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Simple MCBR models of chemical evolution: an application to the thin and the thick disk

Simple MCBR models of chemical evolution are extended to the limit of dominant gas inflow or outflow with respect to gas locked up into long-lived stars and remnants. For an assigned empirical differential oxygen abundance distribution, which can be linearly fitted, a family of theoretical curves is built up with assigned prescriptions. For curves with increasing cut parameter, the gas mass fraction locked up into long-lived stars and remnants is found to attain a maximum and then decrease towards zero as the flow tends to infinity, while the remaining parameters show a monotonic trend. The theoretical integral oxygen abundance distribution is also expressed. An application is performed to the empirical distribution deduced from two different samples of disk stars, for both the thin and the thick disk. The constraints on formation and evolution are discussed in the light of the model. The evolution is tentatively subdivided into four stages, A, F, C, E. The empirical distribution related to any stage is fitted by all curves for a wide range of the cut parameter. The F stage may safely be described by a steady inflow regime, implying a flat theoretical distribution, in agreement with the results of hydrodynamical simulations. Finally, (1) the change of fractional mass due to the extension of the linear fit to the empirical distribution, towards both the (undetected) low-metallicity and high-metallicity tail, is evaluated and (2) the idea of a thick disk-thin disk collapse is discussed, in the light of the model.