Temperature and Density Determinations

Temperature and Density Determinations - zone model is...

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Temperature and Density Determinations: All line ratios that are not fixed by atomic parameters are to some extent dependent on both temperature and electron density. However, for certain ratios the dependence on one is much stronger, and these are particularly valuable as diagnostics of physical conditions in nebulae. For temperature determinations, we want to measure the ratio of electrons with different energies - so we look for pairs of lines of the same species with very different excitation energies. The most-used example is [O III] with lines at 4363 and 4959+5007 Å. As long as the density is below the critical density for the intermediate level from which the two strong lines arise, the populations of all the levels involved reflect the electron energy distribution and thus the electron temperature. As an analytic approximation, The line ratio is about 1000 at 6000 K, dropping to 100 at 14000 K. However, beware that a one-
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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.

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