institute\\
3700 San Martin Drive\\
Baltimore, MD 21218\\
(bohlin@stsci.edu)\\
\and
T.~P.\ Stecher\\
\\
Code 680\\
Goddard Space Flight Center\\
Greenbelt, MD 20771\\
(stecher@uit.gsfc.nasa.gov)}
\pub{The Astrophysical Journal}
\recacc{26 December 1996}{18 March 1997}
\maketitle

\abstract{We compare the detailed distributions of HI, H$\alpha$, and
150~nm far-UV continuum emission in the spiral arms of M81 at a
resolution of $9''$ (linear resolution 150~pc at 3.7~Mpc distance).  The
bright H$\alpha$ emission peaks are always associated with peaks in the
far-UV emission. The converse is not always true; there are many regions
of far-UV emission with little corresponding H$\alpha$.  The HI and the
far-UV are always closely associated, in the sense that the HI is often
brightest around the edges of the far-UV emission.

The effects of extinction on the morphology are small, even in the 
far-UV.  Extensive far-UV emission, often with little coresponding
H$\alpha$, indicates the presence of many ``B-stars'', which produce
mostly non-ionizing UV photons.  These far-UV photons dissociate a small
fraction of an extensive layer of H$_2$ into HI.

The observed morphology can be understood if ``chimneys'' are common in
the spiral arms of M81, where holes are blown out of the galactic disk,
exposing the bright HII regions and the corresponding far-UV associated
with vigorous star formation.  These ``naked'' star-forming regions show
little obscuration.  H$_2$ is turned in HI by UV photons impinging on the
interior surfaces of these chimneys.

The intensity of the far-UV radiation measured by UIT can dissociate the
underlying H$_2$ with a typical density of $\sim10$~H nucleii~cm$^{-3}$
to produce the observed amount of HI in the spiral arms of M81.  Except
for the thin surface layers locally heated in these photo-dissociation
regions close to the far-UV sources, the bulk of the molecular gas in
the inner disk of M81 is apparently too cold to produce much
$^{12}$CO(1-0) emission.}

\end{document}