arXiv subjects

Nicole Vassh

Publications and source records attributed to Nicole Vassh.

At least 19 recordsLinked to original sources

arXiv2026

Identifying observable MeV lines from the decays of weak and main $r$-process isotopes in mergers

We consider predictions for the MeV gamma-ray spectrum emitted by the $β$ decays of freshly synthesized isotopes from a neutron star merger at timescales of relevance for post-merger (days) and remnant (years) emission. We develop a search algorithm to identify observable spectral peaks and then determine if a specific isotope has a dominant emission line producing the spectral feature. We predict emission spectra using nucleosynthesis calculations which consider nuclear models with distinct masses, $β$-decays, and fission properties as well as variations on main ($A>130$) and weak ($A<130$) $r$-process astrophysical conditions. We tabulate all lines from decaying isotopes that our procedure identifies and provide the predicted range in time over which each line could be visible. We find that Rh-106 presents a unique opportunity to distinguish between main and weak $r$-process emission, as our calculated spectrum above $\sim 1$ MeV for an event dominated by the weak $r$ process is identical to the Rh-106 emission spectrum from $\sim$ 0.2 to $\sim$17 years. We further find emission from species such as Hf-181, Ta-182, Ta-184, and Re-188 offers the potential to be able to distinguish between nuclear models. We investigate whether the 2.6 MeV strong gamma-ray line from Tl-208 is predicted to be robustly observable across calculation variations on both timescale of days and years. We find Tl-208 to consistently shine through on the order of years, though it can face competition from Ga-72 and La-140 at early times ($\sim$ days). We additionally highlight numerous isotopes of interest for observation and nuclear experiment.

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arXiv2025

Constraining Nuclear Mass Models Using r-process Observables with Multi-objective Optimization

Predicting nuclear masses is a longstanding challenge. One path forward is machine learning (ML) which trains on experimental data, but can suffer large errors when extrapolating toward neutron-rich species. In nature, such masses shape observables for the rapid neutron capture process (r-process), which in principle could inform ML models. Here we introduce a multi-objective optimization approach using the Pareto Front algorithm. We show that this technique, capable of identifying models which generate r-process abundances aligning with both Solar and stellar data, is a promising method to select ML models with reliable extrapolation power.

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arXiv2025

Classifying metal-poor stars with machine learning using nucleosynthesis calculations

We apply the capabilities of machine learning (ML) to discern patterns in order to classify metal-poor stars. To do so, we train an ML model on a bank of nucleosynthesis calculations derived from hydrodynamic simulations for events such as neutron star mergers where the rapid ($r$) neutron capture process can take place. Likewise we consider a bank of calculations from simulations of the slow ($s$) neutron capture process and also consider a few calculations for the intermediate ($i$) neutron capture process. We demonstrate that the ML does well overall in recognizing the $s$ process from the $r$ process, and after training on theoretical calculations ML stellar assignments match conventional labels 87% of the time. We highlight that this method then points to stars that could benefit from additional observational measurements. We also demonstrate that the ML assigns some of the presently considered $i$-process stars to instead be of $r$ or $s$ in origin, but likewise, finds stars currently labeled as $s$ to be potentially more aligned with $i$ enrichment. This first application of ML to classify metal-poor star enrichment using theoretical nucleosynthesis calculations thus reveals the promise, and some challenges, associated with this new data-driven path forward.

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arXiv2025

Multidisciplinary Science in the Multimessenger Era

Astrophysical observations of the cosmos allow us to probe extreme physics and answer foundational questions on our universe. Modern astronomy is increasingly operating under a holistic approach, probing the same question with multiple diagnostics including how sources vary over time, how they appear across the electromagnetic spectrum, and through their other signatures, including gravitational waves, neutrinos, cosmic rays, and dust on Earth. Astrophysical observations are now reaching the point where approximate physics models are insufficient. Key sources of interest are explosive transients, whose understanding requires multidisciplinary studies at the intersection of astrophysics, gravity, nuclear science, plasma physics, fluid dynamics and turbulence, computation, particle physics, atomic, molecular, and optical science, condensed matter and materials science, radiation transport, and high energy density physics. This white paper provides an overview of the major scientific advances that lay at the intersection of physics and astronomy and are best probed through time-domain and multimessenger astrophysics, an exploration of how multidisciplinary science can be fostered, and introductory descriptions of the relevant scientific disciplines and key astrophysical sources of interest.

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arXiv2024

Motivations for Early High-Profile FRIB Experiments

This white paper is the result of a collaboration by those that attended a workshop at the Facility for Rare Isotope Beams (FRIB), organized by the FRIB Theory Alliance (FRIB-TA), on Theoretical Justifications and Motivations for Early High-Profile FRIB Experiments. It covers a wide range of topics related to the science that will be explored at FRIB. After a brief introduction, the sections address: (II) Overview of theoretical methods, (III) Experimental capabilities, (IV) Structure, (V) Near-threshold Physics, (VI) Reaction mechanisms, (VII) Nuclear equations of state, (VIII) Nuclear astrophysics, (IX) Fundamental symmetries, and (X) Experimental design and uncertainty quantification.

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arXiv2023

Element abundance patterns in stars indicate fission of nuclei heavier than uranium

The heaviest chemical elements are naturally produced by the rapid neutron-capture process (r-process) during neutron star mergers or supernovae. The r-process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments, so must be extrapolated using nucleosynthesis models. We examine element abundances in a sample of stars that are enhanced in r-process elements. The abundances of elements Ru, Rh, Pd, and Ag (atomic numbers Z = 44 to 47, mass numbers A = 99 to 110) correlate with those of heavier elements (63 <= Z <= 78, A > 150). There is no correlation for neighboring elements (34 <= Z <= 42 and 48 <= Z <= 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in r-process events.

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arXiv2023

Thallium-208: a beacon of in situ neutron capture nucleosynthesis

We demonstrate that the well-known 2.6 MeV gamma-ray emission line from thallium-208 could serve as a real-time indicator of astrophysical heavy element production, with both rapid (r) and intermediate (i) neutron capture processes capable of its synthesis. We consider the r process in a Galactic neutron star merger and show Tl-208 to be detectable from ~12 hours to ~10 days, and again ~1-20 years post-event. Detection of Tl-208 represents the only identified prospect for a direct signal of lead production (implying gold synthesis), arguing for the importance of future MeV telescope missions which aim to detect Galactic events but may also be able to reach some nearby galaxies in the Local Group.

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arXiv2022

The R-Process Alliance: Abundance Universality among Some Elements at and between the First and Second R-Process Peaks

We present new observational benchmarks of rapid neutron-capture process (r-process) nucleosynthesis for elements at and between the first (A ~ 80) and second (A ~ 130) peaks. Our analysis is based on archival ultraviolet and optical spectroscopy of eight metal-poor stars with Se (Z = 34) or Te (Z = 52) detections, whose r-process enhancement varies by more than a factor of 30 (-0.22 <= [Eu/Fe] <= +1.32). We calculate ratios among the abundances of Se, Sr through Mo (38 <= Z <= 42), and Te. These benchmarks may offer a new empirical alternative to the predicted solar system r-process residual pattern. The Te abundances in these stars correlate more closely with the lighter r-process elements than the heavier ones, contradicting and superseding previous findings. The small star-to-star dispersion among the abundances of Se, Sr, Y, Zr, Nb, Mo, and Te (<= 0.13 dex, or 26%) matches that observed among the abundances of the lanthanides and third r-process-peak elements. The concept of r-process universality that is recognized among the lanthanide and third-peak elements in r-process-enhanced stars may also apply to Se, Sr, Y, Zr, Nb, Mo, and Te, provided the overall abundances of the lighter r-process elements are scaled independently of the heavier ones. The abundance behavior of the elements Ru through Sn (44 <= Z <= 50) requires further study. Our results suggest that at least one relatively common source in the early Universe produced a consistent abundance pattern among some elements spanning the first and second r-process peaks.

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arXiv2022

Origin of Plutonium-244 in the Early Solar System

We investigate the origin in the early Solar System of the short-lived radionuclide 244Pu (with a half life of 80 Myr) produced by the rapid (r) neutron-capture process. We consider two large sets of r-process nucleosynthesis models and analyse if the origin of 244Pu in the ESS is consistent with that of the other r and slow (s) neutron-capture process radioactive nuclei. Uncertainties on the r-process models come from both the nuclear physics input and the astrophysical site. The former strongly affects the ratios of isotopes of close mass (129I/127I, 244Pu/238U, and 247Pu/235U). The 129I/247Cm ratio, instead, which involves isotopes of a very different mass, is much more variable than those listed above and is more affected by the physics of the astrophysical site. We consider possible scenarios for the evolution of the abundances of these radioactive nuclei in the galactic interstellar medium and verify under which scenarios and conditions solutions can be found for the origin of 244Pu that are consistent with the origin of the other isotopes. Solutions are generally found for all the possible different regimes controlled by the interval ($δ$) between additions from the source to the parcel of interstellar medium gas that ended up in the Solar System, relative to decay timescales. If r-process ejecta in interstellar medium are mixed within a relatively small area (leading to a long $δ$), we derive that the last event that explains the 129I and 247Cm abundances in the early Solar System can also account for the abundance of 244Pu. Due to its longer half life, however, 244Pu may have originated from a few events instead of one only. If r-process ejecta in interstellar medium are mixed within a relatively large area (leading to a short $δ$), we derive that the time elapsed from the formation of the molecular cloud to the formation of the Sun was 9-16 Myr.

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arXiv2022

The need for a local nuclear physics feature in the neutron-rich rare-earths to explain solar $r$-process abundances

We apply Markov Chain Monte Carlo to predict the masses required to form the observed solar $r$-process rare-earth abundance peak. Given highly distinct astrophysical outflows and nuclear inputs, we find that results are most sensitive to the $r$-process dynamics (i.e. overall competition between reactions and decays), with similar mass trends predicted given similar dynamics. We show that regardless of whether fission deposits into the rare-earths or not, our algorithm consistently predicts the need for a local nuclear physics feature of enhanced stability in the neutron-rich lanthanides.

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arXiv2021

129I and 247Cm in Meteorites Constrain the Last Astrophysical Source of Solar r-process Elements

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron-capture process (r process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives ($\simeq$ 15.6 Myr) of the radioactive r-process nuclei 129I and 247Cm preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic 129I / 247Cm = 438 $\pm$ 184 to nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.

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arXiv2021

Markov Chain Monte Carlo Predictions of Neutron-rich Lanthanide Properties as a Probe of $r$-process Dynamics

Lanthanide element signatures are key to understanding many astrophysical observables, from merger kilonova light curves to stellar and solar abundances. To learn about the lanthanide element synthesis that enriched our solar system, we apply the statistical method of Markov Chain Monte Carlo to examine the nuclear masses capable of forming the $r$-process rare-earth abundance peak. We describe the physical constraints we implement with this statistical approach and demonstrate the use of the parallel chains method to explore the multidimensional parameter space. We apply our procedure to three moderately neutron-rich astrophysical outflows with distinct types of $r$-process dynamics. We show that the mass solutions found are dependent on outflow conditions and are related to the $r$-process path. We describe in detail the mechanism behind peak formation in each case. We then compare our mass predictions for neutron-rich neodymium and samarium isotopes to the latest experimental data from the CPT at CARIBU. We find our mass predictions given outflows that undergo an extended (n,$γ$)$\rightleftarrows$($γ$,n) equilibrium to be those most compatible with both observational solar abundances and neutron-rich mass measurements.

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arXiv2020

MeV Gamma Rays from Fission: A Distinct Signature of Actinide Production in Neutron Star Mergers

Neutron star mergers (NSMs) are the first verified sites of rapid neutron capture (r-process) nucleosynthesis, and could emit gamma rays from the radioactive isotopes synthesized in the neutron-rich ejecta. These MeV gamma rays may provide a unique and direct probe of the NSM environment as well as insight into the nature of the r process, just as observed gammas from the 56Ni radioactive decay chain provide a window into supernova nucleosynthesis. In this work, we include the photons from fission processes for the first time in estimates of the MeV gamma-ray signal expected from an NSM event. We consider NSM ejecta compositions with a range of neutron richness and find a dramatic difference in the predicted signal depending on whether or not fissioning nuclei are produced. The difference is most striking at photon energies above ~3.5 MeV and at a relatively late time, several days after the merger event, when the ejecta is optically thin. We estimate that a Galactic NSM could be detectable by a next generation gamma-ray detector such as AMEGO in the MeV range, up to ~10^4 days after the merger, if fissioning nuclei are robustly produced in the event.

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arXiv2020

Probing the fission properties of neutron-rich actinides with the astrophysical $r$ process

We review recent work examining the influence of fission in rapid neutron capture ($r$-process) nucleosynthesis which can take place in astrophysical environments. We briefly discuss the impact of uncertain fission barriers and fission rates on the population of heavy actinide species. We demonstrate the influence of the fission fragment distributions for neutron-rich nuclei and discuss currently available treatments, including recent macroscopic-microscopic calculations. We conclude by comparing our nucleosynthesis results directly with stellar data for metal-poor stars rich in $r$-process elements to consider whether fission plays a role in the so-called `universality' of $r$-process abundances observed from star to star.

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arXiv2020

Co-production of light and heavy $r$-process elements via fission deposition

We apply for the first time fission yields determined across the chart of nuclides from the macroscopic-microscopic theory of the Finite Range Liquid Drop Model to simulations of rapid neutron capture ($r$-process) nucleosynthesis. With the fission rates and yields derived within the same theoretical framework utilized for other relevant nuclear data, our results represent an important step toward self-consistent applications of macroscopic-microscopic models in $r$-process calculations. The yields from this model are wide for nuclei with extreme neutron excess. We show that these wide distributions of neutron-rich nuclei, and particularly the asymmetric yields for key species that fission at late times in the $r$ process, can contribute significantly to the abundances of the lighter heavy elements, specifically the light precious metals palladium and silver. Since these asymmetric yields correspondingly also deposit into the lanthanide region, we consider the possible evidence for co-production by comparing our nucleosynthesis results directly with the trends in the elemental ratios of metal-poor stars rich in $r$-process material. We show that for $r$-process enhanced stars palladium over europium and silver over europium display mostly flat trends suggestive of co-production and compare to the lanthanum over europium trend which is often used to justify robustness arguments in the lanthanide region. We find that such robustness arguments may be extendable down to palladium and heavier and demonstrate that fission deposition is a mechanism by which such a universality or robustness can be achieved.

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arXiv2020

Sandblasting the $\textit{r}$-Process: Spallation of Ejecta from Neutron Star Mergers

Neutron star mergers (NSMs) are rapid neutron capture ($\textit{r}$-process) nucleosynthesis sites that expel matter at high velocities, from $0.1c$ to as high as $0.6c$. Nuclei ejected at these speeds are sufficiently energetic to initiate spallation nuclear reactions with interstellar medium particles. We adopt a thick-target model for the propagation of high-speed heavy nuclei in the interstellar medium, similar to the transport of cosmic rays. We find that spallation may create observable perturbations to NSM isotopic abundances, particularly around the low-mass edges of the $\textit{r}$-process peaks where neighboring nuclei have very different abundances. The extent to which spallation modifies the final NSM isotopic yields depends on: (1) the ejected abundances, which are determined by the NSM astrophysical conditions and the properties of nuclei far from stability, (2) the ejecta velocity distribution and propagation in interstellar matter, and (3) the spallation cross-sections. Observed solar and stellar $\textit{r}$-process yields could thus constrain the velocity distribution of ejected neutron star matter, assuming NSMs are the dominant $\textit{r}$-process source. We suggest avenues for future work, including measurement of relevant cross sections.

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arXiv2020

Spallation of r-Process Nuclei Ejected from a Neutron Star Merger

Neutron star mergers (NSMs) are rapid neutron capture (r-process) nucleosynthesis sites, which eject materials at high velocities, from 0.1c to as high as 0.6c. Thus the r-process nuclei ejected from a NSM event are sufficiently energetic to initiate spallation reactions with the interstellar medium (ISM) particles. With a thick-target model for the propagation of high-speed heavy nuclei in the ISM, we find that spallation reactions may shift the r-process abundance patterns towards solar data, particularly around the low-mass edges of the r-process peaks where neighboring nuclei have very different abundances. The spallation effects depend both on the astrophysical conditions of the r-process nuclei and nuclear physics inputs for the nucleosynthesis calculations and the propagation process. This work extends that of [Wang et al.(2019)] by focusing on the influence of nuclear physics variations on spallation effects.

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arXiv2020

Characterizing $r$-Process Sites through Actinide Production

Of the variations in the elemental abundance patterns of stars enhanced with $r$-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in $r$-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an $r$-process event may be. Furthermore, if an $r$-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of $r$-process site is needed to explain all levels of observed relative actinide enhancements.

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