photo rates for molecules
Title:
Photodissociation and photoionisation of atoms and molecules of astrophysical interest
Authors:
Heays, A. N.; Bosman, A. D.; van Dishoeck, E. F.
Publication:
eprint arXiv:1701.04459
Publication Date:
01/2017
Origin:
ARXIV
Keywords:
Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies
Comment:
Corrected some inconsistent table/figure data. Significant change: Zn photoionisation rate corrected. Accepted for publication by A&A
Bibliographic Code:
2017arXiv170104459H
Abstract
A new collection of photodissociation and photoionisation cross sections for 102
atoms and molecules of astrochemical interest has been assembled, along with a
brief review of the basic processes involved. These have been used to calculate
dissociation and ionisation rates, with uncertainties, in a standard ultraviolet
interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The
new ISRF rates generally agree within 30% with our previous compilations, with a
few notable exceptions. The reduction of rates in shielded regions was calculated
as a function of dust, molecular and atomic hydrogen, atomic C, and self-
shielding column densities. The relative importance of shielding types depends
on the species in question and the dust optical properties. The new data are
publicly available from the Leiden photodissociation and ionisation database.
Sensitivity of rates to variation of temperature and isotope, and cross section
uncertainties, are tested. Tests were conducted with an interstellar-cloud
chemical model, and find general agreement (within a factor of two) with the
previous iteration of the Leiden database for the ISRF, and order-of-magnitude
variations assuming various kinds of stellar radiation. The newly parameterised
dust-shielding factors makes a factor-of-two difference to many atomic and
molecular abundances relative to parameters currently in the UDfA and KIDA
astrochemical reaction databases. The newly-calculated cosmic-ray induced
photodissociation and ionisation rates differ from current standard values up to a
factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates
alter the equilibrium abundances of abundant dark cloud abundances by up to a
factor of two. The partial cross sections for H2O and NH3 photodissociation
forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-
dependent branching ratios.