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When such reactions are exothermic they have zero

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Unformatted text preview: e have Q(He0):Q(H0)>fHe. In this case, the He+ zone fills the entire ionized nebula. The He+ zone cannot extend outside of the H+ zone, since H0 absorbs the >24.6 eV photons, so in these cases a single ionization zone develops with a boundary determined by the flux of >24.6 eV photons. B. He2+ ZONES We now consider nebulae with central sources emitting photons above 54.4 eV. These could include planetary nebulae, AGN, and the postulated supermassive metal ­free stars (“Population III”) that may have existed in the first galaxies. As one might guess, this leads to the formation of a He2+ zone surrounding the central source, whose radius is given by 4 32 πr3 n H f He (1 + 2 f He )α B (He + , T ) = Q(He + ). 3 We have r3<r1 even for source temperatures T*~105 K. € 4. Metal Ionization Metal atoms are rare in nature: for example, solar abundances2 are (by number) O:H = (4.9±0.6)×10−4, C:H = (2.7±0.3)×10−4, and even lower for everything else. Therefore as far as the ionization structure is concerned, they have no major effect and merely respond to their environment. (As far as the thermal structure and emitted spectra are concerned they will be very important.) Their ionization state in each zone is determined by their ionization potential, photoionizations and recombinations, and charge ­transfer reactions. A. FUNDAMENTAL PROCESSES Metal atoms can recombine in two ways. One is the standard radiative recombination to any state of the atom, e.g.: O 2 + (2p 2 3 P e ) + e− → O + (2p 2 4 So ) + γ . In some cases, the rate can be enhanced by dielectronic recombination, which is the inverse of autoionization followed by radiative decay, e.g.: € C 2 + (2s2 1Se ) + e− ↔ C + (2s2p( 3 P o )3d 2 F o ), C + (2s2p( 3 P o )3d 2 F o ) → C + (2s2p 2 2 De ) + γ , C + (2s2p 2 2 De ) → C + (2s2 2p 2 P o ) + γ . Note that the initial ion is unstable against autoionization, but sometimes decays radiatively. € Photoionization can proceed either by direct photoionization, or excitation to an autoionizing state. Note that photoionization depends on the availability of photons above the ionization energy of the atom in question. There is a cutoff energy in the spectrum of such photons of 13.6 eV (in an H I region), 24.6 eV (in an H+/He0 zone), 54.4 eV (in a He+ zone), or ∞ (in a He2+ zone). Finally, charge transfer, in which an ion collides with a neutral atom and transfers an electron, can be important: O + + H ↔ O + H + . When such reactions are exothermic, they have zero activation energy (due to the induced dipole attraction) and do not require a photon to be radiated. Thus their rate coefficients € δ(T) tend to be of order 10−9 cm3 s−1, i.e. in neutral regions they are much faster t...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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