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Emission lines and the far

Emission lines and the far - subscript B denotes case B the...

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Emission lines and the far-UV spectrum . Since recombination lines (and a few others) are powered ultimately by parts of the spectrum we can't see directly, they may give our only information on what happens between the UV and soft X-ray ranges. The treatment works like measurement of Zanstra temperatures for nuclei of planetary nebulae: for a nebula optically thick in the Lyman continuum (which they almost always are), the luminosity in a particular emission line is a constant known fraction of that in the Lyman continuum: balancing ionizations and recombinations, over the emitting volume V. The integration is carried up to a limit set by the amount of helium; an He ++ zone will block radiation below (912/4) Å before it encounters much hydrogen. The
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Unformatted text preview: subscript B denotes case B, the assumption of an optically thick nebula. Since, for example, the Hβ luminosity is we have in numbers of photons N (Hβ) = N(ion) α eff β / α B . which gives a fixed ratio between line and continuum intensities. For a given continuum shape, this gives a fixed (distance-independent) equivalent width (with respect to the ionizing continuum's optical tail) (Shuder 1981 ApJ 244, 12; Yee 1980 ApJ 241, 894). Similar arguments may be applied for He II emission, and for a few high-ionization forbidden lines like [Fe XIV] that require lots of far-UV to be produced....
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