Formation and Fractionation of CO (carbon monoxide) in diffuse clouds
  observed at optical and radio wavelengths
  We modelled \HH\ and CO formation incorporating the fractionation and
selective photodissociation affecting CO when \AV\ $\la2$mag. UV absorption
measurements typically have N(\cotw)/N(\coth) $\approx 65$ that are reproduced
with the standard UV radiation and little density dependence at n(H)
$\approx32-1024\pccc$: Densities n(H) $\la256\pccc$ avoid overproducing CO.
Sightlines observed in mm-wave absorption and a few in UV show enhanced \coth\
by factors of 2-4 and are explained by higher n(H) $\ga256\pccc$ and/or weaker
radiation. The most difficult observations to understand are UV absorptions
having N(\cotw)/N(\coth) $>$100 and N(CO)$\ga10^{15}\pcc$. Plots of \WCO\ vs.
N(CO) show that \WCO\ remains linearly proportional to N(CO) even at high
opacity owing to sub-thermal excitation. \cotw\ and \coth\ have nearly the same
curve of growth so their ratios of column density/integrated intensity are
comparable even when different from the isotopic abundance ratio. For
n(H)$\ga128\pccc$, plots of \WCO\ vs N(CH) are insensitive to n(H), and
\WCO/N(CO)$\approx1\Kkms/(10^{15}~{\rm CO}\pcc)$: This compensates for small
CO/\HH\ to make \WCO\ more readily detectable. Rapid increases of N(CO) with
n(H), N(H) and N(\HH) often render the CO bright, ie a small CO-\HH\ conversion
factor. For n(H) $\la64\pccc$ CO enters the regime of truly weak excitation
where \WCO $\propto$n(H)N(CO). \WCO\ is a strong function of the average \HH\
fraction and models with \WCO=1\Kkms\ fall in the narrow range \mfH2\=0.65-0.8,
or \mfH2\=0.4-0.5 at \WCO\=0.1\Kkms. The insensitivity of easily-detected CO
emission to gas with small \mfH2\ implies that even deep CO surveys using broad
beams may not discover substantially more emission.
