CO emission and CO hotspots in diffuse molecular gas. (arXiv:2005.10270v1 [astro-ph.GA])

CO emission and CO hotspots in diffuse molecular gas. (arXiv:2005.10270v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Liszt_H/0/1/0/all/0/1">Harvey S Liszt</a>

We observed $lambda$3mm cotw, coth, coei, hcop, HCN and CS emission from
diffuse molecular gas along sightlines with EBV $approx$ 0.1 – 1 mag.
Directions were mostly chosen for their proximity to sightlines toward
background mm-wave continuum sources studied in hcop absorption, at positions
where maps of cotw at 1arcmin resolution showed surprisingly bright
integrated CO J=1-0 emission WCO = 5-12 K-kms but we also observed in L121
near zoph. Coherence emerges when data are considered over a broad range of
cotw and coth brightness. WCO/Wth and N(cotw)/N(coth) are 20-40 for
WCO $la 5$ K-kms and N(CO) $la 5times 10^{15}pcc$, increasing with much
scatter for larger WCO or N(CO). N(coth)/N(coei) $> 20-40$ ($3sigma$) vs.
an intrinsic ratio $^{13}$C/$^{18}$O = 8.4, from a combination of selective
photodissociation and enhancement of coth. The observations are understandable
if cotw forms from the thermal recombination of hcop with electrons, after
which the observed coth forms via endothermic carbon isotope exchange with
$^{13}$Cp. WCS/WCO increases abruptly for WCO $ga 10$ K-kms and
WCS/Whcop is bimodal, showing two branches having N(CS)/N(hcop) $approx 5$
and 1.25. Because CO formation and hcop excitation both involve collisions
between hcop and ambient electrons, comparison of the CO and hcop emission
shows that the CO hotspots are small regions of enhanced N(CO) occupying only a
small fraction of the column density of the medium in which they are embedded.
hcop/CO and HCN/CO brightness ratios are 1-2% with obvious implications for
determinations of the true dense gas fraction.

We observed $lambda$3mm cotw, coth, coei, hcop, HCN and CS emission from
diffuse molecular gas along sightlines with EBV $approx$ 0.1 – 1 mag.
Directions were mostly chosen for their proximity to sightlines toward
background mm-wave continuum sources studied in hcop absorption, at positions
where maps of cotw at 1arcmin resolution showed surprisingly bright
integrated CO J=1-0 emission WCO = 5-12 K-kms but we also observed in L121
near zoph. Coherence emerges when data are considered over a broad range of
cotw and coth brightness. WCO/Wth and N(cotw)/N(coth) are 20-40 for
WCO $la 5$ K-kms and N(CO) $la 5times 10^{15}pcc$, increasing with much
scatter for larger WCO or N(CO). N(coth)/N(coei) $> 20-40$ ($3sigma$) vs.
an intrinsic ratio $^{13}$C/$^{18}$O = 8.4, from a combination of selective
photodissociation and enhancement of coth. The observations are understandable
if cotw forms from the thermal recombination of hcop with electrons, after
which the observed coth forms via endothermic carbon isotope exchange with
$^{13}$Cp. WCS/WCO increases abruptly for WCO $ga 10$ K-kms and
WCS/Whcop is bimodal, showing two branches having N(CS)/N(hcop) $approx 5$
and 1.25. Because CO formation and hcop excitation both involve collisions
between hcop and ambient electrons, comparison of the CO and hcop emission
shows that the CO hotspots are small regions of enhanced N(CO) occupying only a
small fraction of the column density of the medium in which they are embedded.
hcop/CO and HCN/CO brightness ratios are 1-2% with obvious implications for
determinations of the true dense gas fraction.

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