r-PROCESS LANTHANIDE PRODUCTION AND HEATING RATES IN KILONOVAE

Abstract

r-process nucleosynthesis in material ejected during neutron star mergers may lead to radioactively powered transients called kilonovae. The timescale and peak luminosity of these transients depend on the composition of the ejecta, which determines the local heating rate from nuclear decays and the opacity. Kasen et al. and Tanaka & Hotokezaka pointed out that lanthanides can drastically increase the opacity in these outflows. We use the new general-purpose nuclear reaction network SkyNet to carry out a parameter study of r-process nucleosynthesis for a range of initial electron fractions Y e , initial specific entropies s, and expansion timescales τ. We find that the ejecta is lanthanide-free for Y e  0.22−0.30, depending on s and τ. The heating rate is insensitive to s and τ, but certain, larger values of Y e lead to reduced heating rates, due to individual nuclides dominating the heating. We calculate approximate light curves with a simplified gray radiative transport scheme. The light curves peak at about a day (week) in the lanthanide-free (-rich) cases. The heating rate does not change much as the ejecta becomes lanthanide-free with increasing Y e , but the light-curve peak becomes about an order of magnitude brighter because it peaks much earlier when the heating rate is larger. We also provide parametric fits for the heating rates between 0.1 and 100 days, and we provide a simple fit in Y e , s, and τ to estimate whether or not the ejecta is lanthanide-rich.

Neutrino-heated winds from millisecond protomagnetars as sources of the weak r-process

“…2). We use the nuclear reaction network SkyNet (Lippuner & Roberts 2015) for the nucleosynthesis calculation. The composition starts out in nuclear statistical equilibrium (NSE), a good approximation for the initial temperature T ∼ > 7 × 10 9 K. Given the extrapolated density as a function of time (see §2.1), SkyNet then evolves the abundances of 7843 nuclear species under the influence of over 140,000 nuclear reactions.…”

Is the Line-Like Optical Afterglow Sed of GRB 050709 Due to a Flare?

“…In comparing with the remarkable progresses made on numerical simulation in these few years (e.g., Roberts et al 2011;Korobkin et al 2012;Kasen et al 2013;Piran et al 2013;Grossman et al 2013;Lippuner & Roberts 2015), the macronova/kilonova sample increases rather slowly. In 2013 the first macronova/kilonova candidate, mainly based on one single Hubble Space Telescope (HST) F160W-band detection, was identified in sGRB 130603B (Tanvir et al 2013;Berger et al 2013).…”

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“…Table II and Fig. 4 characterize the outflow properties relevant to the r process, including Y e , s, and the expansion time scale t exp ¼ r=v, where v denotes the three-velocity (e.g., [68]). These quantities are evaluated for each tracer particle at the last time t ¼ t 5 GK when the temperature of the particle drops below 5 GK.…”

“…Full nuclear reaction network calculations with SKYNET [68] were performed on the tracer particles in a postprocessing step, starting at t ¼ t 10 GK . Figure 5 shows that the solar abundances [70] are well reproduced throughout the mass number (A) range from the second r-process peak (A ∼ 130) to the rare-earth peak (A ∼ 165) to the third r-process peak (A ∼ 195).…”