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Therefore we expect carbon or at least the gasphase

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Unformatted text preview: H0. As one moves into a PDR, the molecular hydrogen becomes self shielding at € column densities N(H2)>1014 cm−2. That is, the radiation in the Lyman and Werner series lines has already been absorbed, so that the FUV radiation that filters into the PDR becomes less efficient (it must excite weaker H2 lines or operate farther from line center). The rate of photodissociation is then suppressed by a self ­shielding factor, ȹ 1014 cm−2 ȹ 0.75 β ss ≈ ȹ ȹ . ȹ N (H 2 ) Ⱥ • € 4 In practice, at the surface of a PDR (i.e. at the H0/H+ boundary) the reaction rates heavily favor atomic over molecular gas. As one moves deeper into the PDR, the self ­shielding of the small amount of H2 reduces βss to values <<1 and eventually the gas can become mostly molecular. 3. The Physics of PDRs A. METALS Carbon is important because its ionization energy (11.2 eV) is less than that of hydrogen (13.6 eV). Therefore, the radiation that filters through the H0 zone can still ionize carbon. We found earlier that the unscreened radiation from the central star typically results in an ionized:neutral ratio for H in the H II region of ~few × 103, and a similar calculation applies to carbon in the PDR. Therefore we expect carbon (or at least the gas ­phase portion) to be primarily in the form of C II. This implies an electron abundance xe ~ 10−4 even though the H and He are neutral. As one goes deeper into the PDR, dust absorbs the FUV photons and some self ­shielding of C I takes place. When the FUV is extincted by a factor of ~104 (10 magnitudes, or AV~4) the carbon becomes mainly neutral. In the C0 zone, metals with lower ionization potentials (S, Si, Fe, Mg, Na) can be the dominant electron donors. The transition to neutral carbon is also characterized by the formation of the molecule CO. This is a tightly bound molecule...
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