BIMODIAL DISTRIBUTION OF GALACTIC DISK STARS ON THE [α/Fe]–[Fe/H] PLANE AS POSSIBLE EVIDENCE OF DISCONTINUOUS RADIAL MIGRATION HISTORY

Abstract

We investigate the role of radial migration history of stars in chemical evolution of a disk galaxy, in particular in understanding the origin of their bimodal distribution on the [α/Fe]-[Fe/H] plane. For this purpose, we examine the three different models with no, continuous, and discontinuous radial migration, respectively. We find that for the model with radial migration, the [α/Fe] ratios of stars in outer disk regions decrease more rapidly with time than the model without radial migration, because the associated net transfer of intermediate and old disk stars from inner to outer disk regions increases the rate of Type Ia relative to that of Type II supernovae in the latter regions. Moreover, in the model assuming rapid and discontinuous radial migration, its effect on the stellar abundances at larger radii is significant enough to provide the large difference in the evolution of stars on the [α/Fe]-[Fe/H] plane between inner and outer disk regions. As a result we obtain the bimodal distribution of disk stars on the [α/Fe]-[Fe/H] plane as observed in the Galactic stellar disk, thereby implying that the event of discontinuous radial migration may play a key role in reproducing the observed bimodality of stars on this abundance-ratio diagram. We discuss possible mechanisms causing such discontinuous radial migration in the early evolution of the Galactic disk, including the event of minor merging of a relatively massive satellite onto the stellar disk.

Metallicity Distribution of Disk Stars and the Formation History of the Milky Way

“…Additionally, we note that a similar effect of radial migration on the stellar distribution on the [α/Fe]-[Fe/H] plane is also confirmed in Toyouchi & Chiba (2016), which can reproduce a bimodal stellar distribution on the [α/Fe]-[Fe/H] plane. According to the previous model calculation, a discontinuous radial migration of disk stars drastically decreases N Ia /N II in the inner disk regions, where the high-[α/Fe] sequence stars formed, and temporarily slows down the decrease of [α/Fe], consequently leading to the high-[α/Fe] peak of stars.…”

“…In this paper, we adopt a standard one-dimensional chemical evolution model along a galactic disk, as studied in our previous work (Toyouchi & Chiba 2016). In this model, we calculate baryonic mass evolution for a radial range from R = 0 to R out (= 16 kpc) with a grid of ∆R = 1 kpc over t = 0 to t p (= 12 Gyr) with a grid of ∆t = 50 Myr.…”

“…Investigating the origin of the observed radial dependence of the MDFs with semi-analytic models for studying chemical evolution of disk galaxies will provide new important implications for the formation histories of the Galactic stellar disk. Such chemical evolution models have been studied in many previous papers (e.g., Schönrich & Binney 2009;Kubryk et al 2015a;Toyouchi & Chiba 2016). In this work, we present a new semi-analytic chemical evolution model, which explicitly takes into account gas infall, outflow, re-accretion, and radial migration.…”

Variations in α-element Ratios Trace the Chemical Evolution of the Disk

“…Although the α-enriched component of the Milky Way has come to be associated with a "hotter disk" and the solar-α component often associated with a "cooler disk" (Bensby et al 2003;Navarro et al 2011;Bovy et al 2012b), the origin of these two populations, and whether they are unique, is still debated (Haywood et al 2016;Toyouchi & Chiba 2016;Bovy et al 2012a). Note that these hotter and cooler populations do not necessarily follow the same morphology as have previously been identified as the "thick" and "thin" disks (Gilmore & Reid 1983).…”

The age-metallicity structure of the Milky Way disc with APOGEE

“…There are ongoing theoretical attempts to reproduce features of the disk in terms of spatial and elemental abundance structure (e.g. Brook et al 2004, Chiappini et al 2009, Brook et al 2012, Minchev et al 2013, Martig et al 2014a,b, Andrews et al 2015, Toyouchi & Chiba 2016, Ma et al 2017, and these will be an important component in developing our understanding of these results.…”