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lecture notes7

Since a pdr defined broadly absorbs much of the

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Unformatted text preview: (bond energy 11 eV) and its formation can be catalyzed by a variety of nonradiative molecular reactions starting from O, H2, and C/C+. Examples include the sequence involving radicals: O + H 2 → OH + H C + + OH → CO + + H CO + + H → CO + H + . The radicals and CO are susceptible to photodissociation. € B. HEATING The heating sources in PDRs are all associated either directly or indirectly with the young stars and their UV/optical radiation. The major processes are: Photoelectric effect on dust: Dust grains (including the PAHs) can undergo the photoelectric effect, i.e. the removal of electrons from solids. This is really the same phenomenon as photoionization but applied to bulk material. The ejected electron carries kinetic energy that heats the gas. Carbon photoionization: This is the analogue of photoionization of H or He in H II regions. It is generally of minor importance compared to dust. 5 Collisional de excitation of H2: Hydrogen molecules excited in the Lyman and Werner bands can decay back to excited vibrational levels of the ground electronic state. At the high densities in PDRs, and given the quadrupole nature of the radiative decays, the H2 molecules may collisionally de ­excite, dumping this vibrational energy into the kinetic degrees of freedom of the gas. Cosmic rays: These are a subdominant source of heating at the surface of the PDR, but will be significant when we investigate molecular clouds because they can penetrate into the deep interior where UV/optical radiation is shielded. Since a PDR (defined broadly) absorbs much of the incident ultraviolet radiation through dust, one might expect the photoelectric effect to dominate. This is usually the case at low AV (indeed it appears to be the case for the diffuse phases of the...
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