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

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2. Emission line intensities

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


If the Contribution Function [FORMULA] of the line is defined as



  1. - [FORMULA] is the relative upper level population;

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

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

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

  5. - [FORMULA] 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.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000