Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies

Wanajo, Shinya; Hirai, Yutaka; Prantzos, Nikos

doi:10.1093/mnras/stab1655

Cited by 32 publications

(21 citation statements)

References 186 publications

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“…Given the moderate impact of the FFI on the disk outflow, it is natural to think of this effect as introducing an uncertainty band to theoretical predictions for the ejecta properties. Our calculations corroborate other work [43,44], indicating that an overall uncertainty of ∼ 10% in ejected mass, electron fraction, and velocity, as well as a factor 2 in lanthanide/actinide mass fraction can be used as a rule-of-thumb uncertainty in parameter inference from and/or upper limits on multi-messenger observations and galactic abundance studies (e.g., [108][109][110][111][112][113][114][115][116]). A similar uncertainty level is associated with spatial resolution of grid-based simulations of post-merger remnants (e.g., [48]).…”

Section: Summary and Discussionsupporting

confidence: 89%

The Fast Flavor Instability in Hypermassive Neutron Star Disk Outflows

Fernández¹,

Richers²,

Mulyk³

et al. 2022

Preprint

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We examine the effect of neutrino flavor transformation by the fast flavor instability (FFI) on longterm mass ejection from accretion disks formed after neutron star mergers. Neutrino emission and absorption in the disk set the composition of the disk ejecta, which subsequently undergoes r-process nucleosynthesis upon expansion and cooling. Here we perform 28 time-dependent, axisymmetric, viscous-hydrodynamic simulations of accretion disks around hypermassive neutron stars (HMNSs) of variable lifetime, using a 3-species neutrino leakage scheme for emission and an annular-lightbulb scheme for absorption. We include neutrino flavor transformation due the FFI in a parametric way, by modifying the absorbed neutrino fluxes and temperatures, allowing for flavor mixing at various levels of flavor equilibration, and also in a way that aims to respect the lepton-number preserving symmetry of the neutrino self-interaction Hamiltonian. We find that for a promptly-formed black hole (BH), the FFI lowers the average electron fraction of the disk outflow due to a decrease in neutrino absorption, driven primarily by a drop in electron neutrino/antineutrino flux upon flavor mixing. For a long-lived HMNS, the disk emits more heavy lepton neutrinos and reabsorbs more electron neutrinos than for a BH, with a smaller drop in flux compensated by a higher neutrino temperature upon flavor mixing. The resulting outflow has a broader electron fraction distribution, a more proton-rich peak, and undergoes stronger radiative driving. Disks with intermediate HMNS lifetimes show results that fall in between these two limits. In most cases, the impact of the FFI on the outflow is moderate, with changes in mass ejection, average velocity, and average electron fraction of order ∼ 10%, and changes in the lanthanide/actinide mass fraction of up to a factor ∼ 2.

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Section: Summary and Discussionsupporting

confidence: 89%

The Fast Flavor Instability in Hypermassive Neutron Star Disk Outflows

Fernández¹,

Richers²,

Mulyk³

et al. 2022

Preprint

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“…With respect to calculating nucleosynthetic yields for astrophysical r-process events [48][49][50][51][52][53] (as could be used in, e.g., galactic chemical evolution or population synthesis studies [54]), we note that isomers considered in this work lie on isobaric β-decay series that are generally populated well after nuclear reactions have subsided in response to cooling temperatures and falling free neutron densities in these environments [55]. The most conspicuous effects of these isomers in the r process would involve the delaying or acceleration of the radioactive decay of nuclei with respect to baseline calculations that omit isomers altogether [36].…”

Section: Introductionmentioning

confidence: 99%

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

et al. 2021

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Nuclear isomers are populated in the rapid neutron capture process (r process) of nucleosynthesis. The r process may cover a wide range `of temperatures, potentially starting from several tens of GK (several MeV) and then cooling as material is ejected from the event. As the r-process environment cools, isomers can freeze out of thermal equilibrium or be directly populated as astrophysically metastable isomers (astromers). Astromers can undergo reactions and decays at rates very different from the ground state, so they may need to be treated independently in nucleosythesis simulations. Two key behaviors of astromers—ground state ↔ isomer transition rates and thermalization temperatures—are determined by direct transition rates between pairs of nuclear states. We perform a sensitivity study to constrain the effects of unknown transitions on astromer behavior. Detailed balance ensures that ground → isomer and isomer → ground transitions are symmetric, so unknown transitions are equally impactful in both directions. We also introduce a categorization of astromers that describes their potential effects in hot environments. We provide a table of neutron-rich isomers that includes the astromer type, thermalization temperature, and key unmeasured transition rates.

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“…Similar diffusive and diffusive-like modes of mass transfer have been utilized in other recent GCE models (Beniamini & Hotokezaka 2020;Wanajo et al 2021); however, a modified treatment wherein stars are formed at and around the midplane while r -process injection sites occur in the halo might constitute an even more refined analysis of the data. This type of setup would be more reflective of the expected distributions of rprocess events and star-forming regions, and as such it would be illuminating to see by how much, if at all, the delay times extracted from the element abundances differ for such a prescription.…”

Section: Implications For Gce Modelsmentioning

confidence: 89%

R-process Rain from Binary Neutron Star Mergers in the Galactic Halo

Amend,

Zrake,

Hartmann

2022

Preprint

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Compact binary mergers involving at least one neutron star are promising sites for the synthesis of r -process elements found in stars and planets. However, mergers are generally thought to take place at high galactic latitudes, far from any star-forming regions of the host galaxy. It is thus important to understand the physical mechanisms involved in transporting enriched material from the gas environments of the galactic halo to the star-forming disk. We investigate these processes, starting from an explosive injection event and its interaction with the halo gas medium. We show that the total outflow mass in compact binary mergers is far too low for the material to travel to the disk in a ballistic fashion. Instead, the enriched ejecta is swept into a shell, which decelerates over 1 − 10 pc scales, and becomes corrugated by the Rayleigh-Taylor instability. The corrugated shell is denser than the ambient medium, and breaks into clouds which then sink toward the disk. The sinking clouds lose thermal energy through radiative cooling, and are also ablated by shearing instabilities. We present a dynamical heuristic that models these effects, and would predict the delay times for delivery to the disk. However, we find that turbulent mass ablation is extremely efficient, and leads to the total fragmentation of sinking r -process clouds over ∼ 10 5 yr. We thus predict that enriched material from halo injection events quickly assimilates into the gas medium of the halo, and that enriched mass flow to the disk could only be accomplished by turbulent diffusion or large-scale inflowing mass currents.

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Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies

Cited by 32 publications

References 186 publications

The Fast Flavor Instability in Hypermassive Neutron Star Disk Outflows

The Fast Flavor Instability in Hypermassive Neutron Star Disk Outflows

Sensitivity of Neutron-Rich Nuclear Isomer Behavior to Uncertainties in Direct Transitions

R-process Rain from Binary Neutron Star Mergers in the Galactic Halo

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