Molecular Gas and Dark Neutral Medium in the Outskirts of Chamaeleon
  %context More gas is inferred to be present in molecular cloud complexes than
can be accounted for by HI and CO emission, a phenomenon known as dark neutral
medium (DNM) or CO-dark gas for the molecules. %aims To see if molecular gas
can be detected in Chamaeleon when gas column densities in the DNM were
inferred and CO emission was not detected. % methods We took 3mm absorption
profiles of HCO+ and other molecules toward quasars across Chamaeleon, 1 of
which had detectable CO emission. We derived N(H2) assuming N(HC+)/N(H2) =
3x10^{-9}. %results With the possible exception of 1 weak continuum target HCO+
absorption was detected in all directions, \cch\ in 8 and HCN in 4 directions.
The sightlines divide in 2 groups according to their DNM content with 1 group
of 8 directions having N(DNM) \ga 2x10^{20} \pcc and another group of 5
directions having N(DNM) < .5x10^{20}\pcc. The groups have comparable <N(HI)>
in Chamaeleon 6-7 x 10^{20}\pcc and <N(H)/E(B-V) ~ 6-7x10^{21}\pcc/mag. They
differ in having quite different <E(B-V)> 0.33 vs 0.18 mag, <N(DNM)> 3.3 vs .14
x 10^{20}\pcc and <2N(H2)> = 5.6 vs 0.8 x 10^{20} \pcc. Gas at more positive
velocities is enriched in molecules and DNM. %conclusion Overall the quantity
of H2 inferred from HCO+ fully accounts for the previously-inferred DNM along
the studied sightlines. H2 is concentrated in the high-DNM group, where the
molecular fraction is 46% vs. 13% otherwise and 38% overall. Thus, neutral gas
in the outskirts of the complex is mostly atomic but the DNM is mostly
molecular. Saturation of the HI emission may occur along 3 of the 4 sightlines
having the largest DNM column densities but there is no substantial reservoir
of 'dark' atomic or molecular gas that remains undetected as part of the
inventory of dark neutral medium.
