Abstract
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 ≲ 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 ∼ 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 ∼6 kpc (i.e., also undergoing dilution), and has none of the characteristics of inner disk stars.
Highlights
The picture we propose is different from the standard gas infall schemes in the following ways: Thick disk growth
The initial growth of the metal content of the interstellar medium (ISM) is explained by a massive population of stars which we associate with the thick disk
We argue that the high level of turbulence and feedback in the ISM at the epoch of thick disk formation allowed the outskirts of the thick disk to have a chemical
Summary
Cold gas accretion (Dekel & Birnboim 2006; Woods et al 2014; Tillson et al 2015), which in the last ten years has become the new paradigm describing how galaxies acquire their gas, predicts that considerable gas accretion occurs along a few dark matter filaments (Birnboim & Dekel 2003; Kereš et al 2005, 2009; Ocvirk et al 2008; Agertz et al 2009; Cornuault et al 2018), driving large amounts of fuel in the inner parts of galaxies, permitting the early buildup of large disks (Genzel et al 2006, 2017; Toft et al 2017), and possibly leading to the formation of large gas reservoirs (Davé et al 2012; Papovich et al 2011; Hopkins et al 2014; Suess et al 2017). We explore the simple idea that if the formation of the thick disk is a global process (i.e., not inside-out), the enrichment it provides cannot be accounted for in proportion to its local mass fraction, but that it results from the chemical evolution of an entire massive population (a few 1010 M ) of the inner disk In this scheme, the solar ring, at the outskirts of the thick disk, may have been enriched by this massive stellar population of the thick disk, due to the efficient mixing within the ISM that prevailed at this epoch, solving the long standing G-dwarf problem. We refer the reader to the study of Buder et al (2019) and the GALAH survey
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