Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disc ejecta from neutron star mergers

Abstract

We investigate the nucleosynthesis of heavy elements in the winds ejected by accretion disks formed in neutron star mergers. We compute the element formation in disk outflows from hypermassive neutron star (HMNS) remnants of variable lifetime, including the effect of angular momentum transport in the disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations to model the ejecta, and compute r-process nucleosynthesis with tracer particles using a nuclear reaction network containing ∼ 8000 species. We find that the previously known strong correlation between HMNS lifetime, ejected mass, and average electron fraction in the outflow is directly related to the amount of neutrino irradiation on the disk, which dominates mass ejection at early times in the form of a neutrino-driven wind. Production of lanthanides and actinides saturates at short HMNS lifetimes ( 10 ms), with additional ejecta contributing to a blue optical kilonova component for longer-lived HMNSs. We find good agreement between the abundances from the disk outflow alone and the solar r-process distribution only for short HMNS lifetimes ( 10 ms). For longer lifetimes, the rare-earth and third r-process peaks are significantly under-produced compared to the solar pattern, requiring additional contributions from the dynamical ejecta. The nucleosynthesis signature from a spinning black hole (BH) can only overlap with that from a HMNS of moderate lifetime ( 60 ms). Finally, we show that angular momentum transport not only contributes with a late-time outflow component, but that it also enhances the neutrino-driven component by moving material to shallower regions of the gravitational potential, in addition to providing additional heating.

Out of Neutron Star Rubble Comes Gold

“…There is also excellent agreement with observed abundances in metal-poor stars [71][72][73]. We find an overproduction at A ¼ 132 as observed in [32,33]. Below the second r-process peak, we recover the trends of the observed solar abundance pattern, but overall underproduce these nuclei, which is consistent with the absence of a significant high-Y e tail Y e > 0.25 (Fig.…”

“…The nucleosynthesis yields show that these outflows are sufficiently neutron-rich to trigger a strong r process and are well able to reproduce observed solar abundances and observed r-process abundances in metal poor stars from the second to the third r-process peak. Significant contributions to abundances below the second r-process peak can also come from BNS mergers leading to an accretion disk around a metastable hot neutron star, which, due to its strong neutrino emission, may raise Y e in part of the outflow material [32,33,74]. Note added.-Recently a BNS merger has been detected by the Advanced LIGO and Virgo Collaborations [75].…”

“…References [29] and [30] employed an approximate leakage scheme to account for neutrino cooling and a "light-bulb" irradiation model for the neutrino heating, while [31] used an energy-dependent twomoment closure scheme for the transport of electron neutrinos and antineutrinos. These works found unbound outflows with electron fractions in the range Y e ∼ 0.2-0.4 [29,31], sufficient to produce the entire mass range of r-process elements [31][32][33]. The total fraction of the original disk mass unbound in outflows ranged from ∼5% for a nonspinning BH, to ∼30% for high BH spin χ BH ≃ 0.95 [30,31].…”

Properties of Kilonovae from Dynamical and Post-merger Ejecta of Neutron Star Mergers

“…These components are as a whole denoted as "post-merger" ejecta in this paper. The post-merger ejecta can consist of less neutron rich material than in the dynamical ejecta Martin et al 2015;Wu et al 2016;Lippuner et al 2017); neutrino absorption as well as a high temperature caused by viscous heating makes ejected material less neutron rich or electron fraction Y e (number of protons per nucleon) higher. If the ejecta are free from Lanthanide elements, the emission from post-merger ejecta can be brighter and bluer, which can be called "blue kilonova" (Metzger & Fernández 2014;Kasen et al 2015).…”

The r-process Pattern of a Bright, Highly r-process-enhanced Metal-poor Halo Star at [Fe/H] ∼ −2

“…Neutron star mergers (NSMs) have long been suspected to be a site of the r-process (e.g., Lattimer & Schramm 1974;Rosswog et al 2014;Lippuner et al 2017). The recent detection of GW170817 (Abbott et al 2017) and subsequent rprocess nucleosynthesis (e.g., Chornock et al 2017) demonstrates that these conditions can indeed be met in NSMs.…”