Calculation of Abundances

VALD requires that the temperature, microturbulence, and the log of the surface gravity be given to construct the model star.  It allows it to create the correct line depths for the spectral lines.  However, it does not require that abundances be entered into the query.  When no elements are entered into VALD it defaults to the solar abundances.  Central depth is the most important piece of information given by the query.  Matching the lines of the VALD query to the lines of the data, the line depths can then be placed in a ratio.  This ratio also carries over to the abundances.  If, for instance, a star had central depths which where 50% the size of the query’s depths, 50% would be converted to a decimal (.5) and then the log of that decimal would be added to the abundance given by the query and the abundance of the data is given.  The log of the fraction is used to calculate abundances because the abundances are in a logarithmic notation as well.  The abundances of the stars were calculated using the central depths.  
                                                                                                                                                                                                                   Spectrum of a metal-poor star     Credit: NOAO

Just like pH, the abundances compare the amount of an element to the amount of hydrogen in a star.  It is based on every 1 X 10¹² hydrogen atoms.  If a star had 1.0 X 10^7.5 atoms of iron for every 1 X 10¹² hydrogen atom, its abundance would be -4.5, or 7.5-12, the exponents of the amounts of iron and hydrogen.  The iron example is actually the classic indicator of metallicity.  [Fe/H] plays a big role in determining if a star is metal poor or not.

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