r-PROCESS NUCLEOSYNTHESIS IN DYNAMICALLY EJECTED MATTER OF NEUTRON STAR MERGERS

By S. Goriely, A. Bauswein, Hans-Thomas Janka. 2011

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Although the rapid neutron-capture process, or r-process, is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60, the astrophysical site of this process has not been identified yet. Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars (NSs) and within milliseconds afterward. For the first time we make use of relativistic hydrodynamical simulations of such events, defining consistently the conditions that determine the nucleosynthesis, i.e., neutron enrichment, entropy, early density evolution and thus expansion timescale, and ejecta mass. We find that 10−3–10−2 M☉ are ejected, which is enough for mergers to be the main source of heavy (A ≳ 140) galactic r-nuclei for merger rates of some 10−5 yr−1. While asymmetric mergers eject 2–3 times more mass than symmetric ones, the exact amount depends weakly on whether the NSs have radii of ∼15 km for a “stiff” nuclear equation of state (EOS) or ∼12 km for a “soft” EOS. r-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model, we expect high emission in the B-V-R bands for 1–2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a319ba9ac2bcda40f935f79d583f3a4bb1d02843

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Citations in the corpus

The long-term evolution of neutron star merger remnants { II. Radioactively powered transients (2013, background): “…relatively independent of details and has a simple temporal dependence: it is constant within roughly the first second and subsequently decays according to a power law ∝ t−α, where α = 1.3, similar to the results found by other groups (Metzger et al. 2010; Goriely et al. 2011; Roberts et al. 2011).”
WHAT IS THE MOST PROMISING ELECTROMAGNETIC COUNTERPART OF A NEUTRON STAR BINARY MERGER? (2011, background): “…of this association, the mergers are expected to be accompanied by isotropic thermal emission, powered by the radioactive decay of heavy elements in the merger ejecta (Li & Paczyński 1998; hereafter LP98; Kulkarni 2005; Rosswog 2005; Metzger et al. 2010; Roberts et al. 2011; Goriely et al. 2011).”
The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars (2017, background): “…an r-process site, and since the first nucleosynthesis calculations (Rosswog et al. 1998; Freiburghaus et al. 1999) a slew of other studies (e.g., Goriely et al. 2011; Korobkin et al. 2012; Just et al. 2015; Mendoza-Temis et al. 2015) have confirmed their suitability for the production of the…”
Europium production: neutron star mergers versus core-collapse supernovae (2013, background): “Another major source of r-process elements might be neutron star mergers (Lattimer & Schramm 1974, 1976; Lattimer et al. 1977; Freiburghaus et al. 1999; Goriely et al. 2011; Robertset al. 2011).”
Neutron-powered precursors of kilonovae (2014, background): “…consequence of the merger process appears to be the ejection of neutron rich matter, either during the early, short-lived dynamical phase (e.g. Goriely et al. 2011) or in delayed outflows from the remnant accretion torus (e.g. Fernández & Metzger 2013; Perego et al. 2014; Just et al. 2014).”
The elemental composition of the Sun - III. The heavy elements Cu to Th (2014, background): “Currently the main contenders are corecollapse supernovae (e.g. in connection with a neutrino wind, Wanajo 2013) or mergers of neutron stars with either black holes or other neutron stars (Goriely et al. 2011), but achieving the necessary neutron fluxes and entropies is challenging in either case.”
On the astrophysical robustness of the neutron star merger r-process (2012, background): “Differences with Goriely et al. (2011) in lighter heavy elements are not particularly relevant because these can also be produced in other astrophysical events, and the overall yields of the elements with A < 120 that we obtain are much smaller compared to the heavy robust r-process elements…”
RADIATIVE TRANSFER SIMULATIONS OF NEUTRON STAR MERGER EJECTA (2013, methodology): “The NS merger ejecta is one of the most promising sites for r-process nucleosynthesis (e.g., Lattimer & Schramm 1974, 1976; Eichler et al. 1989; Freiburghaus et al. 1999; Roberts et al. 2011; Goriely et al. 2011; Korobkin et al. 2012; Bauswein et al. 2013).”
Binary Neutron Star Mergers: Mass Ejection, Electromagnetic Counterparts, and Nucleosynthesis (2018, background): “…that are thought to be a likely site for the production of r-process nuclei (Lattimer & Schramm 1974; Symbalisty & Schramm 1982; Eichler et al. 1989; Meyer 1989; Freiburghaus et al. 1999; Goriely et al. 2011; Korobkin et al. 2012; Wanajo et al. 2014; Just et al. 2015; Thielemann et al. 2017).”
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers (2014, methodology): “…Roberts et al. 2011; Korobkin et al. 2012; Grossman et al. 2014; Rosswog et al. 2014), and – compared to models that disregard neutrino effects (e.g. Goriely et al. 2011b; Bauswein et al. 2013b) – the Ye in the ejecta is slightly higher when neutrino emission is included (e.g. Korobkin et al.…”
The impact of individual nuclear properties on $r$-process nucleosynthesis (2015, methodology): “s trajectory has an articially reduced electron fraction of Y e = 0:31 which yields a main rprocess, A>120. Trajectory (d) is a neutron star-neutron star merger from Bauswain and Janka as in Ref. [32]. This trajectory is highly neutron rich and undergoes several ssion cycles before the rprocess e”
Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disc ejecta from neutron star mergers (2017, background): “…for A & 130 by virtue of fission cycles (e.g., Goriely et al. 2005), with little sensitivity to binary parameters or the equation of state (e.g., Goriely et al. 2011; Korobkin et al. 2012; Bauswein et al. 2013), depending instead on nuclear physics properties such as the fission fragment…”
Kilonova from post-merger ejecta as an optical and near-infrared counterpart of GW170817 (2017, background): “When two NSs merge, a small part of NS material (∼ 10−3 − 10−2M⊙) is ejected into interstellar space (e.g., Rosswog et al. 1999; Ruffert & Janka 2001; Goriely et al. 2011; Hotokezaka et al. 2013; Bauswein et al. 2013).”
NEUTRINO-DRIVEN WINDS IN THE AFTERMATH OF A NEUTRON STAR MERGER: NUCLEOSYNTHESIS AND ELECTROMAGNETIC TRANSIENTS (2015, methodology): “…physics input, this type of ejecta yields a very robust, “strong r-process” (Sneden et al. 2008) pattern due to efficient fission cycling (Panov et al. 2008; Goriely et al. 2011; Korobkin et al. 2012; Petermann et al. 2012; Bauswein et al. 2013; Wanajo et al. 2014; Goriely 2015; Just et al. 2015a).”
DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, background): “Ji et al. concluded that the r-process material originated in a single neutron-star merger (e.g., Goriely et al. 2011; Ramirez-Ruiz et al. 2015) or any other rare site with high r-process yields (∼ 10−4.5 M of Eu per event).”