Unformatted text preview: zone model is seldom appropriate; a small amount of hot gas may skew the results. Also, high-density gas will show relatively strong λ 4363 because the other lines are collisionally de-excited. There is a similar set of lines for the next ion down in the same isoelectronic sequence, [N II] with lines at 5755, 6548, and 6584 Å. For densities, we want pairs of lines from energy levels with nearly the same excitation energy but quite different A-values; now the thermal populations will be the same to within the statistical weights, and the line intensities will tell what fraction of atoms in each level are collisionally de-excited before radiating. The premier examples are [S II] λ 6717,6731 and [O II] λ 3726,3729. Cruder determinations are possible using the fact that lines with critical densities far below the actual density will be invisibly weak - this is how densities in broad-line regions are determined....
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.
- Fall '10