We present a scenario of the chemical enrichment of the solar neighborhood that solves the G-dwarf problem by taking into account constraints on a larger scale .
We argue that the Milky Way disk within 10 kpc has been enriched to solar metallicity by a massive stellar population : the thick disk , which itself formed from a massive turbulent gaseous disk .
While the inner disk , R \lesssim 6 kpc , continued this enrichment after a quenching phase ( 7-10 Gyr ) , at larger distances radial flows of gas diluted the metals left by the thick disk formation at a time we estimate to be 7-8 Gyr ago , thus partitioning the disk into an inner and outer region characterized by different chemical evolutions .
The key new consideration is that the pre-enrichment provided by the thick disk is not related to the mass fraction of this stellar population at the solar radius , as is classically assumed in inside-out scenarios , but is actually related to the formation of the entire massive thick disk , due to the vigorous gas phase mixing that occurred during its formation .
Hence , the fact that this population represents only 15-25 % of the local stellar surface density today , or 5-10 % of the local volume density , is irrelevant for “ solving ” the G-dwarf problem .
The only condition for this scenario to work is that the thick disk was formed from a turbulent gaseous disk that permitted a homogeneous – not radially dependent – distribution of metals , allowing the solar ring to be enriched to solar metallicity .
At the solar radius , the gas flowing from the outer disk combined with the solar metallicity gas left over from thick disk formation , providing the fuel necessary to form the thin disk at the correct metallicity to solve the G-dwarf problem .
Chemical evolution at R > 6 kpc , and in particular beyond the solar radius , can be reproduced with the same scheme .
We suggest that the dilution , occurring at the fringe of the thick disk , was possibly triggered by the formation of the bar and the establishment of the outer Lindblad resonance ( OLR ) , enabling the inflow of metal poorer gas from the outer disk to R \sim 6 kpc , presumably the position of the OLR at this epoch , and at the same time isolating the inner disk from external influence .
These results imply that the local metallicity distribution is not connected to the gas accretion history of the Milky Way .
Finally , we argue that the Sun is the result of the evolution typical of stars in the disk beyond \sim 6 kpc ( i.e. , also undergoing dilution ) , and has none of the characteristics of inner disk stars .