Neutron Star Mergers are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies

Abstract

There are many candidate sites of the r-process: core-collapse supernovae (CCSNe; including rare magnetorotational core-collapse supernovae), neutron star mergers (NSMs), and neutron star/black hole mergers. The chemical enrichment of galaxies—specifically dwarf galaxies—helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference—how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [α/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than CCSNe in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] versus [Fe/H] seen for [Fe/H] ≾ -1.6 when compared to the [Mg/Fe] versus [Fe/H] trend. We conclude that NSMs are the most likely source of r-process enrichment in dwarf galaxies at early times.

Additional Information

© 2018. The American Astronomical Society. Received 2018 July 16; revised 2018 October 17; accepted 2018 October 20; published 2018 December 11. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. DGE-1745301 and the National Science Foundation under grant No. AST-1614081. Facility: Keck:II (DEIMOS). - Software: MOOG (Sneden 1973), spec2d pipeline (Cooper et al. 2012; Newman et al. 2013), scipy (Jones et al. 2001).

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Accepted Version - 1809.04597.pdf

Published - Duggan_2018_ApJ_869_50.pdf

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