Astron. Astrophys. 360, 1148-1156 (2000)

2. Emission line intensities

The number of photons observed in optically thin spectral line is given by

If the Contribution Function of the line is defined as

where

1. - is the relative upper level population;

2. - is the relative abundance of the ion (ion fraction ); this quantity is taken from the literature under the assumption of ionization equilibrium;

3. - is the abundance of the element X relative to hydrogen;

4. - is the hydrogen abundance relative to the electron density ( for fully ionized plasmas);

5. - is the Einstein coefficient for spontaneous emission.

and the Differential Emission Measure (DEM) is introduced as

the number of photons emitted in a spectral line may be expressed as

Throughout this work, use is made of the CHIANTI database (Dere et al. 1997; Landi et al. 1999a) and, for some ions not found in CHIANTI, of the Arcetri Spectral Code (Landi & Landini 1998a) to calculate theoretical line emissivities.

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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