Neutron-capture elements in dwarf galaxies

Abstract

Context. We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving 13 classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total, this study includes 380 stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. Aims. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies. We investigate the mass dependence of the individual systems on the production of α-elements, but also try to shed light on the long-standing puzzle of the dominant production site of r-process elements. Methods. We used data from the Keck observatory archive and the ESO reduced archive to reanalyze stars from these 13 classical dSph and UFD galaxies. We automatized the step of obtaining stellar parameters, but ran a full spectrum synthesis (1D, local thermal equilibrium) to derive all abundances except for iron to which we applied nonlocal thermodynamic equilibrium corrections where possible. Results. The homogenized set of abundances yielded the unique possibility of deriving a relation between the onset of type Ia supernovae and the stellar mass of the galaxy. Furthermore, we derived a formula to estimate the evolution of α-elements. This reveals a universal relation of these systems across a large range in mass. Finally, we show that between stellar masses of 2.1 × 107 M⊙ and 2.9 × 105 M⊙, there is no dependence of the production of heavy r-process elements on the stellar mass of the galaxy. Conclusions. Placing all abundances consistently on the same scale is crucial to answering questions about the chemical history of galaxies. By homogeneously analyzing Ba and Eu in the 13 systems, we have traced the onset of the s-process and found it to increase with metallicity as a function of the galaxy’s stellar mass. Moreover, the r-process material correlates with the α-elements indicating some coproduction of these, which in turn would point toward rare core-collapse supernovae rather than binary neutron star mergers as a host for the r-process at low [Fe/H] in the investigated dSph systems.

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

“…It has been known that the knee position of [𝛼/Fe] varies depending on the luminosity (and thus stellar mass) of dwarf galaxies (Tolstoy et al 2009;Reichert et al 2020). In fact, our model predicts the knee position of [Mg/Fe] at lower metallicity for a less-massive galaxy, in particular for case 1, as also found in the reference galaxies.…”

“…Although the observation of a radioactively powered electromagnetic emission (kilonova, Li & Paczyński 1998;Metzger et al 2010) associated with the neutron star merger (NSM) GW170817 (Abbott et al 2017) implies the production of neutron-capture elements such as Sr (Watson et al 2019;Domoto et al 2021) and lanthanides (Arcavi et al 2017;Chornock et al 2017;Nicholl et al 2017;Tanaka et al 2017), no trace of heavy r-process elements such as Eu and Th has been found in the kilonova ejecta. This can be due to difficulties in identification of elements in highly Doppler-shifted spectra as well as a lack of the relevant atomic data; however, it is suggested that the overall collapsars (Siegel et al 2019) or magneto-rotational supernovae (MRSN, Winteler et al 2012;Nishimura et al 2015;Reichert et al 2020); but see Fujibayashi et al 2020b andMösta et al 2018 for implications of the former and latter case, respectively.…”

Evidence for r-process Delay in Very Metal-poor Stars

“…The [Ba/Mg] abundances of very metal-poor stars in some dwarf galaxies also support the existence of significant time delays. Although the sample is small and dwarf galaxies have a different SFH, most stars are on the same track as the MW stars (Figure 4 and see also Reichert et al 2020). Therefore the "delay" is likely a common feature, which can be tested with larger samples of the Ba abundances of dwarf galaxies.…”

Mono-enriched stars and Galactic chemical evolution

“…Low-α halo stars have likely been accreted from dwarf galaxies, which, compared to the MW, offer less gas and in turn form a larger population of lower-mass supernovae, thereby explaining the lower α−abundances (see e.g. McWilliam et al 2013;Reichert et al 2020, for different views).…”