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3 6 lowlying excitations except for the extremely weak

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Unformatted text preview: ISM as well). We will therefore focus on the photoelectric heating.1 We define the efficiency of photoelectric heating ε to be the fraction of the absorbed photon energy hν that goes into ejected electrons. This is given by hν − W − Z d e 2 / a ε =Y , hν where Zd is the charge of the grain, W is the work function, and Y is the electron yield (i.e. probability that an absorbed photon actually ejects an electron). The € charge is significant because in PDRs, the photoelectric process tends to leave grains with a net positive charge, until the electrostatic potential prevents further electrons from leaving and/or attracts electrons from the ambient medium to the grain surface. The yield varies, dropping to near zero for large grains because the photons can penetrate into the deep interior of the grain, while the electrons released there stop due to collisions before they can exit the grain. For the PAHs, the yield is of order unity. However, even a singly charged PAH +) tends to have a second ionization energy exceeding 13.6 eV, so PAH0 (PAH molecules dominate the PAH photoelectric heating. The fraction of PAHs that are neutral tends to be a few tenths (determined by the γ parameter, i.e. the ratio of photon to electron collision rates). The net efficiency of photoelectric heating varies slowly as a function of T and γ, but values of order ε~0.01 are typical. Given the typical UV dust absorption cross section of ~10−21 cm2 per H nucleus, and typical energy flux of ~10−3G0 erg cm−2 s−1, this suggests a heating rate of ~10−26G0 erg/s/H atom. B. COOLING The species in PDRs available for cooling are H, O, and C+; and at greater depths CO. The fine structure lines...
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