FRACTIONAL YIELDS INFERRED FROM HALO AND THICK DISK STARS

Abstract

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 (Nissen and Schuster 2010, Ramirez et al. 2012), namely LH (N = 24, low-α halo), HH (N = 25, high-α halo), KD (N = 16, thick disk), and OL (N = 2, globular cluster outliers). 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 are displayed along a "main sequence", [Q,O] = [aQ, bQ, ΔbQ], leaving aside the two OL stars, which, in most cases (e.g. Na), lie outside. The unit slope, aQ = 1, implies Q is a primary element synthesised via SNII progenitors in the presence of a universal stellar initial mass function (defined as simple primary element). In this respect, Mg, Si, Ti, show âQ = 1 within -+2^σâQ; Cr, Fe, Ni, within -+3^σâQ; Na, Ca, within -+r^σâQ, r > 3. The empirical, differential element abundance distributions are inferred from LH, HH, KD, HA = HH + KD 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, ^pQ/^pO, 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, CQ=ZQ/ZaQ O, are determined regardless of 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 the presence of chemical evolution, while the latter is preferred for HA subsample.