Fractional yields inferred from halo and thick disk stars

R. Caimmi

Published 2013-06-28, updated 2013-12-19Version 3

Linear [Q/H]-[O/H] relations, Q = Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, are inferred from a sample (N=67) of recently studied FGK-type dwarf stars in the solar neighbourhood including different populations. Regression line slope and intercept estimators and related variance estimators are determined. With regard to the straight line, [Q/H] = aQ [O/H] + bQ, sample stars display along a "main sequence", [Q,O] = [aQ,bQ, Delta bQ], leaving aside the two OL stars which, in most cases (e.g., Na), lie outside. A unit slope, aQ = 1, implies Q is a primary element synthesised via SNII progenitors in presence of universal stellar initial mass function (defined as simple primary element). To this respect, Mg, Si, Ti, show aQ = 1 within 2 sigma(aQ); Cr, Fe, Ni, within 3 sigma(aQ); Na, Ca, exceeding 3 sigma(aQ). The empirical, differential element abundance distributions are inferred from different subsamples, where related regression lines represent their theoretical counterparts within the framework of simple MCBR (multistage closed box + reservoir) chemical evolution models. Hence the fractional yields are determined and (as an example) a comparison is shown with their theoretical counterparts inferred from SNII progenitor nucleosynthesis under the assumption of a power-law stellar initial mass function. The generalized fractional yields are determined regardless from the chemical evolution model. The ratio of outflow to star formation rate is compared for different populations, in the framework of simple MCBR models. The opposite situation of element abundance variation entirely due to cosmic scatter is also considered under reasonable assumptions. The related differential element abundance distribution fits to the data as well as its counterpart inferred in the opposite limit of instantaneous mixing in presence of chemical evolution.