Francis X. Timmes

⁸⁵Kr is an s-process branch point nucleus: the competition between neutron capture and β decay influences the production of nearby nuclides. Its evolution is complicated by the existence of a long-lived isomer that β-decays on a much shorter timescale than the ground state. We calculate the effective thermal transition rates between the ground and isomeric states along with estimates of their thermal β-decay rates in a suite of nucleosynthesis computations to assess the consequences of this isomer on isotopic abundances. A central finding in this study is that the s-process flow with the isomer included under astrophysical conditions relevant for the s-process creates a shift in abundance yield of the important radioactivity ⁸⁷Rb by a factor of ∼2.4 relative to the case when the ⁸⁵Kr is taken to be internally equilibrated when neutron exposures approach the main branch limit. On the other hand, the ⁸⁸Sr/⁸⁶Sr ratio is little affected because the bulk of the flow through ⁸⁵Kr is still via beta decay, whether the isomer is taken into account or not.

When a star undergoes core collapse, a vast amount of energy is released in a ∼10 s long burst of neutrinos of all species. Inverse beta decay in the star’s hydrogen envelope causes an electromagnetic cascade that ultimately results in a flare of gamma rays—an “echo” of the neutrino burst—at the characteristic energy of 0.511 MeV. We study the phenomenology and detectability of this flare. Its luminosity curve is characterized by a fast, seconds-long rise and an equally fast decline, with a minute- or hour-long plateau in between. For a near-Earth star (distance D ≲ 1 kpc) the echo will be observable at near future gamma-ray telescopes with an effective area of 10³ cm² or larger. Its observation will inform us on the envelope size and composition. In conjunction with the direct detection of the neutrino burst, it will also give information on the neutrino emission away from the line of sight and will enable tests of neutrino propagation effects between the stellar surface and Earth.

We present a set of 1194 Type-1 Rosseland-mean opacity tables for four different metallicity mixtures. These new Los Alamos OPLIB atomic radiative opacity tables are an order of magnitude larger in number than any previous opacity table release, and span regimes where previous opacity tables have not existed. For example, the new set of opacity tables expands the metallicity range to Z = 10⁻⁶ to Z = 0.2, which allows improved accuracy of opacities at low and high metallicity, increases the table density in the metallicity range Z = 10⁻⁴ to Z = 0.1 to enhance the accuracy of opacities drawn from interpolations across neighboring metallicities, and adds entries for hydrogen mass fractions between X = 0 and X = 0.1 including X = 10⁻², 10⁻³, 10⁻⁴, 10⁻⁵, 10⁻⁶ that can improve stellar models of hydrogen deficient stars. We implement these new OPLIB radiative opacity tables in MESA and find that calibrated solar models agree broadly with previously published helioseismic and solar neutrino results. We find differences between using the new 1194 OPLIB opacity tables and the 126 OPAL opacity tables range from ≈20% to 80% across individual chemical mixtures, up to ≈8% and ≈15% at the bottom and top of the solar convection zone respectively, and ≈7% in the solar core. We also find differences between standard solar models using different opacity table sources that are on par with altering the initial abundance mixture. We conclude that this new, open-access set of OPLIB opacity tables does not solve the solar modeling problem, and suggest the investigation of physical mechanisms other than the atomic radiative opacity.

Erratum: “On Stellar Evolution in a Neutrino Hertzsprung–Russell Diagram” (2020, ApJ, 893, 133), Ebraheem Farag, F. X. Timmes, Morgan T. Chidester, Kelly M. Patton, R. Farmer Erratum: “On Stellar Evolution in a Neutrino Hertzsprung–Russell Diagram” (2020, ApJ, 893, 133), Ebraheem Farag, F. X. Timmes, Morgan T. Chidester, Kelly M. Patton, R. Farmer

We explore neutrino emission from nonrotating, single-star models across six initial metallicities and 70 initial masses from the zero-age main sequence to the final fate. Overall, across the mass spectrum, we find metal-poor stellar models tend to have denser, hotter, and more massive cores with lower envelope opacities, larger surface luminosities, and larger effective temperatures than their metal-rich counterparts. Across the mass–metallicity plane we identify the sequence (initial CNO → ¹⁴N → ²²Ne → ²⁵Mg → ²⁶Al → ²⁶Mg → ³⁰P → ³⁰Si) as making primary contributions to the neutrino luminosity at different phases of evolution. For the low-mass models we find neutrino emission from the nitrogen flash and thermal pulse phases of evolution depend strongly on the initial metallicity. For the high-mass models, neutrino emission at He-core ignition and He-shell burning depends strongly on the initial metallicity. Antineutrino emission during C, Ne, and O burning shows a strong metallicity dependence with ²²Ne(α, n)²⁵Mg providing much of the neutron excess available for inverse-β decays. We integrate the stellar tracks over an initial mass function and time to investigate the neutrino emission from a simple stellar population. We find average neutrino emission from simple stellar populations to be 0.5–1.2 MeV electron neutrinos. Lower metallicity stellar populations produce slightly larger neutrino luminosities and average β decay energies. This study can provide targets for neutrino detectors from individual stars and stellar populations. We provide convenient fitting formulae and open access to the photon and neutrino tracks for more sophisticated population synthesis models.

We present MESA-Web,a cloud resource with an online interface to the Modules for Experiments in Stellar Astrophysics (MESA) software instrument. MESA-Web allows learners to evolve stellar models without the need to download and install MESA. Since being released in 2015, MESA-Web has delivered over 17,000 calculations to over 2,200 unique learners and currently performs about 11 jobs per day. MESA-Web can be used as an educational tool for stars in the classroom or for scientific investigations. We report on new capabilities of MESA-Web introduced since its 2015 release including learner-supplied nuclear reaction rates, custom stopping conditions, and an expanded selection of input parameters. To foster collaboration we have created a Zenodo MESA-Web community hub, where instructors can openly share examples of using MESA-Web in the classroom. We discuss two examples in the current community hub. The first example is a lesson module on Red Giant Branch stars that includes a suite of exercises designed to fit a range of learners and a Jupyter workbook for additional analysis. The second example is lesson materials for an upper-level Astronomy majors course in Stars and Radiation that includes an assignment verifying some of the expected trends that are presented in a popular stellar physics textbook.

We consider the combined effects that overshooting and the ¹²C(α, γ)¹⁶O reaction rate have on variable white dwarf (WD) stellar models. We find that carbon–oxygen (CO) WD models continue to yield pulsation signatures of the current experimental ¹²C(α, γ)¹⁶O reaction rate probability distribution function when overshooting is included in the evolution. These signatures hold because the resonating mantle region, encompassing ≃0.2 M _⊙ in a typical ≃0.6 M _⊙ WD model, still undergoes radiative helium burning during the evolution to a WD. Our specific models show two potential low-order adiabatic g-modes, g ₂ and g ₆, that signalize the ¹²C(α, γ)¹⁶O reaction rate probability distribution function. Both g-mode signatures induce average relative period shifts of ΔP/P = 0.44% and ΔP/P = 1.33% for g ₂ and g ₆, respectively. We find that g ₆ is a trapped mode, and the g ₂ period signature is inversely proportional to the ¹²C(α, γ)¹⁶O reaction rate. The g ₆ period signature generally separates the slower and faster reaction rates, and has a maximum relative period shift of ΔP/P = 3.45%. We conclude that low-order g-mode periods from CO WDs may still serve as viable probes for the ¹²C(α, γ)¹⁶O reaction rate probability distribution function when overshooting is included in the evolution.

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.

Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.