Abstracts 2022

For a neutron star binary system to merge within a Hubble time the progenitor system must undergo a common envelope phase to form the close binary that will collide [1]. The common envelope phase consists of a compact remnant, in this scenario a neutron star, that is engulfed by the envelope of its companion, during which an accretion disk is believed to form around the neutron star. This disk provides a new location for nucleosynthesis to occur as high temperatures and densities are reached. An initial post processing nucleosynthesis (PPN) investigation has shown that this area might provide rapid proton capture nucleosynthesis that is potentially ejected into the interstellar medium. Alongside this an investigation into the impact that different reaction rate libraries and PPN codes might have on resulting abundances was also undertaken. A comparison between different generations of reaction libraries will also be presented, displaying the impact that outdated rates can have when modelling a complex scenario such as a common envelope system. As well as this, a second comparison between different PPN codes will be presented which have shown differing results for both simple and complex stellar models.

Blue (BP) and Red (RP) Photometer low-resolution spectral data is one of the new products introduced in the third Gaia Data Release. Spectra covering the wavelength range [330,1050] nm are published for almost 220 million objects, mostly brighter than G=17.65. Signal to noise ratio varies significantly over the wavelength range covered, depending on magnitude and colour of the observed objects. This presentation will give an overview of the processing strategies that allow to convert satellite raw data into calibrated spectra, show highlights from the internal scientific validation of the results and offer some recommendations and instructions on the available data products and software tools.

In this talk I will show some recent developments of the binary_c framework, a semi-analytic rapid binary stellar evolution code with a focus on nucleosynthetic yields, and /binary_c-python/, the population synthesis wrapper around binary_c. Recently we have implemented methods to generate standardised output of nested data structures that can contain many quantities like transient event rates or chemical yields from different sources and for arbitrary delay times. We are working on generating a suite of simulations and datasets with up-to-date physics and prescription choices for (binary) stellar evolution. These datasets can then be used by other codes or frameworks, giving them access to results of the latest population synthesis results. (see the talk by Rob Yates for an example of this). Another development is the implementation of event-based-logging, which allows the standardised output of specific events, like a mass-transfer event or a supernova, for each binary system. This output type is useful to study specific physics changes and how they affect the evolution of binary systems, and how these events are related to each other in the evolutionary history of the binary. I will conclude with some example use cases like transient event rates over cosmological timescales and outlining future ideas for the code.

Our knowledge of stellar evolution is limited by uncertainties arising from complex multi-dimensional processes in stellar interiors, usually reproduced in 1D stellar evolution models with simplified prescriptions. 3D hydrodynamics models can improve these prescriptions by studying realistic multi-D processes, usually for a short timerange (minutes or hours). Recent advances in computing resources are starting to enable longer 3D simulations, more compatible with the evolutionary timescales. In this talk, I will present results coming from a new set of hydrodynamics simulations of a massive-star neon-burning shell, run continuously from early development to fuel exhaustion. I will discuss the implications for convective boundary mixing and entrainment, with the possibility of extracting an entrainment law for 1D models.

19F, the only stable isotope of fluorine, has many different potential sites and channels of production. As a result, modelling the chemical evolution of fluorine and attempting to narrow down a dominant site of production is particularly challenging. The most commonly discussed sites of fluorine production are: AGB stars, Wolf-Rayet stars, the v-process in core-collapse supernovae, novae and rotating massive stars; with AGB, Wolf-Rayet and rotating massive stars being the most likely to be the dominant source of fluorine. We use the galactic chemical evolution code OMEGA+ to investigate the chemical evolution of fluorine in the Milky Way. We explore the impact of a variety of AGB and massive star yield sets, with a particular focus on varying the prescription for the initial rotational velocity of massive stars. We find models which include rotating massive stars with vrot=300kms-1 provide the best fit to observations, while a combination of initial rotational velocities provides a more physical solution. We also investigate the fluorine abundance of Wolf-Rayet stars and rule them out as a dominant contributor to the galactic fluorine abundance.

In a binary system, the envelope of the primary can expand - for example as the star enters its red giant phase - and overflow its Roche Lobe. The envelope material accretes onto the secondary star and, if the material moves onto the star faster than it can be accreted, the secondary star can be engulfed entirely by the envelope of the primary - forming a common envelope. If the secondary star is a neutron star, there is an opportunity for nucleosynthesis to occur in the accretion disk formed around the neutron star. When the material is ejected from the accretion disk, some of it is able to move back into the companion - significant since in most neutron star accretion scenarios the material cannot escape the gravity of the neutron star - and this material could be ejected into the interstellar medium when the primary goes through its supernova. As such, understanding the nucleosynthesis of this system is of great interest. Keegans et al. 2019 explored this nucleosynthesis for the first time with simplistic trajectories, and found that this scenario could be important to galactic chemical evolution. The work presented is developed from Keegans et al. 2019, using more detailed trajectories which take into account that angular momentum is required to form an accretion disk. Within the trajectories, variables such as accretion rate, and how far into the accretion disk the material moves prior to ejection are explored. In this talk, ongoing research exploring nucleosynthesis in accreting neutron star common envelopes is presented.

Giant Molecular Clouds (GMCs) are the main sites of star formation in galaxies and are home to the majority of HII regions - typically small regions of the molecular clouds that have been photoionized by newly formed massive stars. The gas-phase metallicities of such HII regions are tracing the latest phase of a galaxy’s chemical evolution. We have conducted a spectroscopic survey of HII regions in the Andromeda galaxy (M31) with the MMT/Hectospec, complemented by a novel dust continuum and CO survey of GMCs, conducted with the Submillimeter Array. Using a BPT diagram we classified our sources as HII regions or otherwise from their emission line ratios. We have calculated metallicities from strong-line diagnostics for around 250 HII regions, values ranging from solar to super-solar. From these, we have calculated a value for the radial gradient of oxygen abundance, which is consistent with literature values within error. We investigate the significant scatter around this radial trend, occurring especially within the 12 kpc ring where the majority of star-forming regions are found, suggesting that this environment is not well-mixed. This is not due to galaxy-scale trends but occurs even between nearby clouds. Furthermore, by combining the MMT and SMA results of the same GMCs, we obtain a unique direct test of the metallicity dependence of the SMA-derived alpha CO factor, i.e., the crucial factor to convert CO emission to mass. Preliminary results suggest that there is no directly observable trend of the alpha CO factor with metallicity, in spite of a considerable dynamic range of metallicities covered in this experiment.

In this talk, I will present brand-new results on the implementation of binary star properties from BINARY_C into the L-GALAXIES galaxy evolution simulation. BINARY_C is a stellar evolution and nucleosynthesis framework designed to model both single and binary stars. L-GALAXIES is a semi-analytic simulation of galaxy evolution which can efficiently model many millions of galaxies from very high redshift to the present day. I will begin by discussing the implementation itself, which utilises ensemble population properties generated using BINARY_C-PYTHON. The chemical yields (for a wide range of elements and sources), as well as supernova and other transient rates, from these ensembles are used as inputs into L-GALAXIES, allowing us to study galaxy chemical evolution (GCE) in a statistically significant way with much more realism than is possible when using simple single-star yields. I will then present some preliminary results from this implementation, including (a) the impact of binary stars on the general galaxy population, (b) the radial distribution of various chemical elements in the gas and stars of the Milky Way, and (c) the abundance and provenance of Al26 in the solar neighbourhood. I will conclude by outlining some of the future work planned for BINARY_C + L-GALAXIES, including charting the rates of neutron star mergers and other transient phenomena as a function of time and metallicity throughout in the Universe.

The Barium (Ba) star phenomenon is unmissable from the study and understanding of nucleosynthetic processes occuring in asymptotic giant branch (AGB) stars. Ba stars belong to binary systems, where the former AGB polluted the companion, a less evolved star, which became enriched with material produced through the slow neutron capture process (s process). While the AGB has evolved to a white dwarf, the currently observed Ba star preserves the abundance pattern of the AGB, allowing us to test the imprints of the s process. Comparing different AGB nucleosynthetic models and Ba star abundances, we are able to constrain the effect of the initial rotation velocity or the neutron source in the interior of the AGB star. Here I will present the results for individual Ba stars using a simplified method of normalising the models to the determined [Ce/Fe] abundances and calculating dilution factors for each star and model. The results of the comparison of models with initial AGB masses from an independent source and the analysis of 28 Ba giant star abundances confirm that the polluting AGBs are of low mass (< 4 MSun). There is a good agreement between the models and the abundance pattern for most of the stars, with some peculiarities at the first s-process peak. Nb, Mo and Ru values higher than the model predictions indicate the operation of a different nucleosynthesis path, which needs further investigation.

The cosmic production of the short-lived radioactive nuclide 26Al is crucial for our understanding of the evolution of stars and galaxies. However, simulations of the stellar sites producing 26Al are still weakened by significant nuclear uncertainties. We re-evaluated the 26Al(n, p)26Mg and 26Al(n, α)23Na reactivities from 0.01 GK to 4 GK, based on the recent nTOF measurement combined with theoretical predictions and a previous measurement at higher energies, and test their impact on stellar nucleosynthesis. We computed the nucleosynthesis of low- and high-mass stars using the Monash nucleosynthesis code, the NuGrid mppnp code, and the FUNS stellar evolutionary code. Our low-mass stellar models cover the 2-3 M mass range with metallicities between Z=0.01 and 0.02, and their predicted 26Al/27Al ratios are compared to 62 meteoritic SiC grains. For high-mass stars, we test our reactivities on two 15 M models with Z=0.006 and 0.02. The new reactivities allow low-mass AGB stars to reproduce the full range of 26Al/27Al ratios measured in SiC grains. The final 26Al abundance in high-mass stars, at the point of highest production, varies by a factor of 2.4 when adopting the upper, or lower, limit of our rates. However, stellar uncertainties still play an important role in both mass regimes. The new reactivities visibly impact both low- and high-mass stars nucleosynthesis and allow a general improvement in the comparison between stardust SiC grains and low-mass star models. Concerning explosive nucleosynthesis, an improvement of the current uncertainties between T9∼0.3 and 2.5 is needed for future studies.

With the search for r-process and rp-process element sources in order to match their observed abundances, common envelopes and the nucleosynthesis within neutron star systems may be a place to look for this dilemma. This research looks to introduce CE systems and the nucleosynthesis that may occur within accreting neutron stars and its role within galactic chemical evolution.

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.