Kavli Affiliate: Alexander P. Ji
| First 5 Authors: Anya E. Nugent, Alexander P. Ji, Wen-fai Fong, Hilay Shah, Freeke van de Voort
| Summary:
Neutron star (NS) mergers are currently the only observed source of
$r$-process production in the Universe. Yet, it is unclear how much $r$-process
mass from these mergers is incorporated into star-forming gas to enrich stars.
This is crucial to consider as all other $r$-process mass estimates in the
Universe beyond Earth are based on stellar $r$-process abundances. Here, we
explore the extent to which merger location and host galaxy properties affect
the incorporation of $r$-process elements into star-forming gas, and quantify
an “enrichment" timescale to account for this process. To put this timescale
in context, we analyze a population of 12 gamma-ray bursts (GRBs) with probable
associations to $r$-process kilonovae (GRB-KNe) and 74 short GRBs without
claimed KNe, including new non-parametric star formation histories for the
GRB-KN hosts. We find enrichment timescales for this sample are between
$approx7$ Myr$-1.6$ Gyr, suggesting that environmental enrichment is delayed
from NS merger occurrence. Moreover, we find a correlation between the amount
of environmental enrichment from a single event and increasing host specific
star formation rate (sSFR), and little correlation with stellar mass and GRB
galactocentric offset. Environments with low sSFRs ($<10^{-10.5}$ yr$^{-1}$),
which comprise 18% of short GRB hosts and the host of GW170817, will have
little to no capacity for stellar enrichment. Our results indicate that not all
$r$-process from NS mergers is incorporated into newly-forming stars, and
instead some remains “lost" to the CGM or IGM. Future studies should consider
these losses to understand the total contribution from NS mergers to the
Universe’s $r$-process budget.
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