Effect of photoionization of PAH molecules on the heating of HI interstellar gas
L. B. D'Hendecourt, A. Leger;
AaA, 1987, 180, L9
ABSTRACT:It is proposed that the photoelectric effect on polycyclic aromatic hydrocarbons (PAH) molecules can be the dominant source of heating of the
interstellar H I gas, if their photoionization cross-section is sufficient.
Such a
quantity has not yet been measured for large PAHs, but is estimated from data on
benzene.
The ionization properties of PAHs are reviewed and their ionization
fraction in various environments are computed.
The heat input from the ejected
photoelectrons is calculated, and it can be the dominant heating source for the H I
clouds and the intercloud medium if about 10 percent of the cosmic carbon is
locked in PAH molecules.
Such a value is consistent with previous estimates
of their abundance.
This situation differs from the photoelectric
effect on grains, which has been pointed out to be inefficient in the hot
intercloud medium.
KEYWORDS: cyclic hydrocarbons, gas heating, hydrogen, interstellar gas, molecular clouds, photoionization, photoabsorption, ultraviolet radiation
CODE: dhendecourt87