Journal
ASTROPHYSICAL JOURNAL
Volume 841, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-4357/aa6f12
Keywords
ISM: molecules; molecular processes; photon-dominated region (PDR)
Categories
Funding
- National Aeronautics and Space Administration (NASA)
Ask authors/readers for more resources
Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the dense gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H-2 molecules,similar or equal to 10(5) for HCN, yields the requirements for electron excitation to be of practical importance if n (H-2) <= 10(5.5) cm(-3) and X(e(-)) >= 10(-5), where the numerical factors reflect the critical values n(c)(H-2) and X*(e(-)). This indicates that in regions where a large fraction of carbon is ionized, X(e(-)) will be large enough to make electron excitation significant. The situation is in general similar for other high-density tracers, including HCO+, CN, and CS. But there are significant differences in the critical electron fractional abundance, X*(e(-)), defined by the value required for equal effect from collisions with H-2 and e-. Electron excitation is, for example, unimportant for CO and C+. Electron excitation may be responsible for the surprisingly large spatial extent of emission from dense gas tracers in some molecular clouds. The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO+ ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available