Title: Actinide-boosting r process in black-hole–neutron-star merger ejecta
Authors: Shinya Wanajo, Sho Fujibayashi, Kota Hayashi, Kenta Kiuchi, Yuichiro Sekiguchi, Masaru Shibata
First Author’s Institution: Max-Planck-Institut für Gravitationsphysik, Potsdam-Golm, Germany
Status: Accepted to Physical Review Letters [closed access]
Origins of Heavy Metal(s)
The origins of the elements is an ongoing field of research. This is of particular interest for “actinide-boosted” stars: stars that have an abnormally high concentration of actinide metals. Actinides comprise the second sub-row below the periodic table, with ~90-100 protons, and are the heaviest naturally-occurring elements on Earth. Stars are capable of fusing nuclei up to iron (Fe), but nuclei of heavier elements have to be created with other methods and in more extreme environments. In particular, some of the very heavy elements (lead, thorium, uranium, etc.) are extremely difficult to synthesize. The main way these elements are created is via the rapid neutron-capture process, or r-process. In this process, a nucleus is bombarded with neutrons and quickly captures a large number of them. This neutron-rich nucleus then undergoes β decay, converting a neutron into a proton and spitting out an electron (and an electron antineutrino). This converts the nucleus into a heavier element, and undergoing several cycles of this process creates some of the heaviest (semi-)stable elements in the periodic table.
The main ingredient necessary for the r-process is a large supply of free neutrons (“free” meaning not in an atomic nucleus). This is typically difficult to come by because free neutrons are unstable; left unattended, they will undergo β decay into protons and electrons with a half-life of about 10 minutes. One of the most promising candidates for this supply of neutrons is a neutron star, a gravitationally-bound ball of neutrons packing about 2 solar masses worth of material into a space the size of a large city. Astrophysicists think that disrupting the neutron star by having it merge with another neutron star or a black hole could lead to the right conditions for the r-process to generate very heavy elements. Today’s authors are investigating the creation of these heavy elements in the ejected material from the merger of a neutron star and a black hole to determine if this could be a pathway to actinide-boosted stars.
Heavy Metal Production
Today’s authors use a simulation code that accounts for a variety of important components:
- General relativity, which is critical for properly modeling gravitational physics near compact objects like black holes and neutron stars;
- hydrodynamics, which handles the fluid-like nature of the neutron star material and tracks the abundances of different elements;
- magnetic fields, which are important for modeling turbulence in the material ejected during the merger; and
- neutrinos, which are very small fundamental particles created in β decays that can carry away significant amounts of energy.
The merger itself lasts about 1-2 seconds, but since the authors are interested in the long-term effects, the simulation considers a timespan of a full year, so any short-lived radioactive isotopes are assumed to have decayed into lead (Pb) by the end of the simulation.
The first quantity the researchers consider is the fraction of protons in the nucleus, Ye. The Ye composition of the merger ejecta mass at the end of the simulation is shown in Figure 1. The sharp peak at low values of Ye correspond to neutron-rich isotopes created by the r-process, while the broader peak at higher values of Ye correspond to more stable isotopes created after decays. While the lower peak is similar for all models, each model shows a more distinct distribution at larger values.
They also look at the abundances, shown in Figure 2. They find that the dynamical ejecta (the material emitted during the merger) is the primary source of the neutron-rich nuclei with a mass number of A > 130. Conversely, the post-merger ejecta is the main source of the lighter nuclei created by the r-process with A < 130.
The authors also investigate the abundances of each element in Figure 3. In this case, the abundances are considered relative to that of europium (Eu), which is a stable element and is the standard for measuring the over-abundance of actinide elements. All of the models they test have higher abundances of metalloids (the “staircase” of elements between metals and non-metals on the right side of the periodic table) like arsenic, selenium, and bromine; higher abundances of metals like tellurium; and large amounts of metals like platinum, mercury, and uranium. All of these models indicated a final thorium-to-europium ratio of Th/Eu ≳ 1, which is a strong indicator of being actinide-boosted.
The authors highlight a few main conclusions of this paper. First, black-hole–neutron-star mergers are viable sites of the r-process and the creation of heavy metals like actinides. Second, measurements of Ye distributions of these systems could be used to constrain the neutron star equation of state, which is important for understanding how highly relativistic matter behaves. Third, mergers could produce enough actinides like thorium and uranium to explain the observed levels in actinide-boosted stars. The authors note that black-hole–neutron-star mergers are only partially responsible for creating r-process elements, as studies have already shown that neutron star mergers contribute to the creation of these elements. The authors also note that these results may be slightly different in neutron star mergers, particularly in the case where a neutron star remains after the merger instead of a black hole, due to how this would affect the ejecta mass and its composition.
Astrobite edited by Annelia Anderson
Featured image credit: Carl Knox, OzGrav/Swinburne
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