Opened 3 years ago

#336 new enhancement

H2 formation on PAHs

Reported by: Gary J. Ferland Owned by: nobody
Priority: major Milestone:
Component: chemical network Version: trunk
Keywords: Cc:



H2 formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen
Boschman, L.; Cazaux, S.; Spaans, M.; Hoekstra, R.; Schlathölter, T.

Aims: Molecular hydrogen is the most abundant molecule in the Universe. It is 
thought that a large portion of H2 forms by association of hydrogen atoms to 
polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on 
total H2 formation rates in photodissociation regions (PDRs) and assess the 
effect of these formation rates on the total cloud structure. 

Methods: We set up a chemical kinetic model at steady state in a PDR 
environment and included radiative transfer to calculate the chemistry at 
different depths in the PDR. This model includes known dust grain chemistry for 
the formation of H2 and a H2 formation mechanism on PAHs. Since H2 
formation on PAHs is impeded by thermal barriers, this pathway is only efficient 
at higher temperatures (T> 200 K). At these temperatures the conventional route 
of H2 formation via H atoms physisorbed on dust grains is no longer feasible, 
so the PAH mechanism enlarges the region where H2 formation is possible. 

Results: We find that PAHs have a significant influence on the structure of 
PDRs. The extinction at which the transition from atomic to molecular hydrogen 
occurs strongly depends on the presence of PAHs, especially for PDRs with a 
strong external radiation field. A sharp spatial transition between fully 
dehydrogenated PAHs on the outside of the cloud and normally hydrogenated 
PAHs on the inside is found. As a proof of concept, we use coronene to show 
that H2 forms very efficiently on PAHs, and that this process can reproduce the 
high H2 formation rates derived in several PDRs.

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