Astron. Astrophys. 319, 630-636 (1997)

3. AAP - abundance analysis procedure

The program AAP is a new tool which was developed for an easy, largely automatic and user friendly abundance analysis. In its present version AAP uses model atmospheres with scaled solar abundances, based on precalculated opacity distribution functions. A description of AAP, including a user's manual, is given in Gelbmann (1995).

The procedure makes use of the Vienna Atomic Line Database VALD (Piskunov et al. 1995), to compile spectral line lists of selected or all chemical elements. VALD is a set of critically evaluated lists of astrophysically important atomic transition parameters and includes supporting extraction software.

The main menu items of AAP are:

Calculate model atmosphere: computes atmospheres with chosen effective temperature, surface gravity, metallicity, and microturbulent velocity.

Select wavelength range: defines the desired wavelength range and starts PRESELECT and SELECT, written by Kupka and Piskunov.

PRESELECT scans through all line lists of VALD and creates an output table which contains the wavelength, the Kurucz-code of the element with ionization stage, the gf-value, the lower and upper energy level, the effective Landé-factor, and the three line broadening constants for each line found. This list contains all data needed by AAP to calculate synthetic spectra in the given wavelength region.

SELECT calculates the core depths for all lines in the preselected list and selects only those lines which contribute significantly to the synthetic spectrum. Spectral lines with a line depth smaller than a chosen limit are rejected.

Generate line list: creates a subset of the previously selected line list of a particular chemical element in a given wavelength range. Blended lines with a chosen ratio in line depth of the components (typically more than 30%) within a chosen interval (typically 0.3Å) are rejected. In addition it is possible to eliminate all spectral lines which are in critical wavelength regions like the wings of the Hydrogen Balmer lines or close to telluric lines. Hence, only those lines which are considered to be sufficiently free from blends are included in this subset.

Synthesize lines: calculates a synthetic spectrum with SYNTH and ROTATE, written by Piskunov (1992), and takes into account various broadening mechanisms. Either all spectral lines of all chemical elements in a chosen wavelength range are used, or all spectral lines of a line list of a particular chemical element. To compute Balmer profiles the program BALMER9 (Kurucz 1993) is used.

It is possible to fit automatically an unblended synthetic spectral line to an observed line. This autofit algorithm minimizes the difference between the observed and the synthetic spectrum, i.e. the area between the two spectra, with the method of the golden section search (Press et al. 1986). The free parameter for this minimization is the element abundance. Hence, an accepted fit of a synthetic line to the observations gives a `line' abundance for a chosen atmosphere (see Fig. 4). The final element abundance results as a mean from individual fits to all unblended spectral lines of that element.

Plot abundances: produces diagrams of `line' abundances versus atomic line parameters for a given chemical element. Since different atomic line parameters are sensitive to different stellar parameters, it is possible to check the consistency of the chosen atmospheric parameters. If no trends are found between `line' abundances and the equivalent widths (primarily sensitive to ), the lower energy levels (), the ionization stages (), and the effective Landé factors (magnetic field), the chosen atmosphere is assumed to be correct. A more detailed description of the explained method is given in Wehrse (1988), especially with respect to the achievable accuracies (systematic errors) and with more detailed considerations on secondary dependencies of atomic line parameters on various stellar parameters. An example of such an analysis based on an incorrectly chosen microturbulent velocity is given in Fig. 1. Diagrams for the finally adopted atmospheric parameters are shown in Figs. 5 and 6 (different scale!).

Fig. 1. Iron `line' abundances plotted versus the equivalent widths, based on a model atmosphere with of 2 km s-1 which is too large (for comparison see Fig. 6). The dashed line is a linear least squares fit.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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