lecture notes9

E lots of neutral hydrogen to reabsorb lyman series

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Unformatted text preview: his is followed by a sequence of radiative decays to successively lower values of n (with l changing by ±1 at each step). This sequence is subject to the € constraint that Lyman lines (decay to n=1) are blocked within the assumption of Case B (i.e. lots of neutral hydrogen to re ­absorb Lyman series radiation). The sequence of decays thus ends in one of two ways: Decay chain ends at 2s: In this case, the atom decays to 1s via emission of two photons. Decay chain ends at 2p: In this case, a Lyman ­α photon is produced. It begins to resonantly scatter within the nebula, i.e. it is re ­absorbed and re ­emitted in accordance with: H(1s) + γ Lyα → H(2p) → H(1s) + γ Lyα . The resonant scattering process terminates when: (i) the photon diffuses out of the nebula; (ii) the photon receives a Doppler boost in scattering off an H atom on the € tail of the Maxwellian (or turbulent) velocity distribution, thereby nudging its frequency far enough from the Lyman ­α line center that it can escape; (iii) in the case of an expanding nebula, the photon may redshift out of the line; (iv) the photon is absorbed by dust; (v) the photon is absorbed by a metal atom or ion; (vi) the transient H(2p) atom undergoes a collision or is photoionized. The intensity of emergent Lyman ­α radiation depends in detail on whether the ultimate fate of the photon is escape (i,ii,iii) or destruction (iv,v,vi). An immediate consequence of this argument is that since every recombination results in a decay chain that terminates at n=2, exactly one Balmer photon (either line or continuum) is generated in each recombination. This result can be modified in some cases (particularly due to dust), but it is qualitatively useful...
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