A type of → interstellar medium cloud in which
→ carbon (C) becomes almost completely
molecular due to relatively high → extinction.
The chemistry is qualitatively different from that of
→ diffuse molecular clouds, as the
→ electron
abundance is very low (→ cosmic-ray ionization
being the dominant source) and the reactive C is replaced by the very stable
→ carbon monoxide (CO).
This regime is found only in → sightlines with
AV > 5-10 mag; not all such sightlines will contain
dense cloud material and if dense cloud material is present it is likely to be
surrounded by → translucent material.
These clouds are typically → <i><a class="linkVoir" href="/terms/self-gravitating/">self-gravitating</a></i>,
and are most often observed by → <i><a class="linkVoir" href="/terms/infrared/">infrared</a></i> absorption and
→ <i><a class="linkVoir" href="/terms/millimeter-wave/">millimeter wave</a></i> emission methods. Their
densities are typically at least 10<SUP>4</SUP> cm<SUP>-3</SUP>, and their
→ <i><a class="linkVoir" href="/terms/kinetic-temperature/">kinetic temperature</a></i>s are typically on the order of 10-50 K in
the quiescent regions. Most of the more than 140 currently known
→ <i><a class="linkVoir" href="/terms/interstellar-molecule/">interstellar molecule</a></i>s
were found through observations of → <i><a class="linkVoir" href="/terms/microwave/">microwave</a></i>→ <i><a class="linkVoir" href="/terms/rotational-transition/">rotational transition</a></i>s
in such clouds, starting with the discovery of OH, followed by a host of other new
detections such as CO, NH<SUB>3</SUB>, H<SUB>2</SUB>O, and H<SUB>2</SUB>CO
(Snow & McCall, 2006, ARA&A 44, 367).
See also: → dense; → molecular;
→ cloud.
