Abstract
The mergers of binary neutron stars, as well as black hole–neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis ...that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining 32 different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics uncertainties typically generate at least one order of magnitude uncertainty in key quantities such as the nuclear heating (one and a half orders of magnitude at 1 day post-merger), the bolometric luminosity (one order of magnitude at 5 days post-merger), and the inferred mass of material from the bolometric luminosity (factor of 8 when considering the 8–10 day region). Since particular nuclear processes are critical for determining the electromagnetic signal, we provide tables of key nuclei undergoing
β
-decay,
α
-decay, and spontaneous fission important for heating at different times, identifying decays that are common among the many nuclear input combinations.
Abstract
Merging neutron stars produce “kilonovae”—electromagnetic transients powered by the decay of unstable nuclei synthesized via rapid neutron capture (the
r
-process) in material that is ...gravitationally unbound during inspiral and coalescence. Kilonova emission, if accurately interpreted, can be used to characterize the masses and compositions of merger-driven outflows, helping to resolve a long-standing debate about the origins of
r
-process material in the Universe. We explore how the uncertain properties of nuclei involved in the
r
-process complicate the inference of outflow properties from kilonova observations. Using
r
-process simulations, we show how nuclear physics uncertainties impact predictions of radioactive heating and element synthesis. For a set of models that span a large range in both predicted heating and final abundances, we carry out detailed numerical calculations of decay product thermalization and radiation transport in a kilonova ejecta with a fixed mass and density profile. The light curves associated with our models exhibit great diversity in their luminosities, with peak brightness varying by more than an order of magnitude. We also find variability in the shape of the kilonova light curves and their color, which in some cases runs counter to the expectation that increasing levels of lanthanide and/or actinide enrichment will be correlated with longer, dimmer, redder emission.
Matter neutrino resonances (MNRs) can occur in environments where the flux of electron antineutrinos is greater than the flux of electron neutrinos. These resonances may result in dramatic neutrino ...flavor transformation. Compact object merger disks are an example of an environment where electron antineutrinos outnumber neutrinos. We study MNR resonances in several such disk configurations and find two qualitatively different types of matter-neutrino resonances: a standard MNR and a symmetric MNR. We examine the transformation that occurs in each type of resonance and explore the consequences for nucleosynthesis.
The Covid-19 pandemic has wrought disruption to everyday life and services, and emerging evidence suggests that those with eating disorders (EDs) are likely to experience marked distress and ...exacerbation of their symptoms. However, little is known around the most relevant factors to symptom change; whether certain emotion regulation and coping strategies are linked to better outcomes; and how people with EDs are adjusting to psychological interventions moving online.
In a mixed-method design, we collected qualitative and quantitative data from 207 (76 males) self-selected UK residents with self-reported ED, who described and ranked impacts of the pandemic on their symptoms. Regression analysis examined whether emotion regulation strategies were associated with self-reported symptom change, ED symptomatology, and negative emotional states. Thematic analysis explored participants' experiences of the pandemic, particularly factors affecting their ED, coping strategies used, and experiences of psychological intervention.
Most participants (83.1%) reported worsening of ED symptomatology, though factors affecting symptom change differed between specific EDs. Emotion regulation, such as having fewer strategies, poorer emotional clarity, and non-acceptance of emotions, explained nearly half of the variance in emotional distress during the pandemic. Qualitative findings indicated that difficult emotions (such as fear and uncertainty), changes to routine, and unhelpful social messages were triggering for participants during the pandemic. While some participants described employing positive coping strategies (such as limiting social media exposure), many reported using ED behaviours (among other maladaptive strategies, like alcohol use) to cope with the pandemic. Finally, loss of treatment support, feeling underserving of support and experiencing a 'detached connection' online were further exacerbating factors for these participants.
While our sample was self-selected and may not represent all people with EDs, our results suggest that people with EDs have been strongly affected by the pandemic. Some aspects of online treatment were found to be beneficial but our findings suggest it also needs some improvement. Our paper discusses implications for online treatment such as taking into account personal circumstances and, in a time where people have limited control over the antecedents of negative emotion, the need to develop skills to manage emotions when they arise.
Californium-254 and Kilonova Light Curves Zhu, Y.; Wollaeger, R. T.; Vassh, N. ...
Astrophysical journal. Letters,
08/2018, Letnik:
863, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Neutron star mergers offer unique conditions for the creation of the heavy elements, and additionally provide a testbed for our understanding of this synthesis known as the r-process. We have ...performed dynamical nucleosynthesis calculations and identified a single isotope, 254Cf, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which 254Cf has the greatest impact to the light curve. Future observations in the mid-infrared, which are bright during this regime, could indicate the production of actinide nucleosynthesis.
Abstract
The rapid neutron capture process (
r
-process) is one of the main mechanisms whereby elements heavier than iron are synthesized, and is entirely responsible for the natural production of ...the actinides. Kilonova emissions are modeled as being largely powered by the radioactive decay of species synthesized via the
r
-process. Given that the
r
-process occurs far from nuclear stability, unmeasured beta-decay rates play an essential role in setting the timescale for the
r
-process. In an effort to better understand the sensitivity of kilonova modeling to different theoretical global beta-decay descriptions, we incorporate these into nucleosynthesis calculations. We compare the results of these calculations and highlight differences in kilonova nuclear energy generation and light-curve predictions, as well as final abundances and their implications for nuclear cosmochronometry. We investigate scenarios where differences in beta-decay rates are responsible for increased nuclear heating on timescales of days that propagates into a significantly increased average bolometric luminosity between 1 and 10 days post-merger. We identify key nuclei, both measured and unmeasured, whose decay rates directly impact nuclear heating generation on timescales responsible for light-curve evolution. We also find that uncertainties in beta-decay rates significantly impact age estimates from cosmochronometry.
β-delayed Fission in r-process Nucleosynthesis Mumpower, M. R.; Kawano, T.; Sprouse, T. M. ...
Astrophysical journal/The Astrophysical journal,
12/2018, Letnik:
869, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present β-delayed neutron emission and β-delayed fission (βdf) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach ...(QRPA+HF) approach. From the initial population of a compound nucleus after β-decay, we follow the statistical decay, taking into account competition between neutrons, γ-rays, and fission. We find a region of the chart of nuclides where the probability of βdf is ∼100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance βdf, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, β-decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of βdf in rapid neutron-capture process (r-process) nucleosynthesis in the tidal ejecta of a neutron star-neutron star merger and show that it is a key fission channel that shapes the final abundances near the second r-process peak.
Astrophysical nucleosynthesis is a family of diverse processes by which atomic nuclei undergo nuclear reactions and decay to form new nuclei. The complex nature of nucleosynthesis, which can involve ...as many as tens of thousands of interactions between thousands of nuclei, makes it difficult to study any one of these interactions in isolation using standard approaches. In this work, we present a new technique, nucleosynthesis tracing, that we use to quantify the relative fraction of nuclear abundances that pass through individual nuclear reaction, decay, and fission processes at any point during nucleosynthesis. Here, we apply this technique to study fission and β– decay as they occur in the rapid neutron capture (r) process of nucleosynthesis.
We consider hot accretion disk outflows from black hole-neutron star mergers in the context of the nucleosynthesis they produce. We begin with a three-dimensional numerical model of a black ...hole-neutron star merger and calculate the neutrino and antineutrino fluxes emitted from the resulting accretion disk. We then follow the element synthesis in material outflowing the disk along parameterized trajectories. We find that at least a weak r-process is produced, and in some cases a main r-process as well. The neutron-rich conditions required for this production of r-process nuclei stem directly from the interactions of the neutrinos emitted by the disk with the free neutrons and protons in the outflow.
The possibility that nucleosynthesis in neutron star mergers may reach fissioning nuclei introduces significant uncertainties in predicting the relative abundances of r-process material from such ...events. We evaluate the impact of using sets of fission yields given by the 2016 GEF code for spontaneous (sf), neutron-induced ((n, f)), and β-delayed (βdf) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energy-dependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF 2016 can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of 254Cf spontaneous fission to the heating at ∼100 d, such that a light curve observation consistent with such late-time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the 254Cf β-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2-1.4 M neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z, N)(n, f) and (Z, N)βdf that control the relative abundances near the second peak. We find the 'hot spots' for β-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N = 184 shell closure and the location of currently available experimental decay data.
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