Carbon burning is a key step in the evolution of massive stars, Type 1a supernovae and superbursts in x-ray binary systems. Nevertheless, our understanding of this critical fusion reaction is not as complete as might be desirable to fully constrain astrophysical models. This limitation centres of the difficulty in determining the $^{12}$C+$^{12}$C fusion cross section at energies corresponding to the Gamow window for these different scenarios as it relies on extrapolation of direct measurements made at higher energies. Such direct fusion measurements are complicated by the presence of resonances at and below the Coulomb barrier. These resonances have traditionally been associated with the formation of short-lived molecular states based on $^{12}$C+$^{12}$C or similar alpha-conjugate systems. Despite study of these resonances over many years, a comprehensive theoretical model accounting for their existence and structure is presently lacking.
Given the difficulties associated with direct fusion studies of the $^{12}$C+$^{12}$C reaction, indirect studies which can identify potential resonances within the respective Gamow windows are of high value. In this respect, a study of the $^{24}$Mg($alpha$,$alpha$’)$^{24}$Mg reaction has identified several 0$^{+}$ states in $^{24}$Mg, close to the $^{12}$C+$^{12}$C threshold, which predominantly decay to $^{20}$Ne(ground state) + $alpha$ [1]. Not only were these states newly identified but surprisingly they were not observed in previously well-studied $^{20}$Ne($alpha$,$alpha_0$)$^{20}$Ne resonance scattering, potentially suggesting that they have a dominant $^{12}$C+$^{12}$C cluster structure. Given the very low angular momentum associated with sub-barrier fusion, these states, which sit in the Gamow window for massive stars, may play a decisive role in $^{12}$C+$^{12}$C fusion. We present estimates of updated $^{12}$C+$^{12}$C fusion reaction rates based on likely parameters for such resonances [1].
A fascinating aspect of the identification of these potential 0$^+$ cluster states in $^{24}$Mg close to the break-up threshold for $^{12}$C+$^{12}$C and similar channels such as $^{16}$O+$^8$Be is the circumstantial similarity to the situation in $^{12}$C with the Hoyle state at the break-up threshold and the critical role that it plays in in helium burning.
Nucleosynthesis yields from sub-Chandrasekhar (sub M-ch) and Chandrasekhar (M-ch) SN Ia progenitors have been discussed and debated for decades on their contributions to iron peak elements in the cosmos. Investigating SNe Ia in ultra-faint dwarf galaxies (UFDs) and dwarf spheroidal galaxies (dSphs) with different star formation and chemical enrichment histories may shed light on the progenitors in different environments. To this end, we incorporate metallicity dependent SN Ia yields from different progenitors within our novel inhomogeneous chemical evolution model, i-GEtool, and compare the predicted chemical abundances to observations in different UFD and dSph galaxies. While the observed [Mn/Mg] ratios increase towards higher metallicities both within single galaxies and when considering galaxies with different metallicity distributions, the observed [Ni/Mg] ratios show a weaker correlation. In my talk, I will show that our models for UFD and dSph can reproduce the observed trends along with their scatter without invoking any contribution from sub M-ch SN Ia progenitors, at variance with previous studies in the literature. I will discuss the implications of our findings for the observed iron peak elemental abundances in the Milky Way halo and disks, outlining our future plan.
The population of isomeric (metastable) excited states in nuclei within astrophysical environments associated with R-process freezeout can affect the final abundance of stable isotopes; these astrophysically relevant isomers are known as `astromers’ [1][2]. Astromers can be populated/depopulated via various electromagnetic mechanisms, generally via low excitation energy, short-lived states above the isomer. The population of these `astromers’ has been studied theoretically using the Planckian photon bath [1] in which a network of photo-nuclear excited states and subsequent relaxations is considered during the population of an astromer from its associated nuclear ground state.
Similarly, an isomer can be depopulated [2] with a much lower, yet albeit comparable electron flux via inelastic electron-scattering processes, which will necessarily also deplete the astrophysical isomer [3]. One such electromagnetic process is `nuclear excitation by electron capture’ (NEEC), which is the inverse of internal conversion, recently reported to deplete isomers terrestrially with a high [4], yet still refuted [5], excitation probability in a radioactive ion-beam scenario.
This presentation focuses around encouraging the development hot-dense-plasma experiments using the photon and electron flux available at current or in-development peta-Watt (PW) laser facilities, which allow experimentation into separating the electromagnetic mechanisms at play in depleting isomers. This will allow us to readily challenge the relevance of astromers in calculating the final abundance of isotopes in the cosmos.
References
[1] G. Wendell Misch et al. “Astromers: Nuclear Isomers in Astrophysics”. In: The Astrophysical Journal Supplement Series 252.1 (Dec. 2020), p. 2. doi: 10.3847/ 1538-4365/abc41d
[2] G. Wendell Misch, T. M. Sprouse, and M. R. Mumpower. “Astromers in the Radioactive Decay of r-process Nuclei”. In: The Astrophysical Journal Letters 913.1 (May 2021), p. L2. doi: 10.3847/2041- 8213/abfb74
[3] J. Carroll and C. Chiara. “Isomer depletion”. In: The European Physical Journal Special Topics (Apr. 2024). doi: 10.1140/epjs/s11734-024-01149-8
[4] C. Chiara et al. “Isomer depletion as experimental evidence of nuclear excitation by electron capture”. In: Nature Publishing Group 554.7691 (2018), pp. 216–218. doi: 10.1038/nature25483.
[5] Y. Wu, C. H. Keitel, and A. P´alffy. “93mMo isomer depletion via beam-based nuclear excitation by electron capture”.
From the chemodynamical properties of tidal debris in the Milky Way (MW), it has been inferred that disrupted dwarf satellites had different chemical abundances at their time of accretion compared with similar-mass dwarf satellites which survive at present day. Specifically, disrupted satellites appear to have had lower [Fe/H] and higher [Mg/Fe] at fixed stellar mass than the surviving ones. In a recent study (Grimozzi, Font & De Rossi 2024), we have used the ARTEMIS simulations to investigate this problem, and determine the evolution of chemical abundances (e.g., the stellar mass-metallicity relation, MZR) with redshift. We have found a strong correlation between the scatter in the MZR of the disrupted dwarfs and their accretion redshift (zacc), as well as with their cold gas fractions at accretion. The slopes of the MZRs of disrupted dwarf satellites are fairly similar at different accretion redshifts, and are comparable with the MZR slope of surviving satellites in the MW today (≈ 0.32). This findings constrain some of the physical processes that regulate the chemical enrichment of dwarf galaxies (for example, the stellar feedback). The simulations also predict strong correlations between averaged properties of the disrupted dwarf populations, such as between «zacc», «[Fe/H]» and «[Mg/Fe]»), which suggests that the chemical abundances of the entire disrupted dwarf population can be used to constrain the merger history of its host. More specifically for the MW, the ARTEMIS simulations predict that the bulk of the disrupted population was accreted at «zacc» ≈ 2, to match the averaged «[Fe/H]» and «[Mg/Fe]». More broadly, our results suggest that one can gain an insight into the formation histories of other MW ‘analogues’, such as M31 or other massive galaxies nearby, provided that chemical abundances ([Fe/H] and [alpha/Fe]) of their debris from disrupted satellites become available.
The workhorse of understanding stellar evolution has been in 1D stellar evolution modelling, where simplified prescriptions of physical processes are implemented to evolve a star over its entire lifetime. While stellar evolution modelling has improved over the decades, their results are still limited by uncertainties in the physics due to complex multi-dimensional processes in stellar interiors. To better understand, and hopefully improve some of these uncertainties, we have run 3D hydrodynamic simulations of the final hour of silicon shell convection of a 14M_solar star prior to core-collapse. I will present these results, and compare them to what was found in 1D stellar evolution calculations. I will discuss how the presence of realistic turbulent mixing affects nuclear burning and how choices of convective overshooting in 1D can affect the final structure of the massive star.
Massive stars are not understood well enough given the important role their evolution and fates play in Galactic Chemical Evolution (GCE). One key uncertainty is convective boundary mixing (CBM), which encompasses the processes by which materials mix across the edge of convective turbulent regions inside stars. As a result of its effects on stellar structure during evolution, CBM also affects nucleosynthesis and consequently stellar yields. To investigate the importance of CBM we have computed two grids of stellar models at Z={10}^{-3} and two different strengths of CBM using the MESA code. The first being the typical CBM value used in literature and the second is based on the results of 3D convection simulations. In this talk, we will present a comparison of the structure of massive stars both during their evolution and at the end of their lives for these two different strengths of CBM to assess the impact of CBM on stellar evolution, SN progenitors and nucleosynthesis with a particular emphasis on the mergers of different burning shells.
Thorne-Żytkow Objects are a class of hybrid stars (Thorne & Żytkow 1977), consisting of a neutron star, surrounded by a diffuse convective envelope.
The formation rates of TŻOs are not well constrained but the presence of a modest population of such objects in the Galaxy could have a significant effect on Galactic Chemical Evolution. Optimistically, a large fraction of X-ray binaries end up as TŻOs, contributing to the abundance of p-process elements.
Farmer et al. (2023) placed a central boundary condition at ~600km above the centre of the star whereas we instead opt to place this at the surface of the neutron star and make use of an accretion prescription set by the opacity at the base of the envelope to model the release of gravitational energy.
We construct a series of models that show differences from those of Cannon and Farmer. Our models’ structures show an analogue of the supergiant-like solutions from TŻ and Cannon et al., these solutions being found across a wide range of masses, including where TŻ found a different, giant-like structure.
We find that the deviation from the Cannon et al. series can be explained by our prescriptions for neutrino generation, while the more significant differences to Farmer et al. are likely a function of the differences in boundary conditions.
We discuss the implications of the possible existence of differing series of structures for TŻOs. We also discuss the implications of our structures for nucleosynthetic pathways, and the further effects on GCE.
We use the (100 Mpc/h)3 Simba-C simulation to examine the chemical abundances in hot intragroup and intracluster gas, by extracting and fitting mock X-ray spectra using the MOXHA pipeline from Jennings & Dave’. As part of our initial testing phase, we used XSPEC to extract the inner-core chemical abundances of O, Ne, Mg, Al, Si, S, Ar, Fe, and Ni from our simulated MOXHA X-ray spectra for seven clusters, seven warm groups, and seven cooler groups. We found increasing chemical abundances for most elements as a function of the halo temperature above kT>1 keV (corresponding to the warmer groups), with exceptions observed for Al, S, and Ar. We also found decreasing [α/Fe] abundance ratios as a function of the halo temperatures. We are in the process of extending the number of haloes to include all simulated haloes within Simba-C with sufficient M500 halo mass and sufficient hot gas for Athena X-IFU 0.5-3.0 keV detections at R500, as well as using the Bayesian X-ray Analysis MCMC simulation to maximise the best-fit likelihood.
Within hierarchical triple stellar systems, there exists a tidal
process unique to them, known as tertiary tides. In this process,
the tidal deformation of a tertiary in a hierarchical triple drains
energy from the inner binary, causing the inner binary’s orbit to
shrink. Previous work has uncovered the rate at which tertiary
tides drain energy from inner binaries, as a function of orbital
and tidal parameters, for hierarchical triples in which the orbits
are all circular and coplanar. However, not all hierarchical triples
have orbits which are circular and coplanar, which requires an
understanding of what happens when this condition is relaxed.
In this paper, we study how eccentricities affect tertiary tides,
and their influence on the subsequent dynamical evolution of the
host hierarchical triple. We find that eccentricities in the outer
orbit undergo tidal circularisation quickly, and are therefore trivial,
but that eccentricities in the inner binary completely change the
behaviour of tertiary tides, draining energy from the outer orbit as
well as the inner orbit. Empirical functions that approximate this
behaviour are provided for ease of implementing this process in
other stellar evolution codes, and the implications of these results
are discussed.
Over the last three years, the rates of all the main nuclear reactions involving the destruction and production of 26Al in stars (26Al(n, p)26Mg, 26Al(n, α)23Na, 26Al(p, γ)27Si and 25Mg(p, γ)26Al) have been re-evaluated thanks to new high-precision experimental measurements of their cross sections at energies of astrophysical interest, considerably reducing the uncertainties in the nuclear physics affecting their nucleosynthesis. We computed the nucleosynthetic yields ejected by the explosion of a high-mass star (20 M⊙, Z = 0.0134) using the FRANEC stellar code, considering two explosion energies, 1.2 × 10^51 erg and 3 × 10^51 erg. We quantify the change in the ejected amount of 26Al and other key species that is predicted when the new rate selection is adopted instead of the reaction rates from the STARLIB nuclear library. Additionally, the ratio of our ejected yields of 26Al to those of 14 other short-lived radionuclides (36Cl, 41Ca, 53Mn, 60Fe, 92Nb, 97Tc, 98Tc, 107Pd, 126Sn, 129I, 36Cs, 146Sm, 182Hf, 205Pb) are compared to early solar system isotopic ratios, inferred from meteorite measurements. The total ejected 26Al yields vary by a factor of ~3 when adopting the new rates or the STARLIB rates. Additionally, the new nuclear reaction rates also impact the predicted abundances of short-lived radionuclides in the early solar system relative to 26Al. However, it is not possible to reproduce all the short lived radionuclide isotopic ratios with our massive star model alone, unless a second stellar source could be invoked, which must have been active in polluting the pristine solar nebula at a similar time of a core-collapse supernova.