Abstracts 2021

I would like to welcome new members of the BRIDGCE network, which includes about 70 members across 19 UK universities now. I will then give a summary of BRIDGCE activities in 2021, and also an update on the NAM session proposal let by Erin Higgins (Armagh).

Understanding nucleosynthesis in astrophysics transients is important. Supernovae, and related events, make significant contribution to the creation of metals and are thus one driver of galactic chemical evolution. Studying the nucleosynthesis yields also plays a major part in constraining progenitor scenarios for some classes of transients - the elemental/isotopic yields encode information about conditions in the explosion itself. In the talk I will discuss how combinations of theoretical and observational work allow us to make progress on understanding nucleosynthesis of key elements in astrophysics explosions associated with compact objects (white dwarfs and neutron stars), and will also highlight some important open issues and ongoing work to address them.

To understand the distribution of rapid neutron capture (r-process) elements, we have to understand the complexity of galaxy formation, including gaseous inflows and outflows and galaxy mergers. We therefore ran a suite of cosmological, magnetohydrodynamical simulations of a Milky Way-mass galaxy with a variety of models for the sources of r-process elements. In this talk, I will focus on neutron star mergers as the dominant source and the effect of neutron star natal kicks on the r-process abundances of stars across the metallicity range. The kicks are, for the first time, included on-the-fly in the simulations. With kicks, neutron star mergers are more likely to occur outside the galaxy disc, but how far the binaries travel before merging also depends on the kick velocity distribution and shape of the delay time distribution for neutron star mergers. The resulting scatter in r-process abundances is much larger with natal kicks, especially at low metallicity, giving rise to more r-process enhanced stars. I will compare our results to observational data and discuss whether or not neutron star mergers can be the only source of r-process elements in the universe. I will argue that it is possible that that the observed scatter in r-process abundances is predominantly caused by natal kicks rather than the usual interpretation that the scatter is set by the rarity of its production source.

Supernova (SN) explosions seed heavy elements synthesised in high-mass stars into the interstellar medium (ISM). Some of atoms ejected by the supernova explosion condense into dust grains in the supernova remnants. It is still in debate how much elements condense into dust in supernova remnants. We discuss the current problem in our understanding of dust formation and metal budget in SNe.

A common envelope event is often used to explain the formation of binary systems that have no other mechanism for production. For two neutron stars to form in a close binary the precursor event is thought to be a common envelope surrounding a neutron star and its companion. Throughout this phase, material from the companion can be accreted onto the neutron star, during which the material undergoes high temperature and pressure changes, which can lead to nucleosynthesis of heavier elements. Due to the composition of the companion this event is thought to be similar to that of an X-ray burst and so could synthesise proton rich heavy elements along the rp-process reaction chains. Whereas the material in an X-ray burst is trapped on the surface of the neutron star, this phase is thought to expel the material into the interstellar medium at the end of the common envelope event. This talk will discuss the results of single zone post processing nucleosynthesis (PPN) studies conducted using NuGrid for a series of trajectories across different accretion rates.

In this talk I will give an over view of the final data release of the Gaia-ESO Survey which includes chemical abundances for some 25 elements for a sample of ~80,000 stars covering the main stellar populations in the Milky Way.

The detailed design and construction of the Cassegrain U-Band Efficient Spectrograph (CUBES) for ESO's VLT is expected to start in early 2022. CUBES will deliver unprecedented sensitivity at ground UV wavelengths, spanning 300-405nm at a resolving power of more than 20,000. I will give an update on the progress of the project and highlight some of the performance predictions for stellar abundances.

Active-target detectors play an important role in low energy nuclear physics. In such detectors, the gas target also acts as the detection gas, eliminating the physical separation between target and detection volumes. TACTIC, an active-target detector, is being jointly developed by University of York and TRIUMF. Our preliminary goal is to study alpha induced reactions at low centre of mass energies.

Currently we are modifying the existing prototype at University of York by using state of the art μ-RWELL GEM configuration. This gas amplification stage enables the detection of charged particles over a larger dynamic range. Moreover, the detector traps the ionization electrons generated by the unreacted beam inside the central cathode. In comparison to other Active-Target detectors, this structure enables it to accommodate high beam intensities without pile-up. The detector will be able to efficiently measure the energy loss of reactions products and reconstruct their tracks inside the detection volume to give a precise location of the reaction vertex. This enables the measurement of excitation function using a single beam energy.

I will present the underlying physics, detailed mechanism and the astrophysical motivation behind the development of the detector. Furthermore, the future goals and development prospects in light of the unique opportunities it can provide in the field of low energy nuclear physics.

I will also present important differences between the three simulations, which can help us constrain the efficiency of star formation, supernova feedback, and gas accretion in galaxies, via an analysis of their metal production, retention, and re-cycling. For example, those simulations which allow a high degree of metal retention in the densest gas are found to reasonably match the cosmic neutral gas ZD, but fail to simultaneously match DLA metallicities.

The chemical evolution of fluorine in the Milky Way poses a challenge as it’s only stable isotope, 19F can potentially be made in a range of stellar environments. The main postulated sites for fluorine production are: AGB stars, Wolf-Rayet stars, the ν-process in core-collapse supernovae, novae and rotating massive stars. In recent years, each source has been investigated in chemical evolution studies as being the primary source of fluorine production. Rotating massive stars have become a promising candidate for being one of the main sources of fluorine production, due to extra mixing in these objects creating more 14N as seed nuclei for 19F production. Therefore, more recent studies have included rotating massive stars in their models. It has been shown that yields of AGB stars in combination with yields of rotating massive stars can start to reproduce recent observations of fluorine in giant stars. We use the galactic chemical evolution code OMEGA+ to create a Milky Way model that can reproduce the recent set of fluorine abundances, including the ‘plateau’ at [Fe/H] ∼ 0.5 dex.

Binary stars evolve into chemically-peculiar objects and are a major driver of the Galactic enrichment of heavy elements. During their evolution they undergo interactions, including tides, that circularize their orbits and synchronize stellar spins, impacting both individual systems and stellar populations.

This work introduces an accurate implementation of equilibrium and dynamical tides in the stellar population code binary_c, relying on Zahn’s theory and MESA model grids. I will first introduce the stellar population code itself, and the overhaul that we call MINT.

Galactic chemical evolution (GCE) describes how elements are distributed within the interstellar medium (ISM) of the galaxy through catastrophic events such as supernovae (SNe). We use various homogeneous and inhomogeneous models to simulate the environment of reactions such that they match observations of the known universe. Presented, is a new inhomogeneous model based on Argast et al. (2002) where we simulate a three dimensional box from which galaxies evolve through various SNe and enrich the system. We show several preliminary results including the dwarf galaxy Draco, where we compare observations with new metallicity gradients. Along with this, we show the star formation rate (SFR), infall time and the ratio of SNII and SNIa. Finally, we explore future prospects with this new inhomogeneous model and examine further uses through future development.

Director of the International Research Network for Nuclear Astrophysics (IReNA) will tell us about the US-based network, activities, and funding opportunities, etc.

Asymptotic Giant Branch (AGB) stars are important contributors to galactic chemical evolution due to their unique nucleosynthesis, which occurs as a result of the thermal pulses and is consequently blown from their surfaces in strong stellar winds. They are considered to be major producers of carbon, nitrogen and of elements heavier than iron by the s-process. However, AGB stars are notoriously difficult to model with full stellar evolution codes, such as MESA, due to the various instabilities and convergence issues that arise during the thermal pulses. This makes the production of grids of AGB stars, with varying masses and metallicities, time consuming due to the large amount of human debugging required. This talk will summarise the various instabilities and convergence found using MESA and discuss ways to evolve past them, such that a grid of AGB models can be reliably run without intervention. From the full models, data will be extracted and transformed into a tabulated grid for use in the population synthesis code binary_c. Populations of stars can then be run through the AGB phase by interpolating parameters from the tabulated grid. These populations will have various uses including calculations of chemical yields and the ability to constrain stellar physics, such as convection, by comparison to observations.

The precise mechanism of SNIa explosions remains an unsolved problem in astrophysics. Previous work in the literature has identified 55Mn as a key discriminator, with 55Mn being preferentially produced in higher mass progenitors. We show that the production of 55Mn is sensitive in all masses of progenitor to the initial metallicity of the system, which weakens the constraint of 55Mn as a tracer for the initial mass. Our suite of 39 metallicity dependent post-processed models presents a unique opportunity to explore isotopic ratios which present a strong constraint on the progenitor mass over all realistic progenitor metallicities. We present isotopic ratios which may provide a mechanism by which the ratio of sub- to Chandrasekhar mass WDs may be determined, given appropriate constraints on the contributions of other stellar sources.

Classical novae are powerful stellar explosions that result from the interaction between a white dwarf (WD) and companion star close binary system. In this scenario, the companion star overfills its Roche-lobe allowing the WD to accrete the companion stars hydrogen-rich outer layers atop its surface until an accretion envelope of sufficient mass is accumulated to initiate nuclear reactions and a subsequent thermonuclear runaway (TNR). Spectroscopy of ejected material from classical novae outbursts show an overabundance of CNO elements and Ne, Na, Mg and Al depending on the composition of the underlying WD (CO- or ONe-rich) which nuclear processing from the TNR alone cannot account for. This suggests that material from the WD substrate is dredged-up into the accreted envelope before the envelope is ejected. In this talk, I will present the results of recent work in which the nucleosynthesis in classical novae with WD material dredge-up is modelled atop a one solar mass WD accreting solar composition material using the 1-dimensional stellar evolution code MESA.

In this talk, I will provide an overview of the recent updates in stellar physics. I will introduce the key processes in stellar models, focusing on the effects of internal mixing mechanisms and stellar outflows. With challenges remaining in the implementation of these processes in theoretical models, I will discuss the related uncertainties and their impact on the evolution and fates of stars.

We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37 high-redshift galaxies at z~2 taken from the KMOS Lensed Emission Lines and VElocity Review (KLEVER) Survey with a comparison sample of local galaxies, taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only a mild enrichment in N/O of 0.1 dex when compared to local galaxies at a given gas-phase metallicity (O/H), but shows a depletion in N/O of 0.36 dex when compared at a fixed stellar mass. We find a strong anti-correlation in local galaxies between N/O and SFR in the stellar mass-N/O plane, similar to the anti-correlation between O/H and SFR found in the mass-metallicity relation (MZR). We use this anti-correlation to construct a fundamental nitrogen relation (FNR), analogous to the fundamental metallicity relation (FMR). We find that KLEVER galaxies are consistent with both the FMR and the FNR. This suggests that the depletion of N/O in high-z galaxies when considered at a fixed stellar mass is driven by the redshift-evolution of the mass-metallicity relation in combination with a near redshift-invariant N/O-O/H relation. Furthermore, the existence of a fundamental nitrogen relation suggests that the mechanisms governing the fundamental metallicity relation must be probed by not only O/H, but also N/O, suggesting pure-pristine gas inflows are not the primary driver of the FMR, and other properties such as gas outflows, variations in galaxy age and star formation efficiency must be important.

Analysis of inclusions in primitive meteorites reveals that several short-lived radionuclides (SLRs) with half-lives 0.1-100 Myr existed in the early Solar System (ESS). We investigate the ESS origin of Pd-107, Cs-135, and Hf-182, which are produced by slow neutron captures (the s-process) in asymptotic giant branch (AGB) stars. We modelled the galactic abundances of these SLRs using the OMEGA+ galactic chemical evolution (GCE) code and two sets of mass- and metallicity-dependent AGB nucleosynthesis yields (Monash and FRUITY). Depending on the ratio of the mean life tau of the SLR to the average length of time between the formation of AGB progenitor gamma, we calculate timescales relevant for the birth of the Sun. If tau/gamma > 2, we predict self-consistent isolation times between 9 and 26 Myr by decaying the GCE predicted Pd-107/Pd-108, Cs-135/Cs-133, and Hf-182/Hf-180 ratios to their respective ESS ratios. The predicted 107-Pd/182-Hf ratio indicates that our GCE models are missing 9-73% of Pd-107 and Pd-108 in the ESS. This missing component may have come from AGB stars of higher metallicity than those that contributed to the ESS in our GCE code. If tau/gamma < 0.3, we calculate instead the time T_LE from the last nucleosynthesis event that added the SLRs into the presolar matter to the formation of the oldest solids in the ESS. For the 2 M_sun Z=0.01 Monash model we find a self-consistent solution of T_LE=25.5 Myr.

[1] Keegans J, Fryer CL, Jones SW, Côté B, Belczynski K, Herwig F, Pignatari M, Laird AM, Diget CA. Nucleosynthetic yields from neutron stars accreting in binary common envelopes. Monthly Notices of the Royal Astronomical Society. 2019 May;485(1):620-39.

The astrophysical origins of the heaviest stable elements that we observe today in the Solar System are still not fully understood. Recent studies have demonstrated that H-accreting white dwarfs (WDs) in a binary system exploding as type Ia supernovae could be an efficient p-process source beyond iron. However, both observational evidence and stellar models challenge the required frequency of these events. In this work, we calculate the evolution and nucleosynthesis in slowly merging carbon-oxygen WDs. As our models approach the Chandrasekhar mass during the merger phase, the 22Ne(α,n)25Mg neutron source reaction is activated in the external layers of the primary WD, where the carbon-rich material accreted from the secondary WD is burned via the 12C+12C reaction, which provides the necessary α-particles via the 12C(12C,alpha)20Ne channel. The resulting neutron capture abundance distribution closely resembles a weak s-process one and peaks at Zr, which is overproduced by a factor of 30 compared to solar. The mass of the most external layers enriched in first-peak s-process elements crucially depends on the 12C+12C reaction rate, ranging between 0.05 Msun and ∼0.1 Msun. These results indicate that slow white dwarf mergers can efficiently produce the lightest p-process isotopes (such as 74Se, 78Kr, 84Sr, 92Mo and 94Mo) via γ-induced reactions if they explode via a delayed detonation mechanism, or eject the unburned external layers highly enriched in first peak s-process elements in the case of a pure deflagration. In both cases, we propose for the first time that slow WD mergers in binary systems may be a new relevant source for elements heavier than iron.