non-LTE effects in abundance determinations
Background
The abundance of elements is a crucial parameter for the understanding of the origin of the elements. The tool used is the spectroscopic analysis of spectral lines. Normally one assumes local thermodynamic equilibrium (LTE) in such analysis. This may be dangerous as deviations from LTE may be substantial.Goal
The goal of the project is to look at the non-LTE formation of lines from a few elements in various types of stars to check whether the assumption of LTE is good or not for lines that are common in abundance analysis.Method
MULTI is used to solve the equations of statistical equilibrium for a given model atom and model atmosphere.Procedure
1. Choose one element. If several students choose this project you should choose different elements. Possible elements are Li, Mg, Si, Na and O (for these we have good model atoms - construction of a model atom from scratch is too laborious for a term project). Choose lines that are of interest. The lines should be in the optical part of the spectrum and reasonably strong but pure absorption lines with no emission peaks.You should copy model atoms (atom.ext) and input file (input.ext) from ~matsc/multi/input:
| Atom | ext |
| Li | li12 |
| Mg | mg13 |
2. Modify the existing model atom to make sure the lines of interest are well treated (modify NQ, Q0 and QMAX in the atom file to get the line profile well covered, use atmos.val3c for this exercise and copy the dscale.ca6_val3c file into dscale.ext_val3c).
3. Choose atmospheric models for the project. Atmospheric models can
be copied from ~matsc/multi/marcs. An overview of existing model
atmospheres:
ls ~matsc/multi/input/marcs/atmos.t**
one example is
atmos.t6000g4.00m0.00x1.0
This is a model atmosphere with an effective temperature of 6000K, lg(g)=4.00, [Fe/H]=0.00, Vmic=1.0 km/s
There are corresponding dscale files (for the atoms "li12" and "mg13" but the same can be used for any atom as starting point).
4. Check the formation of the chosen lines. Compare non-LTE with LTE. Find explanations of the non-LTE effects if they are important and find out how they depend on the atmopsheric parameters (Teff, lgg, [Fe/H])