lecture notes7

Lecture notes7

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Unformatted text preview: of O and C+ are the main coolants since H has no 1 Tielens §3.3 6 low ­lying excitations (except for the extremely weak 21 cm line). Excitation may be via either electron collisions, C + ( 2P1o2 ) + e− → C + ( 2P3o/ 2 ) + e− , / or by neutrals: € C + ( 2P1o2 ) + H → C + ( 2P3o/ 2 ) + H. / The former reaction has the advantage of Coulomb focusing, and hence has a rate coefficient typically ~100 times larger, but in a mostly neutral region such as a € PDR, the latter can be significant. Neither process actually uses the electron spin; rather the 2p electron’s orbital angular momentum is changed, or it is swapped for the free electron or that of the H atom. The excited atoms emit in the [C II] 158 μm and [O I] 63, 146 μm lines. The former has a lower excitation energy (90 K for [C II] 158 μm versus 230 K for [O I] 63 μm), and hence dominates at low temperatures. The latter has a higher critical density (~106 cm−3 for H collisions) so in dense regions at moderate temperatures it may dominate. The cooling rate via the 63 μm line is, in the low density limit, 2.5✕10−29 nH T0.67 e−230 K/T erg/s/H atom. We may set this equal to the heating rate to obtain the equation for the temperature: 0.0025 n HT 0.67e−230 K / T = G0 . This is a very rough approximation, but it illustrates the main point. At constant pressure, nHT = P/k = constant, and the left hand side is a slowly falling € power law (T−0.33) times a rapidly rising exponential. An equilibrium is usually found at temperatures of a few hundred K. There is, however, a maximum of the left ­hand side that is proportional to pressure. If the pressure is too...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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