Astronomy and Astrophysics – Astronomy
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...182.2005b&link_type=abstract
American Astronomical Society, 182nd AAS Meeting, #20.05; Bulletin of the American Astronomical Society, Vol. 25, p.821
Astronomy and Astrophysics
Astronomy
1
Scientific paper
We present an ab initio model of the photoelectric charging of PAHs and PAH clusters in the ISM. These particles have important effects on the heating of a range of ISM from the cold diffuse ISM to warm, dense photodissociation regions (PDRs). Previous treatments of photoelectric heating in the ISM considered the grain population as being composed of a typical grain at an average charge. Our analysis considers a grain size distribution (3--300 Angstroms) and a charge distribution. The rate of gas--grain heat exchange in the interstellar medium depends to a large extent on grain charge, which is set by the balance between the photoelectric ejection of electrons from and the collisional recombination of electrons and ions with the grain surface. The ejected photoelectron imparts energy to the surrounding gas while the subsequent recombination of the electron and ions with the grain removes thermal energy from the gas. The PAH population is found to be predominantly neutral in a range of ISM and is the most efficient medium for photoelectric heating, providing around half of the gas heating. We have formulated an analytic expression for photoelectric heating efficiency varepsilon applicable to a wide range of ISM of gas temperature T_gas, electron density n_e and illuminated by FUV fields G_o times the standard Habing value such that \varepsilon = \frac{3.0\times 10^{-2 ;(Tgas/4000)a}{1 + 2.0 \times 10^{-4} (\frac{GoTgas^{1/2}}{ne} )}} where a=0 for T_gas <= 4000 K and a=--0.9554 for T_gas > 4000 K. Our model gives the total photoelectric heating rate Gamma_pe for PAHs and small grains as \frac{\Gammape{nH} = 10^{-24} Go ;\varepsilon;erg ;s^{-1} ;(H;atom)^{-1}} where n_H is the hydrogen nucleus density. It compares well with observations for the diffuse ISM and the FIRAS observations on COBE.
Bakes E. E.
Tielens Alexander
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