Astron. Astrophys. 347, 401-408 (1999)

2. Theoretical calculations

The energy emitted by an optically thin plasma per unit volume, wavelength and time may be expressed as

where is the plasma emissivity, given by

is the continuum Contribution Function ; is the sum of all the single line Contribution Function found in the selected wavelength band. Each line Contribution Function is given (in erg cm³ s^-1) by

where

• is the relative upper level population of the ion ;

• is the relative abundance of the ion ;

• is the abundance of the element X relative to Hydrogen;

• is the hydrogen abundance relative to the electron density ();

The total radiated energy per unit volume and time E_rad lost by the plasma is then given by the sum of the emission at all wavelengths:

while the total radiated power per unit (total emissivity) of the emitting plasma is given by

Here a brief description of the Arcetri Code is given, for further details the reader is referred to the quoted paper.

The Arcetri Code consists of a set of routines and a database of atomic parameters and transition probabilities necessary to calculate line and continuum emission from an optically thin plasma. The continuum radiation is evaluated according to Landini & Monsignori Fossi 1990 including free-free, free-bound and two-photon processes. Line radiation is calculated for most of the ions of astrophysical interest using two different approximations: (a) solving the statistical equilibrium equations for ion level population or (b) using the Coronal Model Approximation . The Arcetri Code covers the 1-2000 Å spectral range, and calculates line emission for approximately 175 ions. The atomic data and transition probabilities database necessary for using method (a) is the same as for the CHIANTI project (Dere et al. 1997, Landi et al. 1999).

The calculation of the theoretical spectrum of an optically thin plasma depends on a number of parameters: electron density, element abundances, ion fractions, atomic data and transition probabilities for the calculation of level populations for the emitting ions. In the present work we investigate the effects of these parameters in the resulting radiative losses. For this reason we have calculated the theoretical spectrum for several values of the electron density ( cm^-3), using different sets of element abundances (Allen 1973, Feldman 1992, Grevesse & Anders 1991, Meyer 1985 and Waljeski 1994) and ion fraction calculations (Shull & Steenberg 1982, Arnaud & Rothenflug 1985, Arnaud & Raymond 1992, Mazzotta et al. 1998) under the assumption of ionization equilibrium. Also, in order to assess the improvement of the new dataset of atomic parameters and transition probabilities on the radiative losses curves we have performed a comparison between the present results and those obtained with and old version of the Arcetri Code found in Landini & Monsignori Fossi 1990.

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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