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Cooling ages of white dwarfs are routinely determined by mapping effective temperatures and masses to ages using evolutionary models. Typically, the reported uncertainties on cooling ages only consider the error propagation of the uncertainties on the spectroscopically or photometrically determined $T_{\rm eff}$ and mass. However, cooling models are themselves uncertain, given their dependence on many poorly constrained inputs. This paper estimates these systematic model uncertainties. We use MESA to generate cooling sequences of $0.5-1.0 M_{\odot}$ hydrogen-atmosphere white dwarfs with carbon-oxygen cores under different assumptions regarding the chemical stratification of their core, the thickness of their helium envelope, their hydrogen content, and the conductive opacities employed in the calculations. The parameter space explored is constrained by the range of values predicted by a variety of stellar evolution models and inferred from asteroseismological studies. For a $0.6 M_{\odot}$ white dwarf, we find an uncertainty of 0.03 Gyr at 10,000 K (corresponding to a 5% relative uncertainty) and 0.8 Gyr at 4000 K (9%). This uncertainty is significant, as it is comparable to the age uncertainty obtained by propagating the measurement errors on $T_{\rm eff}$ and mass for a typical white dwarf. We also separately consider the potential impact of $^{22}$Ne shell distillation, which plausibly leads to an additional uncertainty of $\sim 1$ Gyr for crystallized white dwarfs. We provide a table of our simulation results that can be used to evaluate the systematic model uncertainty based on a white dwarf's $T_{\rm eff}$ and mass. We encourage its use in all future studies where white dwarf cooling ages are measured., Comment: Submitted to ApJ

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., Comment: accepted to PRL

A few percent of red giants are enriched in Lithium with $A(\mathrm{Li}) > 1.5$. Their evolutionary status has remained uncertain because these Li-rich giants can be placed both on the red-giant branch (RGB) near the bump luminosity and in the red clump (RC) region. However, thanks to asteroseismology, it has been found that most of them are actually RC stars. Starting at the bump luminosity, RGB progenitors of the RC stars experience extra mixing in the radiative zone separating the H-burning shell from the convective envelope followed by a series of convective He-shell flashes at the RGB tip, known as the He-core flash. The He-core flash was proposed to cause fast extra mixing in the stars at the RGB tip that is needed for the Cameron-Fowler mechanism to produce Li. We propose that the RGB stars are getting enriched in Li by the RGB extra mixing that is getting enhanced and begins to produce Li, instead of destroying it, when the stars are approaching the RGB tip. After a discussion of several mechanisms of the RGB extra mixing, including the joint operation of rotation-driven meridional circulation and turbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI), thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity waves, and based on results of (magneto-) hydrodynamics simulations and asteroseismology observations, we are inclined to conclude that it is the mechanism of the AMRI or magnetically-enhanced thermohaline convection, that is most likely to support our hypothesis., Comment: 17 pages, 11 figures, accepted for publication in MNRAS

The inner structure of core-helium burning (CHeB) stars remains uncertain due to the yet unknown nature of mixing at the boundary of their cores. Large convective cores beyond a bare Schwarzschild model are favoured both from theoretical arguments and from asteroseismological constraints. However, the exact nature of this extra mixing, and in particular the possible presence of semiconvective layers, is still debated. In this work, we approach this problem through a new avenue by performing the first full-sphere 3D hydrodynamics simulations of the interiors of CHeB stars. We use the PPMstar explicit gas dynamics code to simulate the inner 0.45 $M_{\odot}$ of a 3 $M_{\odot}$ CHeB star. Simulations are performed using different Cartesian grid resolutions (768$^3$, 1152$^3$ and 1728$^3$) and heating rates. We use two different initial states, one based on MESA's predictive mixing scheme (which significantly extends the core beyond the Schwarzschild boundary) and one based on the convective premixing approach (which exhibits a semiconvective interface). The general behaviour of the flow in the convective core and in the stable envelope (where internal gravity waves are observed) is consistent with our recent simulations of core convection in massive main-sequence stars, and so are the various luminosity scaling relations. The semiconvective layers are dominated by strong internal gravity waves that do not produce measurable species mixing, but overshooting motions from the convective core gradually homogenize the semiconvective interface. This process can possibly completely erase the semiconvective layers, which would imply that CHeB stars do not harbour a semiconvection zone., Comment: Accepted for publication in MNRAS

We present 3-D hydrodynamical simulations of core convection with a stably stratified envelope of a \unit{25}{\Msun} star in the early phase of the main-sequence. We use the explicit gas-dynamics code \code{PPMstar} which tracks two fluids and includes radiation pressure and radiative diffusion. Multiple series of simulations with different luminosities and radiative thermal conductivities are presented. The entrainment rate at the convective boundary, internal gravity waves in and above the boundary region, and the approach to dynamical equilibrium shortly after a few convective turnovers are investigated. We perform very long simulations on $896^3$ grids accelerated by luminosity boost factors $1000$, $3162$ and $10000$. In these simulations the growing penetrative convection reduces the initially unrealistically large entrainment. This reduction is enabled by a spatial separation that develops between the entropy gradient and the composition gradient. The convective boundary moves outward much more slowly at the end of these simulations. Finally, we present a 1-D method to predict the extent and character of penetrative convection beyond the Schwarzschild bounxdary. The 1-D model is based on a spherically-averaged reduced entropy equation that takes the turbulent dissipation as input from the 3-D hydrodynamic simulation and takes buoyancy and all other energy sources and sinks into account. This 1-D method is intended to be ultimately deployed in 1-D stellar evolution calculations and is based on the properties of penetrative convection in our simulations carried forward through the local thermal timescale., Comment: 33 pages, 31 figures; Accepted by ApJ

Supermassive stars are Population III stars with masses exceeding $10^4\,M_{\odot}$ that could be the progenitors of the first supermassive black holes. Their interiors are in a regime where radiation pressure dominates the equation of state. In this work, we use the explicit gas dynamics code PPMstar to simulate the hydrogen-burning core of a $10^4\,M_{\odot}$ supermassive main-sequence star. These are the first 3D hydrodynamics simulations of core convection in supermassive stars. We perform a series of ten simulations at different heating rates and on Cartesian grids with resolutions of $768^3$, $1152^3$ and $1728^3$. We examine different properties of the convective flow, including its large-scale morphology, its velocity spectrum and its mixing properties. We conclude that the radiation pressure-dominated nature of the interior does not noticeably affect the behaviour of convection compared to the case of core convection in a massive main-sequence star where gas pressure dominates. Our simulations also offer support for the use of mixing-length theory in 1D models of supermassive stars., Comment: 9 pages, 9 figures, movies at https://www.ppmstar.org/, accepted for publication in MNRAS

We present the first 3D hydrodynamics simulations of the excitation and propagation of internal gravity waves (IGWs) in the radiative interiors of low-mass stars on the red giant branch (RGB). We use the PPMstar explicit gas dynamics code to simulate a portion of the convective envelope and all the radiative zone down to the hydrogen-burning shell of a 1.2$M_{\odot}$ upper RGB star. We perform simulations for different grid resolutions (768$^3$, 1536$^3$ and 2880$^3$), a range of driving luminosities, and two different stratifications (corresponding to the bump luminosity and the tip of the RGB). Our RGB tip simulations can be directly performed at the nominal luminosity, circumventing the need for extrapolations to lower luminosities. A rich, continuous spectrum of IGWs is observed, with a significant amount of total power contained at high wavenumbers. By following the time evolution of a passive dye in the stable layers, we find that IGW mixing in our simulations is weaker than predicted by a simple analytical prescription based on shear mixing and not efficient enough to explain the missing RGB extra mixing. However, we may be underestimating the efficiency of IGW mixing given that our simulations include a limited portion of the convective envelope. Quadrupling its radial extent compared to our fiducial setup increases convective velocities by up to a factor 2 and IGW velocities by up to a factor 4. We also report the formation of a $\sim 0.2\,H_P$ penetration zone and evidence that IGWs are excited by plumes that overshoot into the stable layers., Comment: 20 pages, 29 figures, movies at https://www.ppmstar.org, resubmitted to MNRAS

Recent photometric observations of massive stars have identified a low-frequency power excess which appears as stochastic low-frequency variability in light curve observations. We present the oscillation properties of high resolution hydrodynamic simulations of a 25 $\mathrm{M}_\odot$ star performed with the PPMStar code. The model star has a convective core mass of $\approx\, 12\, \mathrm{M}_\odot$ and approximately half of the envelope simulated. From this simulation, we extract light curves from several directions, average them over each hemisphere, and process them as if they were real photometric observations. We show how core convection excites waves with a similar frequency as the convective time scale in addition to significant power across a forest of low and high angular degree $l$ modes. We find that the coherence of these modes is relatively low as a result of their stochastic excitation by core convection, with lifetimes on the order of 10s of days. Thanks to the still significant power at higher $l$ and this relatively low coherence, we find that integrating over a hemisphere produces a power spectrum that still contains measurable power up to the Brunt--V\"ais\"al\"a frequency. These power spectra extracted from the stable envelope are qualitatively similar to observations, with same order of magnitude yet lower characteristic frequency. This work further shows the potential of long-duration, high-resolution hydrodynamic simulations for connecting asteroseismic observations to the structure and dynamics of core convection and the convective boundary., Comment: Accepted version (2024 April 26). Originally submitted 2022 November 04

We performed 3D hydrodynamic simulations of the inner $\approx 50\%$ radial extent of a $25\ \mathrm{M_\odot}$ star in the early phase of the main sequence and investigate core convection and internal gravity waves in the core-envelope boundary region. Simulations for different grid resolutions and driving luminosities establish scaling relations to constrain models of mixing for 1D applications. As in previous works, the turbulent mass entrainment rate extrapolated to nominal heating is unrealistically high ($1.58\times 10^{-4}\ \mathrm{M_\odot/yr}$), which is discussed in terms of the non-equilibrium response of the simulations to the initial stratification. We measure quantitatively the effect of mixing due to internal gravity waves excited by core convection interacting with the boundary in our simulations. The wave power spectral density as a function of frequency and wavelength agrees well with the GYRE eigenmode predictions based on the 1D spherically averaged radial profile. A diffusion coefficient profile that reproduces the spherically averaged abundance distribution evolution is determined for each simulation. Through a combination of eigenmode analysis and scaling relations it is shown that in the $N^2$-peak region, mixing is due to internal gravity waves and follows the scaling relation $D_\mathrm{IGW-hydro} \propto L^{4/3}$ over a $\gtrapprox 2\ \mathrm{dex}$ range of heating factors. Different extrapolations of the mixing efficiency down to nominal heating are discussed. If internal gravity wave mixing is due to thermally-enhanced shear mixing, an upper limit is $D_\mathrm{IGW} \lessapprox 2$ to $3\times 10^4\ \mathrm{cm^2/s}$ at nominal heating in the $N^2$-peak region above the convective core., Comment: MNRAS. Clarifications and improvements following referee recommendations. Movies and data access https://www.ppmstar.org. 38 Figs. In v2 the bottom panel of Fig 28 is showing the wrong case (M116 instead of M114). Corrected in v3

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron degenerate ignition events. We strengthen MESA's implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator split nuclear burning mode. We close by discussing major updates to MESA's software infrastructure that enhance source code development and community engagement., Comment: 50 pages, 29 figures; Accepted to ApJS

R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich variable supergiants thought to be the result of dynamically unstable white dwarf mergers. We attempt to model RCBs through all the relevant timescales by simulating a merger event in Octo-tiger, a 3D adaptive mesh refinement (AMR) hydrodynamics code and mapping the post-merger object into MESA, a 1D stellar evolution code. We then post-process the nucleosynthesis on a much larger nuclear reaction network to study the enhancement of s-process elements. We present models that match observations or previous studies in most surface abundances, isotopic ratios, early evolution and lifetimes. We also observe similar mixing behavior as previous modeling attempts which result in the partial He-burning products visible on the surface in observations. However, we do note that our sub-solar models lack any enhancement in s-process elements, which we attribute to a lack of hydrogen in the envelope. We also find that the Oxygen-16/Oxygen-18 isotopic ratio is very sensitive to initial hydrogen abundance and increases outside of the acceptable range with a hydrogen mass fraction greater than $10^{-4}$., Comment: 18 pages, 17 figures, 1 table. To be published in The Astrophysical Journal

The abundances of n-capture elements in the CEMP-r/s stars agree with predictions of intermediate n-density nucleosynthesis, at $N_\mathrm{n}\sim 10^{13}$-$10^{15} \mathrm{cm}^{-3}$, in rapidly-accreting white dwarfs (RAWDs). We have performed Monte-Carlo simulations of this i-process nucleosynthesis to determine the impact of (n,$\gamma$) reaction rate uncertainties of 164 unstable isotopes, from $^{131}$I to $^{189}$Hf, on the predicted abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of $N_\mathrm{n} = 3.16\times 10^{14}\ \mathrm{cm}^{-3}$ and $N_\mathrm{n} = 3.16\times 10^{13}\ \mathrm{cm}^{-3}$ and on a multi-zone model based on a realistic stellar evolution simulation of He-shell convection entraining H in a RAWD model with [Fe/H]=-2.6. For each of the selected elements, we have identified up to two (n,$\gamma$) reactions having the strongest correlations between their rate variations constrained by Hauser-Feshbach computations and the predicted abundances, with the Pearson product-moment correlation coefficients $|r_\mathrm{P}| > 0.15$. We find that the discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-i star CS31062-050 are significantly diminished if the rate of $^{137}$Cs(n,$\gamma)^{138}$Cs is reduced and the rates of $^{141}$Ba(n,$\gamma)^{142}$Ba or $^{141}$La(n,$\gamma)^{142}$La increased. The uncertainties of temperature-dependent $\beta$-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances. One-zone Monte-Carlo simulations can be used instead of computationally time-consuming multi-zone Monte-Carlo simulations in reaction rate uncertainty studies if they use comparable values of $N_\mathrm{n}$ (abridged)., Comment: 15 pages, 16 figures, 3 tables, accepted for publication in MNRAS

Recent spectroscopic measurements of the equivalent widths of the resonant Be II doublet and Ca II K lines and their ratios in expanding nova ejecta indicate surprisingly high abundances of $^7$Be with a typical mass fraction $X_\mathrm{obs}(^7\mathrm{Be}) = 10^{-4}$. This is an order of magnitude larger than theoretically predicted values of $X_\mathrm{theor}(^7\mathrm{Be})\sim 10^{-5}$ for novae. We use an analytical solution of the $^7$Be production equations to demonstrate that $X_\mathrm{theor}(^7\mathrm{Be})$ is proportional to the $^4$He mass fraction $Y$ in the nova accreted envelope and then we perform computations of 1D hydrostatic evolution of the $1.15\,M_\odot$ CO nova model that confirm our conclusion based on the analytical solution. Our assumption of enhanced $^4$He abundances helps to reduce, although not completely eliminate, the discrepancy between $X_\mathrm{obs}(^7\mathrm{Be})$ and $X_\mathrm{theor}(^7\mathrm{Be})$. It is supported by UV, optical and IR spectroscopy data that reveal unusually high values of $Y$ in nova ejecta. We also show that a significantly increased abundance of $^3$He in nova accreted envelopes does not lead to higher values of $X_\mathrm{theor}(^7\mathrm{Be})$ because this assumption affects the evolution of nova models resulting in a decrease of both their peak temperatures and accreted masses and, as a consequence, in a reduced production of $^7$Be., Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letters

The origin of the proton-rich trans-iron isotopes in the solar system is still uncertain. Single-degenerate thermonuclear supernovae (SNIa) with n-capture nucleosynthesis seeds assembled in the external layers of the progenitor's rapidly accreting white dwarf phase may produce these isotopes. We calculate the stellar structure of the accretion phase of five white dwarf models with initial masses >~ 0.85Msun using the stellar code MESA. The near-surface layers of the 1, 1.26, 1.32 and 1.38Msun models are most representative of the regions in which the bulk of the p nuclei are produced during SNIa explosions, and for these models we also calculate the neutron-capture nucleosynthesis in the external layers. Contrary to previous rapidly-accreting white dwarf models at lower mass, we find that the H-shell ashes are the main site of n-capture nucleosynthesis. We find high neutron densities up to several 10^15 cm^-3 in the most massive WDs. Through the recurrence of the H-shell ashes these intermediate neutron densities can be sustained effectively for a long time leading to high neutron exposures with a strong production up to Pb. Both the neutron density and the neutron exposure increase with increasing the mass of the accreting WD. Finally, the SNIa nucleosynthesis is calculated using the obtained abundances as seeds. We obtain solar to super-solar abundances for p-nuclei with A>96. Our models show that SNIa are a viable p-process production site., Comment: Accepted for publication in MNRAS. 49 pages, 17 figures, 5 tables

In Pop III stellar models convection-induced mixing between H- and He-rich burning layers can induce a burst of nuclear energy and thereby substantially alter the subsequent evolution and nucleosynthesis in the first massive stars. We investigate H-He shell and core interactions in 26 stellar evolution simulations with masses $15 - 140\,\mathrm{M}_{\odot}$, using five sets of mixing assumptions. In 22 cases H-He interactions induce local nuclear energy release in the range $ \sim 10^{9} - 10^{13.5}\,\mathrm{L}_{\odot}$. The luminosities on the upper end of this range amount to a substantial fraction of the layer's internal energy over a convective advection timescale, indicating a dynamic stellar response that would violate 1D stellar evolution modelling assumptions. We distinguish four types of H-He interactions depending on the evolutionary phase and convective stability of the He-rich material. H-burning conditions during H-He interactions give $^{12}\mathrm{C}/^{13}\mathrm{C}$ ratios between $\approx 1.5$ to $\sim 1000$ and [C/N] ratios from $\approx -2.3 $ to $\approx 3$ with a correlation that agrees well with observations of CEMP-no stars. We also explore Ca production from hot CNO breakout and find the simulations presented here likely cannot explain the observed Ca abundance in the most Ca-poor CEMP-no star. We describe the evolution leading to H-He interactions, which occur during or shortly after core-contraction phases. Three simulations without a H-He interaction are computed to Fe-core infall and a $140\,\mathrm{M}_{\odot}$ simulation becomes pair-unstable. We also discuss present modelling limitations and the need for 3D hydrodynamic models to fully understand these stellar evolutionary phases., Comment: Accepted for publication in MNRAS

We present two mixing models for post-processing of 3D hydrodynamic simulations applied to convective-reactive i-process nucleosynthesis in a rapidly accreting white dwarf (RAWD) with $\mathrm{[Fe/H]} = -2.6$, in which H is ingested into a convective He shell. A 1D advective two-stream model adopts physically motivated radial and horizontal mixing coefficients constrained by 3D hydrodynamic simulations. A simpler approach uses diffusion coefficients calculated from the same simulations. All 3D simulations include the energy feedback of the $^{12}$C(p,$\gamma$)$^{13}$N reaction from the H entrainment. Global oscillations of shell H ingestion in two of the RAWD simulations cause bursts of entrainment of H and non-radial hydrodynamic feedback. With the same nuclear network as in the 3D simulations, the 1D advective two-stream model reproduces the rate and location of the H burning within the He shell closely matching the 3D simulation predictions, as well as qualitatively displaying the asymmetry of the $X_{\mathrm{H}}$ profiles between the up- and downstream. With a full i-process network the advective mixing model captures the difference in the n-capture nucleosynthesis in the up- and downstream. For example, $^{89}$Kr and $^{90}$Kr with half-lives of 3.18 min and 32.3 s differ by a factor 2-10 in the two streams. In this particular application the diffusion approach provides globally the same abundance distribution as the advective two-stream mixing model. The resulting i-process yields are in excellent agreement with observations of the exemplary CEMP-r/s star CS31062-050., Comment: 18 pages, 17 figures, 2 tables, accepted for publication in MNRAS

Modeling the evolution of the elements in the Milky Way is a multidisciplinary and challenging task. In addition to simulating the 13 billion years evolution of our Galaxy, chemical evolution simulations must keep track of the elements synthesized and ejected from every astrophysical site of interest (e.g., supernova, compact binary merger). The elemental abundances of such ejecta, which are a fundamental input for chemical evolution codes, are usually taken from theoretical nucleosynthesis calculations performed by the nuclear astrophysics community. Therefore, almost all chemical evolution predictions rely on the nuclear physics behind those calculations. In this proceedings, we highlight the impact of nuclear physics uncertainties on galactic chemical evolution predictions. We demonstrate that nuclear physics and galactic evolution uncertainties both have a significant impact on interpreting the origin of neutron-capture elements in our Solar System. Those results serve as a motivation to create and maintain collaborations between the fields of nuclear astrophysics and galaxy evolution., Comment: 8 pages, 2 figures, NPA-IX proceedings

The origin of many elements of the periodic table remains an unsolved problem. While many nucleosynthetic channels are broadly understood, significant uncertainties remain regarding certain groups of elements such as the intermediate and rapid neutron-capture processes, the p-process, or the origin of odd-Z elements in the most metal-poor stars. Canada has a long tradition of leadership in nuclear astrophysics, dating back to the work of Alastair Cameron in the 1950s. Recent faculty hires have further boosted activity in the field, including transient observation and theory, survey science on galactic nucleosynthesis, and nuclear experiments. This white paper contains a brief overview of recent activity in the community, highlighting strengths in each sub-field, and provides recommendations to improve interdisciplinary collaboration. Sustaining Canadian leadership in the next decade will require, on the observational side, access to transient and non-transient surveys like LSST, SKA, or MSE, support for target-of-opportunity observing in current and future Canadian telescopes, and participation in next-generation X-ray telescopes such as ATHENA. State-of-the-art theoretical predictions will require an ambitious succession plan for the Niagara supercomputer to support large parallel jobs. We propose a funding instrument for postdoctoral training that reflects the interdisciplinary nature of nuclear astrophysics research, and the creation of a national collaborative funding program that allows for joint projects and workshop organization., Comment: White paper submitted to the Canadian Long Range Plan 2020. Minor formatting changes relative to submitted version

Several anomalous elemental abundance ratios have been observed in the metal-poor star HD94028. We assume that its high [As/Ge] ratio is a product of a weak intermediate (i) neutron-capture process. Given that observational errors are usually smaller than predicted nuclear physics uncertainties, we have first set up a benchmark one-zone i-process nucleosynthesis simulation results of which provide the best fit to the observed abundances. We have then performed Monte Carlo simulations in which 113 relevant (n,$\gamma$) reaction rates of unstable species were randomly varied within Hauser-Feshbach model uncertainty ranges for each reaction to estimate the impact on the predicted stellar abundances. One of the interesting results of these simulations is a double-peaked distribution of the As abundance, which is caused by the variation of the $^{75}$Ga (n,$\gamma$) cross section. This variation strongly anti-correlates with the predicted As abundance, confirming the necessity for improved theoretical or experimental bounds on this cross section. The $^{66}$Ni (n,$\gamma$) reaction is found to behave as a major bottleneck for the i-process nucleosynthesis. Our analysis finds the Pearson product-moment correlation coefficient $r_\mathrm{P} > 0.2$ for all of the i-process elements with $32 \leq Z \leq 42$, with significant changes in their predicted abundances showing up when the rate of this reaction is reduced to its theoretically constrained lower bound. Our results are applicable to any other stellar nucleosynthesis site with the similar i-process conditions, such as Sakurai's object (V4334 Sagittarii) or rapidly-accreting white dwarfs., Comment: 10 pages, 14 figures, 1 table, accepted for publication by MNRAS

We analyze the production of the element Cr in galactic chemical evolution (GCE) models using the NuGrid nucleosynthesis yields set. We show that the unusually large [Cr/Fe] abundance at [Fe/H] $\approx 0$ reported by previous studies using those yields and predicted by our Milky Way model originates from the merging of convective Si-burning and C-burning shells in a 20 $M_\odot$ model at metallicity $Z=0.01$, about an hour before the star explodes. This merger mixes the incomplete burning material in the Si shell, including $^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. The adopted supernova model ejects the outer 2 $M_\odot$ of the CO core, which includes a significant fraction of the Cr-rich material. When including this 20 $M_\odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE model for stars in between 15 and 25 $M_\odot$, we overestimate [Cr/Fe] by an order of magnitude at [Fe/H] $\approx$ 0 relative to observations in the Galactic disk. This raises a number of questions regarding the occurrence of Si-C shell mergers in nature, the accuracy of different simulation approaches, and the impact of such mergers on the pre-supernova structure and explosion dynamics. According to the conditions in this 1D stellar model, the substantial penetration of C-shell material into the Si-shell could launch a convective-reactive global oscillation, if a merger does take place. In any case, GCE provides stringent constraints on the outcome of this stellar evolution phase., Comment: 8 pages, 5 figures, submitted to ApJL

The production of the neutron-capture isotopes beyond iron that we observe today in the solar system is the result of the combined contribution of the r-process, the s- process and possibly the i-process. Low-mass AGB (2 < M/Msun < 3) and massive (M >10 Msun ) stars have been identified as the sites of the s-process. In this work we consider the evolution and nucleosynthesis of low-mass AGB stars. We provide an update of the NuGrid Set models, adopting the same general physics assumptions but using an updated convective-boundary mixing model accounting for the contribution from internal gravity waves. The combined data set includes the initial masses Mzams/Msun = 2, 3 for Z = 0.03, 0.02, 0.01. These models are computed with the MESA stellar code and the evolution is followed up to the end of the AGB phase. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid mppnp code. The convective boundary mixing model leads to the formation of a 13C-pocket three times wider compared to the one obtained in the previous set of models, bringing the simulation results now in closer agreement with observations. We also discuss the potential impact of other processes inducing mixing, like rotation, adopting parametric models compatible with theory and observations. Complete yield data tables, derived data products and online analytic data access are provided., Comment: Submitted to MNRAS

Gamma-ray astronomy explores the most energetic photons in nature to address some of the most pressing puzzles in contemporary astrophysics. It encompasses a wide range of objects and phenomena: stars, supernovae, novae, neutron stars, stellar-mass black holes, nucleosynthesis, the interstellar medium, cosmic rays and relativistic-particle acceleration, and the evolution of galaxies. MeV gamma-rays provide a unique probe of nuclear processes in astronomy, directly measuring radioactive decay, nuclear de-excitation, and positron annihilation. The substantial information carried by gamma-ray photons allows us to see deeper into these objects, the bulk of the power is often emitted at gamma-ray energies, and radioactivity provides a natural physical clock that adds unique information. New science will be driven by time-domain population studies at gamma-ray energies. This science is enabled by next-generation gamma-ray instruments with one to two orders of magnitude better sensitivity, larger sky coverage, and faster cadence than all previous gamma-ray instruments. This transformative capability permits: (a) the accurate identification of the gamma-ray emitting objects and correlations with observations taken at other wavelengths and with other messengers; (b) construction of new gamma-ray maps of the Milky Way and other nearby galaxies where extended regions are distinguished from point sources; and (c) considerable serendipitous science of scarce events -- nearby neutron star mergers, for example. Advances in technology push the performance of new gamma-ray instruments to address a wide set of astrophysical questions., Comment: 14 pages including 3 figures

Massive-star binaries can undergo a phase where one of the two stars expands during its advanced evolutionary stage as a giant and envelops its companion, ejecting the hydrogen envelope and tightening its orbit. Such a common envelope phase is required to tighten the binary orbit in the formation of many of the observed X-ray binaries and merging compact binary systems. In the formation scenario for neutron star binaries, the system might pass through a phase where a neutron star spirals into the envelope of its giant star companion. These phases lead to mass accretion onto the neutron star. Accretion onto these common-envelope-phase neutron stars can eject matter that has undergone burning near to the neutron star surface. This paper presents nucleosynthetic yields of this ejected matter, using population synthesis models to study the importance of these nucleosynthetic yields in a galactic chemical evolution context. Depending on the extreme conditions in temperature and density found in the accreted material, both proton-rich and neutron-rich nucleosynthesis can be obtained, with efficient production of neutron rich isotopes of low Z material at the most extreme conditions, and proton rich isotopes, again at low Z, in lower density models. Final yields are found to be extremely sensitive to the physical modeling of the accretion phase. We show that neutron stars accreting in binary common envelopes might be a new relevant site for galactic chemical evolution, and therefore more comprehensive studies are needed to better constrain nucleosynthesis in these objects., Comment: 21 pages, 14 figures

The special computational challenges of simulating 3-D hydrodynamics in deep stellar interiors are discussed, and numerical algorithmic responses described. Results of recent simulations carried out at scale on the NSF's Blue Waters machine at the University of Illinois are presented, with a special focus on the computational challenges they address. Prospects for future work using GPU-accelerated nodes such as those on the DoE's new Summit machine at Oak Ridge National Laboratory are described, with a focus on numerical algorithmic accommodations that we believe will be necessary., Comment: 14 pages, 7 figures

We report on our ongoing investigation into the nucleosynthetic and hydrodynamic nature of mixing at the interface between the H- and He-convection zones in massive Pop III stars. Studying a grid of 26 1D stellar evolution simulations with different mixing assumptions, we find that H-He interactions occur in 23/26 cases. We demonstrate the nucleosynthesis expected in a H-He interaction in an 80M$_\odot$. Finally, we describe our progress in simulating a Pop III double convection zone in the PPMStar hydrodynamics code., Comment: NIC 2018 Proceeding

We have modelled the multicycle evolution of rapidly-accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes on their surfaces for $0.7\leq M_\mathrm{RAWD}/M_\odot\leq 0.75$ and [Fe/H] ranging from $0$ to $-2.6$. We have also computed the i-process nucleosynthesis yields for these models. The i process occurs when convection driven by the He-shell flash ingests protons from the accreted H-rich surface layer, which results in maximum neutron densities $N_\mathrm{n,max}\approx 10^{13}$-$10^{15}\ \mathrm{cm}^{-3}$. The H-ingestion rate and the convective boundary mixing (CBM) parameter $f_\mathrm{top}$ adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through 3D hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter $f_\mathrm{top}$ have been determined from 3D hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ($\eta < 10\%$). A new result is that RAWDs with [Fe/H]$< -2$ have $\eta > 20\%$, therefore their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNeIa., Comment: 15 pages, 12 figures, 2 tables, accepted for publication in MNRAS

Interactions between convective shells in evolved massive stars have been linked to supernova impostors, to the production of the odd-Z elements Cl, K, and Sc, and they might also help generate the large-scale asphericities that are known to facilitate shock revival in supernova explosion models. We investigate the process of ingestion of C-shell material into a convective O-burning shell, including the hydrodynamic feedback from the nuclear burning of the ingested material. Our 3D hydrodynamic simulations span almost 3 dex in the total luminosity $L_\mathrm{tot}$. All but one of the simulations reach a quasi-stationary state with the entrainment rate and convective velocity proportional to $L_\mathrm{tot}$ and $L_\mathrm{tot}^{1/3}$, respectively. Carbon burning provides $14\,$--$\,33\%$ of the total luminosity, depending on the set of reactions considered. Equivalent simulations done on $768^3$ and $1152^3$ grids are in excellent quantitative agreement. The flow is dominated by a few large-scale convective cells. An instability leading to large-scale oscillations with Mach numbers in excess of $0.2$ develops in an experimental run with the energy yield from C burning increased by a factor of 10. This run represents most closely the conditions expected in a violent O-C shell merger, which is a potential production site for odd-Z elements such as K and Sc and which may seed asymmetries in the supernova progenitor. 1D simulations may underestimate the energy generation from the burning of ingested material by as much as a factor two owing to their missing the effect of clumpiness of entrained material on the nuclear reaction rate., Comment: 24 pages, 27 figures, accepted for publication in MNRAS

Collaborations in astronomy and astrophysics are faced with numerous cyber infrastructure challenges, such as large data sets, the need to combine heterogeneous data sets, and the challenge to effectively collaborate on those large, heterogeneous data sets with significant processing requirements and complex science software tools. The cyberhubs system is an easy-to-deploy package for small to medium-sized collaborations based on the Jupyter and Docker technology, that allows web-browser enabled, remote, interactive analytic access to shared data. It offers an initial step to address these challenges. The features and deployment steps of the system are described, as well as the requirements collection through an account of the different approaches to data structuring, handling and available analytic tools for the NuGrid and PPMstar collaborations. NuGrid is an international collaboration that creates stellar evolution and explosion physics and nucleosynthesis simulation data. The PPMstar collaboration performs large-scale 3D stellar hydrodynamics simulation of interior convection in the late phases of stellar evolution. Examples of science that is presently performed on cyberhubs, in the areas 3D stellar hydrodynamic simulations, stellar evolution and nucleosynthesis and Galactic chemical evolution, are presented., Comment: 19 pages, 9 figures, ApJ Supplement Special Issue on Data, accepted

Rapidly accreting white dwarfs (RAWDs) have been proposed as contributors to the chemical evolution of heavy elements in the Galaxy. Here, we test this scenario for the first time and determine the contribution of RAWDs to the solar composition of first-peak neutron-capture elements. We add the metallicity-dependent contribution of RAWDs to the one-zone galactic chemical evolution code OMEGA according to RAWD rates from binary stellar population models combined with metallicity-dependent $i$-process stellar yields calculated following the models of Denissenkov et al. (2017). With this approach we find that the contribution of RAWDs to the evolution of heavy elements in the Galaxy could be responsible for a significant fraction of the solar composition of Kr, Rb, Sr, Y, Zr, Nb, and Mo ranging from $2$ to $45\%$ depending on the element, the enrichment history of the Galactic gas, and the total mass ejected per RAWD. This contribution could explain the missing solar Lighter Element Primary Process for some elements (e.g., Sr, Y, and Zr). We do not overproduce any isotope relative to the solar composition, but $^{96}$Zr is produced in a similar amount. The $i$ process produces efficiently the Mo stable isotopes $^{95}$Mo and $^{97}$Mo. When nuclear reaction rate uncertainties are combined with our GCE uncertainties, the upper limits for the predicted RAWD contribution increase by a factor of $1.5-2$ for Rb, Sr, Y, and Zr, and by 3.8 and 2.4 for Nb and Mo, respectively. We discuss the implication of the RAWD stellar evolution properties on the single degenerate Type Ia supernova scenario., Comment: 19 pages, 13 figures, 3 tables. Submitted to ApJ (version 2)

Convective boundary mixing (CBM) in the advanced evolutionary stages of massive stars is not well understood. Structural changes caused by convection have an impact on the evolution as well as the subsequent supernova, or lack thereof. The effects of convectively driven mixing across convective boundaries during the post He core burning evolution of $25\mathrm{M}_{\odot}$, solar-metallicity, non-rotating stellar models is studied using the MESA stellar evolution code. CBM is modelled using the exponentially decaying diffusion coefficient equation, the free parameter of which, $f_{\mathrm{cbm}}$, is varied systematically throughout the course of the stellar model's evolution with values of $(0.002, 0.012, 0.022, 0.032)$. The effect of varying this parameter produces mass ranges at collapse in the ONe, Si, Fe cores of $(1.82\mathrm{M}_{\odot}, 4.36\mathrm{M}_{\odot})$, $(1.67\mathrm{M}_{\odot}, 1.99\mathrm{M}_{\odot})$ and $(1.46\mathrm{M}_{\odot}, 1.70\mathrm{M}_{\odot})$ respectively, with percent differences from the model with minimal CBM as large as 86.3%. At the presupernova stage, the compactness of the stellar cores from O'Connor & Ott (2011), $\xi_M$, exhibit a range of $(0.120, 0.354)$, suggesting that the extent of CBM in the advanced burning stages of massive stars is an important consideration for the explodability and type of compact remnant. The nucleosynthetic yields from the models, most notably C, O, Ne, Mg and Si are also significantly affected by the CBM assumptions, showing non-linear trends with increased mixing. The simulations show that interactions between convective C, Ne and O shells produce significant non-linear changes in the evolution, whereas from the end of Si burning, the structural changes attributed to the CBM are dominated by the growth of the convective C shell. Progenitor structures for all the models are available from HERE (link and DOI to appear)., Comment: 19 pages, 19 figures

The stellar yields for galactic modeling applications (SYGMA) code is an open-source module that models the chemical ejecta and feedback of simple stellar populations (SSPs). It is intended for use in hydrodynamical simulations and semi-analytic models of galactic chemical evolution. The module includes the enrichment from asymptotic giant branch (AGB) stars, massive stars, SNIa and neutron-star mergers. An extensive and extendable stellar yields library includes the NuGrid yields with all elements and many isotopes up to Bi. Stellar feedback from mechanic and frequency-dependent radiative luminosities are computed based on NuGrid stellar models and their synthetic spectra. The module further allows for customizable initial-mass functions and supernova Ia (SNIa) delay-time distributions to calculate time-dependent ejecta based on stellar yield input. A variety of r-process sites can be included. A comparison of SSP ejecta based on NuGrid yields with those from Portinari et al. (1998) and Marigo (2001) reveals up to a factor of 3.5 and 4.8 less C and N enrichment from AGB stars at low metallicity, a result we attribute to NuGrid's modeling of hot-bottom burning. Different core-collapse supernova explosion and fallback prescriptions may lead to substantial variations for the accumulated ejecta of C, O and Si in the first $10^7\, \mathrm{yr}$ at $Z=0.001$. An online interface of the open-source SYGMA module enables interactive simulations, analysis and data extraction of the evolution of all species formed by the evolution of simple stellar populations., Comment: 18 pages, 10 figures, 3 tables, published in ApJS

The first peak s-process elements Rb, Sr, Y and Zr in the post-AGB star Sakurai's object (V4334 Sagittarii) have been proposed to be the result of i-process nucleosynthesis in a post-AGB very-late thermal pulse event. We estimate the nuclear physics uncertainties in the i-process model predictions to determine whether the remaining discrepancies with observations are significant and point to potential issues with the underlying astrophysical model. We find that the dominant source in the nuclear physics uncertainties are predictions of neutron capture rates on unstable neutron rich nuclei, which can have uncertainties of more than a factor 20 in the band of the i-process. We use a Monte Carlo variation of 52 neutron capture rates and a 1D multi-zone post-processing model for the i-process in Sakurai's object to determine the cumulative effect of these uncertainties on the final elemental abundance predictions. We find that the nuclear physics uncertainties are large and comparable to observational errors. Within these uncertainties the model predictions are consistent with observations. A correlation analysis of the results of our MC simulations reveals that the strongest impact on the predicted abundances of Rb, Sr, Y and Zr is made by the uncertainties in the (n,$\gamma$) reaction rates of $^{85}$Br, $^{86}$Br, $^{87}$Kr, $^{88}$Kr, $^{89}$Kr, $^{89}$Rb, $^{89}$Sr, and $^{92}$Sr. (abridged), Comment: 30 pages, 10 figures, 5 tables, accepted for publication by The Journal of Physics G

Based on stellar evolution simulations, we demonstrate that rapidly accreting white dwarfs in close binary systems are an astrophysical site for the intermediate neutron-capture process. During recurrent and very strong He-shell flashes in the stable H-burning accretion regime H-rich material enters the He-shell flash convection zone. $^{12}$C(p,$\gamma)^{13}$N reactions release enough energy to potentially impact convection, and i process is activated through the $^{13}$C($\alpha$,n)$^{16}$O reaction. The H-ingestion flash may not cause a split of the convection zone as it was seen in simulations of He-shell flashes in post-AGB and low-Z AGB stars. We estimate that for the production of first-peak heavy elements this site can be of similar importance for galactic chemical evolution as the s-process production by low-mass AGB stars. The He-shell flashes result in the expansion and, ultimately, ejection of the accreted and then i-process enriched material, via super-Eddington luminosity winds or Roche-lobe overflow. The white dwarf models do not retain any significant amount of the accreted mass, with a He retention efficiency of $\leq 10\%$ depending on mass and convective boundary mixing assumptions. This makes the evolutionary path of such systems to supernova Ia explosion highly unlikely., Comment: 6 pages, 4 figures, 1 table, accepted to publication in ApJ Letters after minor changes

Galactic chemical evolution is a topic that involves nuclear physics, stellar evolution, galaxy evolution, observation, and cosmology. Continuous communication and feedback between these fields is a key component in improving our understanding of how galaxies form and how elements are created and recycled in galaxies and intergalactic space. In this proceedings, we present the current state of the JINA-NuGrid chemical evolution pipeline. It is designed to probe the impact of nuclear astrophysics uncertainties on galactic chemical evolution, to improve our knowledges regarding the origin of the elements in a cosmological context, and to create the required interdisciplinary connections., Comment: 3 pages, 2 figures, submitted to JPS Conference Proceedings, Nuclei in the Cosmos XIV

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged), Comment: 210 pages, 91 figures. Exposure draft. Appendices to the Detailed Science Case can be found at http://mse.cfht.hawaii.edu/docs/

This work investigates the properties of convection in stars with particular emphasis on entrainment across the upper convective boundary (CB). Idealised simulations of turbulent convection in the O-burning shell of a massive star are performed in $4\pi$ geometry on $768^3$ and $1536^3$ grids, driven by a representative heating rate. A heating series is also performed on the $768^3$ grid. The $1536^3$ simulation exhibits an entrainment rate at the upper CB of $1.33\times10^{-6}~M_\odot~\mathrm{s}^{-1}$. The $768^3$ simulation with the same heating rate agrees within 17 per cent. The entrainment rate at the upper convective boundary is found to scale linearly with the driving luminosity and with the cube of the shear velocity at the upper boundary, while the radial RMS fluid velocity scales with the cube root of the driving luminosity, as expected. The mixing is analysed in a 1D diffusion framework, resulting in a simple model for CB mixing. The analysis confirms previous findings that limiting the MLT mixing length to the distance to the CB in 1D simulations better represents the spherically-averaged radial velocity profiles from the 3D simulations and provides an improved determination of the reference diffusion coefficient $D_0$ for the exponential diffusion CB mixing model in 1D. From the 3D simulation data we adopt as the convective boundary the location of the maximum gradient in the horizontal velocity component which has $2\sigma$ spatial fluctuations of $\approx0.17 H_P$ . The exponentially decaying diffusion CB mixing model with $f = 0.03$ reproduces the spherically-averaged 3D abundance profiles., Comment: 21 pages, 24 figures, 1 table. Accepted for publication in MNRAS

We use the OMEGA galactic chemical evolution code to investigate how the assumptions used for the treatment of galactic inflows and outflows impact numerical predictions. The goal is to determine how our capacity to reproduce the chemical evolution trends of a galaxy is affected by the choice of implementation used to include those physical processes. In pursuit of this goal, we experiment with three different prescriptions for galactic inflows and outflows and use OMEGA within a Markov Chain Monte Carlo code to recover the set of input parameters that best reproduces the chemical evolution of nine elements in the dwarf spheroidal galaxy Sculptor. This provides a consistent framework for comparing the best-fit solutions generated by our different models. Despite their different degrees of intended physical realism, we found that all three prescriptions can reproduce in an almost identical way the stellar abundance trends observed in Sculptor. While the three models have the same capacity to fit the data, the best values recovered for the parameters controlling the number of Type Ia supernovae and the strength of galactic outflows, are substantially different and in fact mutually exclusive from one model to another. For the purpose of understanding how a galaxy evolves, we conclude that only reproducing the evolution of a limited number of elements is insufficient and can lead to misleading conclusions. More elements or additional constraints such as the galaxy's star formation efficiency and the gas fraction are needed in order to break the degeneracy between the different modeling assumptions. Our results show that the successes and failures of chemical evolution models are predominantly driven by the input stellar yields, rather than by the complexity of the galaxy model itself. Simple models such as OMEGA are therefore sufficient to test and validate stellar yields., Comment: 16 pages, 9 figures, 1 table, accepted in ApJ

We present hydrodynamic simulations of the common envelope binary interaction between a giant star and a compact companion carried out with the adaptive mesh refinement code ENZO and the smooth particle hydrodynamics code PHANTOM. These simulations mimic the parameters of one of the simulations by Passy et al., but assess the impact of a larger, more realistic initial orbital separation on the simulation outcome. We conclude that for both codes the post-common envelope separation is somewhat larger and the amount of unbound mass slightly greater when the initial separation is wide enough that the giant does not yet overflow or just overflows its Roche lobe. PHANTOM has been adapted to the common envelope problem here for the first time and a full comparison with ENZO is presented, including an investigation of convergence as well as energy and angular momentum conservation. We also set our simulations in the context of past simulations. This comparison reveals that it is the expansion of the giant before rapid in-spiral and not spinning up of the star that causes a larger final separation. We also suggest that the large range in unbound mass for different simulations is difficult to explain and may have something to do with simulations that are not fully converged., Comment: Accepted by MNRAS

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21-23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9- 10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12-13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade., Comment: 131 pages, community white paper based on 2012 JINA Town Meeting in Detroit, MI, and 2014 APS Town Meeting in College Station, TX, corrected author names

We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H]=-1.62+/-0.09) star HD94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s-process; e.g., [Pb/Fe]=+0.79+/-0.32) and rapid neutron-capture process (r-process; e.g., [Eu/Fe]=+0.22+/-0.12), including unusually large molybdenum ([Mo/Fe]=+0.97+/-0.16) and ruthenium ([Ru/Fe]=+0.69+/-0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe]=-0.06+/-0.19). We analyze an archival near-ultraviolet spectrum of HD94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99+/-0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated by the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy., Comment: Accepted for publication in the Astrophysical Journal. (13 pages, 7 figures)

Motivated by recent results in stellar evolution that predict the existence of hybrid white dwarf (WD) stars with a C-O core inside an O-Ne shell, we simulate thermonuclear (Type Ia) supernovae from these hybrid progenitors. We use the FLASH code to perform multidimensional simulations in the deflagration to detonation transition (DDT) explosion paradigm. Our hybrid progenitor models were produced with the MESA stellar evolution code and include the effects of the Urca process, and we map the progenitor model to the FLASH grid. We performed a suite of DDT simulations over a range of ignition conditions consistent with the progenitor's thermal and convective structure assuming multiple ignition points. To compare the results from these hybrid WD stars to previous results from C-O white dwarfs, we construct a set of C-O WD models with similar properties and similarly simulate a suite of explosions. We find that despite significant variability within each suite, trends distinguishing the explosions are apparent in their $^{56}$Ni yields and the kinetic properties of the ejecta. We comment on the feasibility of these explosions as the source of some classes of observed subluminous events., Comment: 14 pages, 19 figures, submitted to the Astrophysical Journal

We used a one-zone chemical evolution model to address the question of how many masses and metallicities are required in grids of massive stellar models in order to ensure reliable galactic chemical evolution predictions. We used a set of yields that includes seven masses between 13 and 30 Msun, 15 metallicities between 0 and 0.03 in mass fraction, and two different remnant mass prescriptions. We ran several simulations where we sampled subsets of stellar models to explore the impact of different grid resolutions. Stellar yields from low- and intermediate-mass stars and from Type Ia supernovae have been included in our simulations, but with a fixed grid resolution. We compared our results with the stellar abundances observed in the Milky Way for O, Na, Mg, Si, Ca, Ti, and Mn. Our results suggest that the range of metallicity considered is more important than the number of metallicities within that range, which only affects our numerical predictions by about 0.1 dex. We found that our predictions at [Fe/H] < -2 are very sensitive to the metallicity range and the mass sampling used for the lowest metallicity included in the set of yields. Variations between results can be as high as 0.8 dex, for any remnant mass prescription. At higher [Fe/H], we found that the required number of masses depends on the element of interest and on the remnant mass prescription. With a monotonic remnant mass prescription where every model explodes as a core-collapse supernova, the mass resolution induces variations of 0.2 dex on average. But with a remnant mass prescription that includes islands of non-explodability, the mass resolution can cause variations of about 0.2 to 0.7 dex depending on the choice of metallicity range. With such a prescription, explosive or non-explosive models can be missed if not enough masses are selected, resulting in over- or under-estimations of the mass ejected by massive stars., Comment: 14 pages, 2 tables, 14 figures, submitted to MNRAS

We use a simple one-zone galactic chemical evolution model to quantify the uncertainties generated by the input parameters in numerical predictions, for a galaxy with properties similar to those of the Milky Way. We compiled several studies from the literature to gather the current constraints for our simulations regarding the typical value and uncertainty of seven basic parameters, which are: the lower and upper mass limits of the stellar initial mass function (IMF), the slope of the high-mass end of the stellar IMF, the slope of the delay-time distribution function of Type Ia supernovae (SNe Ia), the number of SNeIa per Msun formed, the total stellar mass formed, and the final mass of gas. We derived a probability distribution function to express the range of likely values for every parameter, which were then included in a Monte Carlo code to run several hundred simulations with randomly selected input parameters. This approach enables us to analyze the predicted chemical evolution of 16 elements in a statistical way by identifying the most probable solutions along with their 68% and 95% confidence levels. Our results show that the overall uncertainties are shaped by several input parameters that individually contribute at different metallicities, and thus at different galactic ages. The level of uncertainty then depends on the metallicity and is different from one element to another. Among the seven input parameters considered in this work, the slope of the IMF and the number of SNe Ia are currently the two main sources of uncertainty., Comment: 20 pages, 12 figures, 7 tables, accepted in ApJ (v.3 - 11/05/2016)

Massive stars are key sources of radiative, kinetic, and chemical feedback in the universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes---GENEC, KEPLER and MESA---to compute non-rotating stellar models of $15~\mathrm{M}_\odot$, $20~\mathrm{M}_\odot$, and $25~\mathrm{M}_\odot$ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The GENEC and KEPLER models hold physics assumptions used in large grids of published models. The MESA code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by GENEC. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool MPPNP. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than $30\,\%$---smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g., helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models., Comment: 17 pages, 10 figures, 7 tables; accepted for publication in Monthly Notices of the Royal Astronomical Society

When carbon is ignited off-center in a CO core of a super-AGB star, its burning in a convective shell tends to propagate to the center. Whether the C flame will actually be able to reach the center depends on the efficiency of extra mixing beneath the C convective shell. Whereas thermohaline mixing is too inefficient to interfere with the C-flame propagation, convective boundary mixing can prevent the C burning from reaching the center. As a result, a C-O-Ne white dwarf (WD) is formed, after the star has lost its envelope. Such a "hybrid" WD has a small CO core surrounded by a thick ONe zone. In our 1D stellar evolution computations the hybrid WD is allowed to accrete C-rich material, as if it were in a close binary system and accreted H-rich material from its companion with a sufficiently high rate at which the accreted H would be processed into He under stationary conditions, assuming that He could then be transformed into C. When the mass of the accreting WD approaches the Chandrasekhar limit, we find a series of convective Urca shell flashes associated with high abundances of 23Na and 25Mg. They are followed by off-center C ignition leading to convection that occupies almost the entire star. To model the Urca processes, we use the most recent well-resolved data for their reaction and neutrino-energy loss rates. Because of the emphasized uncertainty of the convective Urca process in our hybrid WD models of SN Ia progenitors, we consider a number of their potentially possible alternative instances for different mixing assumptions, all of which reach a phase of explosive C ignition, either off or in the center. Our hybrid SN Ia progenitor models have much lower C to O abundance ratios at the moment of the explosive C ignition than their pure CO counterparts, which may explain the observed diversity of the SNe Ia., Comment: 6 pages, 4 figures, submitted to the Proceedings of Science, XIII Nuclei in the Cosmos,7-11 July, 2014, Debrecen, Hungary