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# Since the grain mass per h atom determined by the

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Unformatted text preview: release it into the gas phase. The rate coefficient for the catalyzed reaction dust 2H → H 2 can be computed assuming that all hydrogen atoms colliding with a dust grain react. The coefficient (units: cm3/s) is € 2 k= 1 2 2 ∫ πa v H 1 dn gr da , n H da where we have used the mean velocity vH of the hydrogen atoms, which is proportional to T1/2. If all grains had the same size a, then we could write this € integral in terms of the grain volume: n V 1 1 3 vH k = πa 2v H gr = πa 2v H 4 gr 3 = Vgr ; 2 nH 2 πa 8a 3 here Vgr is the grain volume per H atom. Since the grain mass per H atom (determined by the abundance of the depleted metals) is ~0.005mH, we have Vgr ~ 3×10−26 cm3. € he typical size is a~0.2 μm, and the mean velocity is 5×105T31/2 cm/s. T Thus we find k ~ 3 × 10−16 T31 / 2 cm3 /s. This simplistic argument appears to be roughly correct, although the actual rate derived from observations of diffuse clouds is a factor of a few lower. € B. H2 PHOTODISSOCIATION AND FLUORESCENCE The destruction of H2 is usually driven by UV radiation, since at molecular cloud or even PDR temperatures collisional dissociation is inefficient. One might at first imagine that the reaction: H 2 + γ → 2H is possible; however as discussed earlier there is no dipole moment. One might then imagine direct photoionization: € H 2 + γ → H + + e− , 2 but the photon energy required is at least 15 eV and hence cannot occur in regions shielded by neutral hydrogen. In fact, the principal i€ nteraction of UV photons with H2 involves excited electronic states. Recall that the ground electronic state of H2, X 1Σ + , consists of both g electrons in the lowest σg orbital....
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## This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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