The Stars in M15 Were Born with the r-process
Abstract
High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (~1 dex) dispersion in the abundances of r-process elements such as Ba and Eu. Neutron star mergers (NSMs) have been proposed as a major source of the r-process. However, most NSM models predict a delay time longer than the timescale for cluster formation. One possibility is that a NSM polluted the surfaces of stars in M15 long after the cluster finished forming. In this case, the abundances of the polluting elements would decrease in the first dredge-up as stars turn on to the RGB. We present Keck/DEIMOS abundances of Ba in 66 stars along the entire RGB and the top of the main sequence. The Ba abundances have no trend with stellar luminosity (evolutionary phase). Therefore, the stars were born with the Ba that they have today, and Ba did not originate in a source with a delay time longer than the timescale for cluster formation. In particular, if the source of Ba was a NSM, it would have had a very short delay time. Alternatively, if Ba enrichment took place before the formation of the cluster, an inhomogeneity of a factor of 30 in Ba abundance needs to be able to persist over the length scale of the gas cloud that formed M15, which is unlikely.
Additional Information
© 2020 The American Astronomical Society. Received 2019 December 2; revised 2020 February 13; accepted 2020 February 20; published 2020 March 2. We are grateful to Marc Kassis, Carlos Álvarez, and Percy Gómez for their essential roles in providing DEIMOS with the 1200B diffraction grating. We thank Paz Beniamini, E. Sterl Phinney, and Shrinivas Kulkarni for directing us to the observational studies of merging times for binary neutron stars. We also thank Dan Kasen and Ryan Foley for helpful discussions, and the anonymous referee for a helpful report. This material is based upon work supported by the National Science Foundation under grant No. AST-1847909. E.N.K. gratefully acknowledges support from a Cottrell Scholar award administered by the Research Corporation for Science Advancement as well as funding from generous donors to the California Institute of Technology. E.R.-R. and P.M. thank the Heising–Simons Foundation, the Danish National Research Foundation (DNRF132), and NSF (AST-1911206 and AST-1852393) for support. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. Facility: Keck:II (DEIMOS) - KECK II Telescope. Software: spec2d (Cooper et al. 2012; Newman et al. 2013), MOOG (Sneden et al. 2012), MESA (Paxton et al. 2011).Attached Files
Published - Kirby_2020_ApJL_891_L13.pdf
Accepted Version - 2002.09495.pdf
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Additional details
- Eprint ID
- 101666
- Resolver ID
- CaltechAUTHORS:20200302-152240292
- AST-1847909
- NSF
- Cottrell Scholar of Research Corporation
- Caltech
- Heising-Simons Foundation
- DNRF132
- Danish National Research Foundation
- AST-1911206
- NSF
- AST-1852393
- NSF
- Created
-
2020-03-02Created from EPrint's datestamp field
- Updated
-
2021-11-16Created from EPrint's last_modified field
- Caltech groups
- Astronomy Department