A simple multistage closed-(box+reservoir) model of chemical evolution

TL;DR

This paper extends simple chemical evolution models to include inflow and outflow, creating multistage models that better fit observed oxygen abundance distributions in the Galactic halo and bulge.

Contribution

It introduces multistage closed-(box+reservoir) models allowing variable inflow and outflow rates across different evolutionary stages.

Findings

Theoretical oxygen abundance distribution is a broken line with slopes related to inflow rates.

Empirical distributions can be approximated by broken lines, fitting the model.

Inner halo mass can be comparable to or exceed the metal-poor bulge mass.

Abstract

Simple closed-box (CB) models of chemical evolution are extended on two respects: (i) simple closed-(box+reservoir) (CBR) models allowing gas outflow from the box into the reservoir or gas inflow into the box from the reservoir with same composition as the preexisting gas and rate proportional to the star formation rate, and (ii) simple multistage closed-(box+reservoir) (MCBR) models allowing different stages of evolution characterized by different inflow or outflow rates. The stellar initial mass function is assumed to be universal, and mass conservation holds for the whole system (box+reservoir) while it is violated for each subsystem (box and reservoir). The theoretical differential oxygen abundance distribution (TDOD) predicted by the model, under the assumption of instantaneous recycling, is a continuous broken line, where different slopes are related to different inflow rates. For an application of the model (a) a fictitious sample is built up from two distinct samples and taken as representative of the inner Galactic halo, and (b) different [O/H]-[Fe/H] empirical relations are deduced from five different samples related to different methods, and two of them are selected for determining the empirical differential oxygen abundance distribution (EDOD) with regard to the fictitious sample. In both cases the EDOD is represented, to an acceptable extent, as a continuous broken line. If the inner halo and the metal-poor bulge (after the inflow stage) are represented by the box and the reservoir, respectively, then the inner halo fractional mass (normalized to the halo) is comparable with, or exceeds by a factor up to 4, the metal-poor bulge fractional mass (normalized to the bulge), for current estimates of the halo-to-bulge mass ratio of about 0.05-0.10.