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Simple closed-box (CB) models of chemical evolution are extended on tworespects: (i) simple closed-(box+reservoir) (CBR) models allowing gas outflowfrom the box into the reservoir or gas inflow into the box from the reservoirwith same composition as the preexisting gas and rate proportional to the starformation rate, and (ii) simple multistage closed-(box+reservoir) (MCBR) modelsallowing different stages of evolution characterized by different inflow oroutflow rates. The stellar initial mass function is assumed to be universal,and mass conservation holds for the whole system (box+reservoir) while it isviolated for each subsystem (box and reservoir). The theoretical differentialoxygen abundance distribution (TDOD) predicted by the model, under theassumption of instantaneous recycling, is a continuous broken line, wheredifferent slopes are related to different inflow rates. For an application ofthe model (a) a fictitious sample is built up from two distinct samples andtaken as representative of the inner Galactic halo, and (b) different[O/H]-[Fe/H] empirical relations are deduced from five different samplesrelated to different methods, and two of them are selected for determining theempirical differential oxygen abundance distribution (EDOD) with regard to thefictitious sample. In both cases the EDOD is represented, to an acceptableextent, 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) iscomparable with, or exceeds by a factor up to 4, the metal-poor bulgefractional mass (normalized to the bulge), for current estimates of thehalo-to-bulge mass ratio of about 0.05-0.10.

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