Standing in the shadow of dark gas: ALMA observations of absorption from
  dark CO in the molecular DNM of Chamaeleon
  % context We had detected J=1-0 HCO+ absorption in 12 directions lacking
detected CO emission in the outskirts of the Chamaeleon complex and on 1
sightline with integrated CO emission 2.4 K-\kms. 8 sightlines had a much
larger mean column density of dark neutral medium (DNM)-gas not represented in
H I or CO emission-and were found to have much higher mean molecular column
density. The 5 other sightlines had little or no DNM and were found to have
much smaller but still detectable N(HCO+). % aims To determine N(CO) along
previously-observed Chamaeleon sightlines and to determine why CO emission was
not detected in directions where molecular gas is present. % methods We took
\cotw\ J=1-0 absorption profiles on 5 sightlines having higher DNM and HCO+
column densities and 1 sightline with smaller N(DNM) and N(HCO+). We converted
the integrated HCO+ optical depths to N(H2) in the weak-excitation limit using
N(HCO+)/N(H2)$=3\times10^{-9}$ and converted the integrated CO optical depths
\ICO\ to CO column density N(CO) $= 1.861\times 10^{15}\pcc \ICO^{1.131}$ as
found along comparable lines of sight previously studied in J=1-0 and J=2-1 CO
absorption&emission. %results CO absorption was detected along the 5 sightlines
in the higher-DNM group, with CO column densities $4\times 10^{13} \pcc<$ N(CO)
$<10^{15}\pcc$ that are generally below the detectability limit of CO emission
surveys. %conclusions In the outskirts of the Chamaeleon complex, the presence
of molecular DNM resulted primarily from small CO column densities at the onset
of CO formation around the HI/\HH\ transition in diffuse molecular gas. CO
relative abundances N(CO)/N(H2) $< 2\times 10^{-6}$ in the outskirts of
Chamaeleon are comparable to those seen in UV absorption toward early-type
stars, including in Chamaeleon.
