Abstracts 2020

I will introduce nuclear astrophysics, and in particular the main challenges in experimentally measuring nuclear reactions of interest in quiescent and in explosive stellar scenarios. I will explain some of the most successful experimental approaches and techniques, and show recent results obtained from them. Finally, I will mention future plans and possibilities that will open up with facilities coming online in the near future.

The origin of the heaviest elements is still a puzzle. For the rapid neutron capture process (r-process), multiple sites have been proposed, e.g., neutron star mergers and (sub-classes) of supernovae. R -process elements have been measured in a large fraction of metal-poor stars. Galactic archaeology studies show that the r-process abundances among these stars vary by over 2 orders of magnitude. On the other hand, abundances in stars with solar-like metallicity do not
differ greatly. This leads to two major open questions: 1. What is the reason for such a huge abundance scatter of r -process elements in the early galaxy? 2. While the large scatter at low metallicities might point to a rare production site, why is there barely any scatter at solar metallicities? In this talk, I will discuss chemical evolution scenarios that provide an explanation for the observed abundance features of r-process elements in our Galaxy. Further, I will explain how adding short lived radioisotopes to the model can help to further constrain the r-process and other processes.

Almost all chemical elements have been made by nucleosynthetic reactions in various kind of stars and have been accumulated along our cosmic history. Among those elements, the origin of phosphorus is of extreme interest because it is known to be essential for life such as we know on Earth. However, current models of (Galactic) chemical evolution under-predict the phosphorus we observe in our Solar System. Here we report the discovery of 16 phosphorus-rich stars with unusual overabundances of O, Mg, Si, Al, and a s-process heavy elemental abundance pattern. Phosphorus-rich stars likely inherit their peculiar chemistry from another nearby stellar source but their intriguing chemical abundance pattern challenge the present stellar nucleosynthesis theoretical predictions. Specific effects such as rotation or advanced nucleosynthesis in convective-reactive regions in massive stars represent the most promising alternatives to explain the existence of phosphorus-rich stars. The phosphorus-rich stars progenitors may significantly contribute to the phosphorus present on Earth today.

The evolution of massive stars is deeply affected by uncertainties connected with the convective boundary mixing (CBM). Only recently, theoretical works are starting to improve 1D stellar evolution codes with the help of 3D hydrodynamic models, which can reproduce, on a shorter timescale, more realistic 3D processes (e.g. convection, rotation, magnetic activity). In order to better understand and constrain some of the CBM uncertainties, we computed a series of 3D hydrodynamical simulations of turbulent convection during neon burning in massive stars with the PROMPI code (using different boosting factors of the driving luminosity: 1, 10 and 100 times the standard energy generation rate). In this talk, I will present preliminary results from these 3D simulations including kinetic behaviour of the fluid elements within the convective region, entrainment rates, and shape of the boundaries estimated from the composition profiles.

Classical Wolf-Rayet (WR) stars are hydrogen-poor massive stars with prominent emission lines. They are therefore often used as benchmarks for massive helium (He) stars in stellar populations. However, their occurrence depends strongly on their environment, in particular on the metallicity (Z). Until now, our theoretical understanding of their feedback is rather fragmentary, hampering robust predictions about stellar populations and the occurrence of WR-type mass loss.
To develop a fundamental understanding of mass loss in massive He stars, we employ a new generation of model atmospheres including a consistent solution of the wind hydrodynamics. This way, we can study the ingredients of He-star winds and unveil the nature of WR-type mass loss. Our results reveal a complex picture with strong, non-linear dependencies on the luminosity-to-mass ratio and Z. Moreover, they provide a theoretical motivation for a population of He stars at low Z, which cannot be detected via WR-type spectral features.
The talk will present the results from our groundbreaking study of massive He-star atmosphere models, yielding the very first mass-loss recipe derived from first principles in this regime. Moreover, we will discuss first efforts to compare the impact of the new mass-loss recipe to typical implementations of WR mass loss in current evolution models. Performing our own studies, we will highlight the consequences of an improved treatment. We finish by outlining observational implications of our study including the need to correctly understand the manifold impacts of the presence and absence of WR-type mass-loss in the Universe.

Abstract Nova-hosting white dwarfs (WDs) have long been considered as the likely progenitors of type-Ia supernovae through the single-degenerate channel of evolution. In this scenario, the WD grows in mass with every nova outburst until it reaches the Chandrasekhar mass limit. Classical Nova eruptions have historically been disregarded as potential candidates for this channel of evolution as they are assumed to either decrease in mass with every outburst or increase in mass too slowly. However, these assumptions are based on single-nova outburst simulations or incorrect nova-modelling methods. The material ejected into the interstellar medium from classical nova events are observed to be enriched in CNO elements and sometimes other intermediate-mass elements such as Ne, Na, Mg and Al depending on the composition of the underlying WD (CO- or ONe-rich). This suggests that the solar-like material transferred from the WDs companion star becomes mixed with the underlying WD. In 1-D nova simulations, this mixing was conventionally modelled by accreting material already pre-enriched with WD matter atop a WD which subsequently lead to unrealistic nova outbursts. To improve upon this, a new method of convectively mixing WD and envelope material has been developed for the stellar evolution code MESA based on the recent results multi-D nova mixing studies. This method will be used to investigate the possibility of a WD growing to the Chandrasekhar mass limit through many classical nova eruptions in a more realistic way to previous studies.

The astrophysical ¹⁵O alpha capture reaction [1] is a key breakout route from the Hot CNO cycle leading to explosive nucleosynthesis via the rp-process on the surface of neutron stars in binary systems. Determining an accurate cross section for the relevant states is critical for a better understanding of the X-ray burst energy production and light-curves [2], as well as other novel binary stellar systems involving neutron stars and their potential impact on nucleosynthesis [3].

An indirect ⁷Li(¹⁵O,t)¹⁹Ne alpha transfer reaction in inverse kinematics has been performed, populating the relevant states at temperatures up to 1GK. In this, we take advantage of the ¹⁵O Radioactive Ion Beam provided at GANIL and the state-of-the art detection system VAMOS + AGATA + MUGAST coupled together for the first time, allowing us an unrivalled selectivity for detecting triple coincidences in this reaction. We will present the experimental set-up and analysis, as well as preliminary results for the strongest populated resonances in ¹⁹Ne. 

[1] M. R. Hall et al. Phys. Rev. C 99, 035805 (2019)
[2] R. H. Cyburt et al. Astrophys. J. 830, 55 (2016)
[3] J. Keegans et al. MNRAS, V. 485, Issue 1, Pages 620–639 (2019)

In this talk, I will introduce the concept of stellar evolution, and how it relates to nucleosynthesis. Stars are Nature's furnaces where elements are made and as such they are the link between nuclear physics and the large-scale make up of the Galaxy. I will discuss some of the issues in bridging this divide.

Within the range 26<Z<47 there is an observed overabundance of elements when compared to model predictions of nucleosynthesis. One possible solution is an extension of the s-process to rapidly rotating metal poor stars. However, there is significant uncertainty in nucleosynthesis models of these stars, primarily in the ratio of ¹⁷O(α,γ)²¹Ne / ¹⁷O(α,n)²⁰Ne. Due to low cross-sections, these reactions cannot be directly measured and must be modelled. These models depend upon the spin-parities of the energy levels of ²¹Ne within the Gamow window. Unfortunately, several of these energy levels have unknown spin-parities. In order to better predict the rates of reaction, an experiment has taken place at HELIOS (Argonne) with the reaction ²⁰Ne(d,p)²¹Ne that aims to determine the spin-parities for the resonances within the Gamow window by comparing Distorted-Wave Born Approximation predictions of angular-distribution to experimental measurements. It will also attempt to determine the neutron widths of these states. The astrophysical motivation behind the experiment and details of the ongoing analysis will be presented.

¹⁶O acts as a neutron poison for the s-process. The neutrons absorbed by ¹⁶O can be freed by the ¹⁷O(α,n)²⁰Ne reaction which competes with the ¹⁷O(α,γ)²¹Ne reaction. Detailed information about ²¹Ne can help to constrain how many neutrons are recovered. We have used the ²⁰Ne(d,p)21Ne reaction to obtain information about neutron widths, spins and parities of ²¹Ne levels.

Many post-asymptotic giant branch (post-AGB) stars, the remnants of evolved red giants that will soon become white dwarfs, are in binary-star systems. Having gone through a phase of mass transfer and quite possibly a common-envelope event, such systems are a laboratory for binary-star astrophysics. Our best theories, however, fail to reproduce their properties: these binary stars have orbits that are often far more eccentric than tidal theory allows. Perhaps this is because many post-AGB stars have large, stable circumbinary discs? These are similar in size and mass to protoplanetary discs, and affect their parent stars' orbits significantly even over their short lifetimes. We construct a model of such circumbinary discs and apply it to the disc around IRAS 08544-4431 which was recently observed by ALMA. Our model fits the disc of IRAS 08544-4431 remarkably well and, given its computational speed, forms a platform from which we can explore whole populations of circumbinary discs in binary stars.

Convection is a key process in stellar evolution which transports energy, mixes matter and shapes the internal structure of stars. Convective boundary mixing, often known as overshoot, increases the extent of convective regions and is essential for the creation of the ¹³C-pocket, the s-process site in low-mass stars. In the case of massive stars, several recent studies have shown a sensitive dependence of the pre-supernova structure and its explosion likelihood on the complex convective history. Despite the importance of convection, its impact in stellar evolution is blurred by missing details and large inconsistencies and uncertainties in 1D stellar evolution codes. One longstanding problem is the treatment of convective boundaries. Here, we will present the effects of some uncertainties, such as the convective boundary location and the amount of convective boundary mixing, and show how they impact the stellar structure, evolution and some implications for nucleosynthesis. Also, we will highlight which 1D model predictions are subject to the largest uncertainties.

Deep observations have detected extended Lya emission nebulae surrounding tens of quasars at redshift 2 to 6. However, the metallicity of such extended haloes is still poorly understood. We perform a detailed analysis on a large sample of 80 quasars at z ∼ 3 based on MUSE-VLT data. We find clear evidence of extended emission of the UV nebular lines such as C IV λ1549 or He II λ1640 for about 20% of the sample, while C III] λ1909 is only marginally detected in a few objects. By stacking the cubes we detect emission of C IV, He II and C III] out to a radius of about 45 kpc. C IV and He II show a radial decline much steeper than Lyα, while C III] shows a shallower profile similar to Lyα in the inner 45 kpc. We infer that the average metallicity of the circumgalactic gas within the central 30–50 kpc is ∼0.5 solar, or even higher. However, we also find evidence of a component of the Lya haloes, which has much weaker metal emission lines relative to Lya. We suggest that the high metallicity of the circumgalactic medium within the central 30–50 kpc is associated with chemical pre-enrichment by past quasar-driven outflows and that there is a more extended component of the CGM that has much lower metallicity and likely associated with near-pristine gas accreted from the intergalactic medium. We show that our observational results are in good agreement with the expectations of the FABLE zoom-in cosmological simulations.

Gaia and a myriad of breathtaking ground-based spectroscopic surveys have placed high-dimensional chemokinematic data of millions of Milky Way stars at our doorstep but a challenge still remains in how to interpret them. The colour-magnitude selection of stars can significantly distort the observed stellar distribution in age, metallicity, and age. I will present how extended distribution function (eDF) modelling can be used to reveal the intrinsic distribution of the stars so that we can constrain the evolutionary history and dark matter content in the Milky Way.

Galactic Archaeology seeks to reveal the formation and evolution history of the Milky Way. This is being pursued by constructing a multi-dimensional chart of the Milky Way using the great wealth of data now available from stellar spectroscopic surveys. The data encompasses a full range of parameters from kinematics and dynamics, stellar properties including chemical abundances, to stellar age and mass. This review will present the impressive suite of current and upcoming surveys and highlight key scientific discoveries.

In this talk, I will present recent work on the evolution of global metallicities and radial metallicity profiles in galaxies from high redshift to the present day. This study combines insights from the newly-improved L-Galaxies 2020 semi-analytic model of galaxy formation (which includes delayed enrichment of multiple chemical elements from SNe and stellar winds) with the latest observational chemical data for gas and stars in and around galaxies. At low redshift, L-Galaxies 2020 is compared to the mass - metallicity relation and radial profiles revealed by IFUs such as MaNGA and MUSE. At higher redshift, direct absorption-line-based metallicity measurements from QSO and GRB DLAs are used. Such measurements are ideal precursors to future high-redshift observations with JWST. Our study reveals that a very efficient direct enrichment of the circumgalactic medium (CGM) by SNe is required in L-Galaxies 2020, in order to simultaneously reproduce the chemical composition found in and around galaxies back to z=2-3. I will discuss how this requirement affects feedback energetics and mass-loading factors, and compare our findings to those from other semi-analytic models and hydrodynamical simulations. Importantly, I will also discuss how a reduction in the maximum SN-II progenitor mass in L-Galaxies 2020 can improve modelled radial profiles at large radii, and also how the assumed stellar yields can have a significant impact on the expected chemical abundances and N/O ratios.

Accretion of material onto the surface of a neutron star is a process that we know occurs in our universe, however the key nuclear reactions that are taking place are not fully understood. One way this accretion may occur is inside a common envelope between a neutron star and its companion. Such a scenario could be a precursor to a neutron star merger and is an interesting mechanism that proposes a new method of synthesising material that could help understand observed abundances in our universe. A study into simulating these regions and reaction processes is ongoing using two different nucleosynthesis simulation codes, to provide information as to which reactions are influential during this mass transfer process. These can then be further investigated with sensitivity studies to deduce which reactions inside the simulations are the most influential. Once these codes are verified they can then be applied to different scenarios. This talk will cover what common envelope accretion is and discuss updates to the current model.

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 CM 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 un-reacted 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.

It has become clear that a significant fraction of stars has one or more companions. Stars in a binary, or higher order, systems often interact by transferring mass, drastically altering their evolution compared to single stars. The transfer of material from one star to the other affects the mass ratio, orbital evolution and can even lead to the stars merging together. This process is not always conservative, and in many systems we expect some mass to have been lost in this process. The transferred material does not always directly hit the accreting object, in certain cases it forms an accretion disk. Viscous processes within the disk transport the angular momentum outward
and tidal interactions with the edge of the disk are expected to transport the excess angular momentum back into the binary orbit. Material at the outer edge potentially carries a very large specific angular momentum, and would some of that material be lost from the disk before being able to return its angular momentum to the orbit, it could alter the orbital evolution, as well as estimates for the mass transfer efficiency. In this talk I will show several results of synthetic populations of Algol systems, comparing different assumptions of disk mass loss.

In binary systems, mass transfer between stars obscures their earlier history. The changes in surface properties are complex and often crudely simplified in stellar population codes.
In this talk, I present a new approach to derive an improved prescription for mass transfer episodes, developed for the binary_c stellar population algorithm. Relying on a dedicated grid of MESA models, I compute the response of the stellar surface to an accretion pulse (that is the injection of mass over a very short time). This yields a map of the mass-radius exponents, that quantify the stability of the transfer, over the whole parameter space.
I will then discuss future developments, to describe episodic accretion as a series of impulses and quantify the occurrence of common-envelope phases or include periastron mass transfer in eccentric systems more accurately.

We will present KLEVER, an ESO Large Programme aimed at investigating dynamics, gas excitation properties and chemical abundances in high redshift galaxies, by means of near-IR spatially resolved spectroscopy. Exploiting KMOS multi-IFU observations in the J,H and K bands we aim to map multiple optical rest-frame emission lines (from [O II]3727 to [S III]9530) in a sample of ~200 galaxies between 1.2 < z < 2.5. The survey targets both gravitationally lensed galaxies in Frontier Fields clusters and non-lensed galaxies in the COMOS and GOODS fields.
We investigate the physical drivers responsible for the evolution in the emission line ratios by assessing whether the offsets from the local relations correlate with different properties like electron density, ionisation parameter and nitrogen abundance.
We also derive full metallicity maps, exploiting different calibrators and evaluate presence and evolution of metallicity gradients.
Although the bulk of the analysed galaxies are characterised by flat gradients, suggesting that efficient feedback and gas mixing processes are in place at these epochs, the irregular and non-axisymmetric patterns often seen in the full 2D metallicity maps suggests to move beyond the classical "radial-averages" approach to get meaningful constraints on galaxy evolution models and allow for fair comparison with prescriptions of high resolution simulations.

I will introduce galactic chemical evolution models and simulations, and how chemical abundances are used for Galactic and extra-galaxies studies. I will also briefly mention the observations of metallicities and elemental abundance ratios of star-forming and early-type galaxies. Similar observations will be available at high redshifts with James Webb Space Telescope, which will provide a strong constraint on the formation and evolutionary histories of galaxies.

The s-process efficiency in low-mass AGB (1.5 < M/Msun < 3) critically depends on how mixing processes in stellar interiors are handled, which are still affected by considerable uncertainties. In this work we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicity using the MESA stellar code. The combined data set includes the initial masses M_zams/M_sun = 2, 3 for Z = 0.001. The nucleosynthesis was calculated for all relevant isotopes in post-processing with the NuGrid mppnp code. Using these new models, we show the impact of the main mixing-processes affecting heavy elements nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low metallicity AGB stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low mettallicity, and its importance is comparable to the convective boundary mixing processes under the convective envelope, which determine the formation and size of the ¹³C-pocket. Additionally, our results indicate that models with very low to none mixing below the He-intershell during thermal pulses, and with a ¹³C-pocket size of at least ~3×10^-4 Msun, are strongly favoured in reproducing observations. Finally, we show how increasing the size of available datasets, comprised of abundance measurements of stars in the metallicity range here considered, is critical to carry a solid validation of low-Z AGB stellar models.

In massive-star binary systems, upon reaching later stages of stellar evolution one star can expand as a giant and envelop its companion. If the star enveloped is a neutron star, then mass will rapidly accrete onto the neutron star. Accretion onto common-envelope-phase neutron stars can result in ejected matter that has undergone burning near the neutron star’s surface [1]. We study the nucleosynthesis yields of this ejected matter using PRISM (Portable Routines for Integrated nucleoSynthesis Modeling) - a program built for time-dependent composition tracking of nucleosynthesis in an astrophysical environment. Building on the work of Keegans et. al (2019), collaborators have altered the input trajectories presented in that paper by adding considerations for angular momentum in the accreted matter, an essential component for the formation of an accretion disk around the neutron star. We are comparing the results of running both the trajectories from Keegans et al. and the updated trajectories through PRISM, looking in particular at relative abundances and mass fractions.
Our end goal is to identify nuclear reactions of interest for future experimental work. We will present the altered input trajectories, as well as preliminary results. 

[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 theoretical HD limit in metal-poor galaxies is generally expected to shift to higher luminosities owing to the physics of metallicity dependent line-driven winds. However, the observations in the Large and Small Magellanic Clouds suggest no such evidence of these winds as the primary factor in setting the HD limit. Recent studies show a downward revision of luminosities in the Galaxy as well, hinting towards a metallicity independent HD limit at about log(L) of 5.5. For this reason, we investigate the HD limit at three different metallicities using the MLT++ routine in MESA and study the impact of overshooting efficiency on our results.

Star-forming galaxies can be transformed into passive systems by a multitude of processes that shut down (i.e. ‘quench’) star formation, such as the halting of cold gas accretion (starvation) or the rapid removal of gas in AGN-driven outflows. However, it remains unclear which processes are the most significant, primary drivers of the star-forming–passive bimodality. In this talk, I will discuss how measurements of the relative level of chemical enrichment in star-forming and passive galaxies can be used to derive valuable new insights into the quenching process, using stellar metallicities as a tracer of quenching mechanisms. Having analysed the stellar metallicities of tens of thousands of local star-forming, green valley and passive galaxies in the Sloan Digital Sky Survey, I will demonstrate that galaxies typically undergo a significant chemical transformation during the quenching process. Passive galaxies are in fact considerably more metal-rich (> 0.2 dex) than star-forming galaxies of the same stellar mass, indicating that the bulk of metals in passive galaxies are formed during the quenching process, rather than during the main sequence evolution of their star-forming progenitors. I will show that this significant difference in stellar metallicity between star-forming and passive galaxies implies that for galaxies at all masses, quenching must have involved an extended phase of starvation (where the accretion of metal-poor gas from the CGM/IGM is halted). I will further highlight how our chemical-enrichment-based technique has been applied to yield new insights into the mass-, environment- and radial-dependence of galaxy quenching.

I will introduce the Cassegrain U-band Efficient Spectrograph (CUBES) that will provide ESO's VLT with unrivalled sensitivity for high-resolution (R~20,000) spectroscopy at ground ultraviolet wavelengths (305-400nm). Near-UV spectroscopy provides access to a tremendous diversity of atomic transitions for neutron-capture elements, light elements, and even light-element molecules (CO, CN, OH) in stellar spectra, enabling new insights into nucleosynthesis and studies of stellar populations. The near UV is also critical in exciting topics in solar system science and extragalactic astronomy. The aim of this presentation is to introduce the technical concept and development plan for the instrument, but also to stimulate discussion/ideas on the interests and priorities from the BRIDGCE community, to feed in to the ongoing development of the CUBES science case and future observational programmes.

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).