\documentstyle[pptwocol]{preprint}
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\begin{document}
\title{A SEARCH FOR THE CO--H\boldmath{$_2$} DIMER IN THE GALAXY}
\author{R.~J.\ Allen, L.\ Loinard\/\thanks{Observatoire de Grenoble, 
Laboratoire d'Astrophysique, Universit\'e Joseph Fourier, B.P.~53X, F-38041 
Grenoble Cedex, France}\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD  21218\\
\and
A.~R.~W.\ McKellar\\
\\
Steacie Institute for Molecular Sciences\\
\\
National Research Council of Canada\\
\\
Ottawa, Ontario K1A 0R6, Canada\\
\and
J.\ Lequeux\\
\\
DEMIRM, Observatoire de Paris\\
\\
61 Avenue de l'Observatoire\\
\\
F-75014 Paris, France}

\pub{Ap.~J}
\recacc{24 January 1997}{18 February 1997}

\maketitle

\abstract{We have used the 30~m IRAM millimeter radio telescope to
search for two low-lying rotational transitions of the CO--H$_2$ dimer
in the emission spectra of 3~Galactic sources and in the absorption
spectrum of a low-latitude extragalactic continuum source.  The
transitions are the 111-$\leftrightarrow$ 000+ line at 109.21~GHz, and
the 110+ $\leftrightarrow$ 101- line at 90.84~GHz.  The search targets
were the TMC~1 and L~134 dark clouds, the L~1157 bipolar outflow, and
the compact radio continuum source 2013+370.  The sectral search covered
the frequency ranges from 90.58--91.10~GHz and from 108.95--109.46~GHz.
Several known emission lines were found including HC$_3$N(12-11),
HC$_3$N(10-9), OCS, SO, and HNC, but no other features were seen in the
spectra.  The rms noise was typically 5~mK per 1~MHz channel
($\approx3$~km~s$^1$).

We conclude that the fractional abundance of the CO--H$_2$ dimer with
respect to the CO monomer is typically $\leq1\times10^{-4}$ if the
excitation temperature is at least a few degrees above the cosmic
background.  This is the most stringent upper limit yet placed on the
presence of a dimer in the ISM.}