Galactic r-process Abundance Feature Shaped by Radial Migration

Abstract

Growing interests in the chemical feature of r-process elements among nearby disk stars represented by the [Eu/Fe] vs. [Fe/H] diagram have sprouted since it can assess the origin of r-process elements through the comparison with theoretical models, including a test as to if neutron star mergers can be the major site of r-process nucleosynthesis. On the other hand, recent studies reveal that local chemistry is strongly coupled with the dynamics of Galactic disk, which predicts that stars radially move on the disk where the observed elemental feature is different at various Galactocentric distances.Here, we show that radial migration of stars across the Galactic disk plays a crucial role in shaping the r-process abundance feature in the solar vicinity. In this proposed scenario, we highlight the importance of migration from the outer disk where [r-process/Fe] of some old stars is predicted to be enhanced to the level beyond the expectation from the observed Galactic Fe and Eu radial gradient, which results in a large span of [r-process/Fe] among nearby disk stars. The variation in the [r-process/Fe] ratio seen across the Galactic disk as well as in dwarf galaxies may be an outcome of different stellar initial mass functions which change the occurrence frequency between supernovae leaving behind neutron stars and ones ending with black holes. Here we propose that enhancement in [Eu/Fe] is attributed to the initial mass function lacking high-mass stars such as > ∼ 25M ⊙ in the scheme for which neutron star mergers are a major source of r-process elements.

Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies

“…In addition to a standard infall model, therefore, we also examine a two-infall model (Chiappini et al 1997), in which the first and second infall episodes are assumed to correspond to those of the thin and thick discs, respectively (Spitoni et al 2019;Tsujimoto & Baba 2019;Palla et al 2020). Note that the two-infall model assumes that the formation of the thick and thin discs are sequential, in which the infalling material in the second episode is assumed to be a mixture of pristine (or low-metallicity) gas and that of the thick disc.…”

Modelling the chemical evolution of Zr, La, Ce, and Eu in the Galactic discs and bulge

“…Recently, the importance of radial migration in shaping also the r-process abundance pattern has been investigated by Tsujimoto & Baba (2019). Concerning the Galactic thick disc, it is characterized by a more intense star formation history than the thin disc.…”

Identification of Absorption Lines of Heavy Metals in the Wavelength Range 0.97–1.32 μm

“…After the discovery of the gravitational wave and the electromagnetic counterpart of GW 170817 (Abbott et al 2017), it has become more evident that the neutron star mergers accompanied by kilonovae are major contributors to r-process elements (Smartt et al 2017;Tanaka et al 2017). However, explaining the chemical evolution of r-process abundances seen in disk stars requires further investigations concerning some complications due, e.g., to the delay-time distributions of the neutron star mergers (Hotokezaka et al 2018), the initial mass functions (Tsujimoto, & Baba 2019), and potential contribution from other origins (Siegel et al 2019). New observational constraints on the r-process enrichment can be expected if the near-infrared diagnostic lines of Eu and Dy are used for measuring the abundances of stars in unexplored regions of the Galactic disk behind severe interstellar extinction.…”

“…The youth of Cepheids offers a crucial advantage in studying the disk chemical evolution. The radial migration of the disk (Sellwood & Binney 2002) affects the current abundance distribution of various elements for stars in the disk (e.g., see a recent study on the r-process abundance in the solar neighborhood by Tsujimoto, & Baba 2019). Cepheids are, however, considered to be located almost at their birth positions in terms of R GC and tell us, at least approximately, the current abundances of star-forming gas.…”