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r-Process Lanthanide Production and Heating Rates in Kilonovae

Lippuner, Jonas and Roberts, Luke F. (2015) r-Process Lanthanide Production and Heating Rates in Kilonovae. Astrophysical Journal, 815 (2). Art. No. 82. ISSN 0004-637X. doi:10.1088/0004-637X/815/2/82.

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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 material after nuclear burning ceases, which determines the local heating rate from nuclear decays and the opacity. Kasen et al. (2013, ApJ, 774, 25) and Tanaka & Hotokezaka (2013, ApJ, 775, 113) 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 Ye lead to reduced heating rates, due to individual nuclides dominating the heating. With a simplified gray radiative transport scheme in spherical symmetry, we estimate the luminosity, time, and effective temperature at the peak of the light curves. We find that the luminosity peaks much earlier, at about a day in the lanthanide-free cases compared to a week in the lanthanide-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 the ejecta is lanthanide-rich or not.

Item Type:Article
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URLURL TypeDescription Paper
Lippuner, Jonas0000-0002-5936-3485
Roberts, Luke F.0000-0001-7364-7946
Additional Information:© 2015 The American Astronomical Society. Received 2015 August 31; accepted 2015 November 4; published 2015 December 11. We thank Dan Kasen for helpful discussions on light-curve modeling and for graciously providing us with temperature-dependent mean opacities for various mixtures of neodymium and iron. We thank Christian Ott for numerous useful discussions and for a careful reading of the manuscript. We thank Brian Metzger for a number of useful comments on the manuscript. And we also thank Shri Kulkarni for discussion about computing observed magnitudes. The calculations presented here were performed on the Caltech "Zwicky" compute cluster (NSF MRI award No. PHY-0960291), on the NSF XSEDE network under allocation TG-PHY100033, and on NSF/NCSA Blue Waters under allocation jr6 (NSF PRAC award No. ACI-1440083). Support for L.R. during this work was provided by NASA through an Einstein Postdoctoral Fellowship grant No. PF3-140114 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060. J.L. is partially supported by NSF under award Nos. TCAN AST-1333520, CAREER PHY-1151197, and AST-1205732, and by the Sherman Fairchild Foundation.
Group:TAPIR, Walter Burke Institute for Theoretical Physics
Funding AgencyGrant Number
NASA Einstein Postdoctoral FellowshipPF3-140114
Sherman Fairchild FoundationUNSPECIFIED
Subject Keywords:gamma-ray burst: general; gravitational waves; nuclear reactions, nucleosynthesis, abundances; stars: neutron
Issue or Number:2
Record Number:CaltechAUTHORS:20150915-083800029
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Official Citation:Jonas Lippuner and Luke F. Roberts 2015 ApJ 815 82
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:60242
Deposited By: Tony Diaz
Deposited On:15 Sep 2015 19:10
Last Modified:10 Nov 2021 22:31

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