TY - JOUR

T1 - The Stars in M15 Were Born with the r-process*

AU - Kirby, Evan N.

AU - Duggan, Gina

AU - Ramirez-Ruiz, Enrico

AU - Macias, Phillip

PY - 2020/3/1

Y1 - 2020/3/1

N2 - High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (similar to 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.

AB - High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (similar to 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.

KW - Globular star clusters

KW - R-process

KW - Nucleosynthesis

KW - RED GIANT BRANCH

KW - HEAVY-ELEMENTS

KW - PROCESS NUCLEOSYNTHESIS

KW - CHEMICAL EVOLUTION

KW - ABUNDANCES

KW - SIMULATIONS

KW - ENRICHMENT

KW - SUBGIANTS

KW - MERGERS

KW - SAMPLE

U2 - 10.3847/2041-8213/ab78a1

DO - 10.3847/2041-8213/ab78a1

M3 - Letter

VL - 891

JO - The Astrophysical Journal Letters

JF - The Astrophysical Journal Letters

SN - 2041-8205

IS - 1

M1 - 13

ER -