Oxygen abundance in local disk and bulge: chemical evolution with a strictly universal IMF

Abstract

This paper has two parts: one about observational constraints related to the empirical differential oxygen abundance distribution (EDOD), and the other about inhomogeneous models of chemical evolution, in particular the theoretical differential oxygen abundance distribution (TDOD). In the first part, the EDOD is deduced from subsamples related to two different samples involving (i) N=532 solar neighbourhood (SN) stars within the range, -1.5<[Fe/H]<0.5, for which the oxygen abundance has been determined both in presence and in absence of the local thermodynamical equilibrium (LTE) approximation (Ramirez et al. in Astron. Astrophys. 465:271, <CitationRef CitationID="CR47">2007</CitationRef>); and (ii) N=64 SN thick disk, SN thin disk, and bulge K-giant stars within the range, -1.7<[Fe/H]<0.5, for which the oxygen abundance has been determined (Melendez et al. in Astron. Astrophys. 484:L21, <CitationRef CitationID="CR36">2008</CitationRef>). A comparison is made with previous results implying use of [O/H]-[Fe/H] empirical relations (Caimmi in Astron. Nachr. 322:241, <CitationRef CitationID="CR13">2001b</CitationRef>; New Astron. 12:289, <CitationRef CitationID="CR14">2007</CitationRef>) related to (iii) 372 SN halo subdwarfs (Ryan and Norris in Astron. J. 101:1865, <CitationRef CitationID="CR51">1991</CitationRef>); and (iv) 268 K-giant bulge stars (Sadler et al. in Astron. J. 112:171, <CitationRef CitationID="CR53">1996</CitationRef>). The EDOD of the SN thick + thin disk is determined by weighting the mass, for assumed SN thick to thin disk mass ratio within the range, 0.1-0.9. In the second part, inhomogeneous models of chemical evolution for the SN thick disk, the SN thin disk, the SN thick + thin disk, the SN halo, and the bulge, are computed assuming the instantaneous recycling approximation. The EDOD data are fitted, to an acceptable extent, by their TDOD counterparts with the exception of the thin or thick + thin disk, where two additional restrictions are needed: (i) still undetected, low-oxygen abundance thin disk stars exist, and (ii) a single oxygen overabundant star is removed from a thin disk subsample. In any case, the (assumed power-law) stellar initial mass function (IMF) is universal but gas can be inhibited from, or enhanced in, forming stars at different rates with respect to a selected reference case. Models involving a strictly universal IMF (i.e. gas neither inhibited from, nor enhanced in, forming stars with respect to a selected reference case) can also reproduce the data to an acceptable extent. Our main conclusions are (1) different models are necessary to fit the (incomplete) halo sample, which is consistent with the idea of two distinct halo components: an inner, flattened halo in slow prograde rotation, and an outer, spherical halo in net retrograde rotation (Carollo et al. in Nature 450:1020, <CitationRef CitationID="CR16">2007</CitationRef>); (2) the oxygen enrichment within the inner SN halo, the SN thick disk, and the bulge, was similar and coeval within the same metallicity range, ...

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

“…On the other hand, the latter value would be against a thick disk -thin disk collapse, unless a massive thick disk (with respect to the thin disk) is recognized. Finally, if a single (thick + thin) disk population exists, the EDOD maintains close to its counterpart inferred both from the PM11n sample, due to the smallness of the PM11k sample, and from the Ra07n sample, as shown in an earlier attempt (Caimmi and Milanese 2009). The family of TDOD curves, considered for each stage of evolution, is characterized by two common points, (φ i , ψ i ), (φ f , ψ f ), which make the boundary of the related linear fit.…”

“…The latter alternative is in contradiction with current observations, which seem to exclude the presence of a massive thick disk, while the former remains available on this respect. If thick disk and thin disk stars belong to a single population (e.g., Bovy et al 2012), no prediction can be made for or against a thick disk -thin disk collapse and the TDOD is expected to be slightly different from its counterpart related to the thin disk (Caimmi and Milanese 2009).…”

“…For this reason, the population of available samples does not exceed a few hundredths at most. For further details refer to earlier attempts (Caimmi 2007(Caimmi , 2008Caimmi and Milanese 2009;C11).…”

“…With regard to Ra07 samples, the EDOD has been inferred in an earlier attempt (Caimmi and Milanese 2009) both in presence and in absence of the LTE approximation and only the latter case shall be considered in the current investigation.…”

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

“…With regard to the Galaxy, the empirical differential oxygen abundance distribution (EDOD) shows that (i) at least two different stages of (chemical) evolution exist, and (ii) each stage can be described using an extended simple CB model, for both the inner halo (C07), the bulge (C07), the thick disk (Caimmi and Milanese 2009, hereafter quoted as CM09), and the thin disk (CM09). In particular, the earlier stage is marked by an initially increasing EDOD.…”