lecture notes9

and density since the fine structure excitations are

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Unformatted text preview: connect to it (10830 Å, 3889 Å). By partially blocking decays to He I 2 3 S1 , this can redistribute the He I triplet recombination line intensities. It can also result in collisional o excitation out of He I 2 3 S1 followed by line emission. € C. RESONANCE FLUORESCENCE € A final process that can affect the spectrum of H II regions is resonance fluorescence – the excitation of one species by line radiation from another, followed by a decay sequence that emits lower ­energy photons. Two important examples are: O III: He + (2p) → He + (1s) + γ 304 A O 2 + (2p 2 3 P2e ) + γ 304 A → O 2 + (2p3d 3 P2o ) [20 fluorescence lines at 2809 … 3811 Å] O I: H( 3p) → H(1s) + γ1026 A O(2p 4 3 P2e ) + γ1026 A → O(2p 3 3d 3 Do ) 3 O(2p 3 3d 3 Do ) → O(2p 3 3p 3 P2e ) + γ11287 A 3 o O(2p 3 3p 3 P2e ) → O(2p 3 3s 3 S1 ) + γ 8446 A € o O(2p 3 3s 3 S1 ) → O(2p 4 3 P2e,1,0 ) + γ1302,1305,1306A Both of these processes can lead to emission in metal lines that are too weak to observe in recombination and have excitation energies that are too high for collisions. 4. Diagnostics We are now in a position to consider diagnostics for the temperature and density of H II regions. We have already described the theory, so we will mostly list the basic ideas behind each diagnostic here. 9 A. TEMPERATURE Diagnostics for the temperature include: Forbidden lines of np2 and np4 ions: These ions, [N II], [O III], [Ne III] and [S III], have a first excited state that emits a doublet (e.g. [O III]λ4959,5007Å) and a higher excited state that emits a single line (e.g. [O III]λ4363Å). There is a rather substantial energy difference, so the ratio of the single to the double line reflects the Boltzmann suppression in the collisional excitation rate (i.e. it increases with temperature). This method has the advantage of using a single species and hence being insensitive to the ionization stage or abundance of the metal. At high densities, it may suffer problems because the doublets are longer ­lived and hence more ea...
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