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

TL;DR

This paper extends simple chemical evolution models to include dominant gas flows and applies them to the thin and thick disks of the galaxy, analyzing their formation stages and gas flow regimes.

Contribution

It introduces an extended MCBR model accounting for dominant gas inflow/outflow and applies it to empirical data of galactic disks, exploring formation stages and evolution constraints.

Findings

The gas mass fraction peaks and then decreases with increasing flow parameter.

The F stage of evolution is consistent with steady inflow, matching hydrodynamical simulations.

The model evaluates the impact of low- and high-metallicity tails on mass fractions.

Abstract

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.