Abstracts 2022
Alexander Hall-Smith (York)
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 A comparison between different generations of reaction libraries will also be presented, displaying the impact that outdated rates can have when modelling a complex scenario such as a common envelope system.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.
James Keegans (Univ. Keele)
Using a new set of metallicity dependent SNIa yields, we identify isotopic abundances which differ between sub-Chandrasekhar and Chandrasekhar mass progenitors across an initial mass fraction of Ne-22 ranging from 0 to 0.1.
This presents a robust constraint on nucleosynthetic tracers of progenitor mass which are insensitive to initial metallicity.
Taking these tracer isotopes, we then investigate the contribution to the solar composition from sub- and Chandrasekhar mass explosions using the one zone GCE code OMEGA.
Alexander J Hackett (Univ. Cambridge)
Thorne- Żytkow Objects (TŻO) are a class of hybrid stars, originally proposed by Thorne & Żytkow (1977), consisting of a neutron star core, surrounded by a diffuse giant envelope. This is analogous to how one may consider the structure of a giant with a degenerate core as a white dwarf embedded in a giant envelope.
Structural and nucleosynthetic models of these objects have not been updated since the models of Cannon et al. 1992/93, all of which are derived from the models of TŻ. Although these models have been shown to be self-consistent, it would be instructive to re-create these models using the mesa stellar evolution
code, without using the models of TŻ a priori.
Using a set of central boundary conditions to mimic the presence of a neutron star, we construct a preliminary series of envelope models that may show some marked differences from those models. Our models’ structures show an expanded analogue of the supergiant-like solutions from TŻ and Cannon et al.
etc, with these solutions being found in a wide range of envelope masses, including in the region where TŻ found a different, giant-like structure.
We discuss our solutions and the implications of the possible existence a parallel series of equilibrium structures for TŻOs. We also discuss the important implications of our structure solutions for possible nucleosynthetic pathways in our models, and the further effects of this on yields of these post-XRB systems,
and on Galactic Chemical Evolution as a whole.
Benjamin Wehmeyer (Konkoly Obs & Univ Hertfordshire)
In addition to the insights gained by studying the galactic evolution of chemical elements, short lived radioisotopes contain additional information on astrophysical nucleosynthesis sites. Meteorites can carry information about the nucleosynthetic conditions in the early Solar System using short lived radioisotopes [1][2], while detections of live isotopes of cosmic origin in the deep sea crust help us understand recent nucleosynthetic processes in the Solar neighborhood [3]. We use a three dimensional, high resolution chemical evolution code to model the conditions at the time of the formation of the Solar System, as well as to explain why different classes of radioisotopes should often arrive conjointly on Earth, even if they were produced indifferent sites. Further, we included radioisotope production into a cosmological zoom-in chemodynamical simulation of a Milky Way-type galaxy, which provides a map of gamma-rays from the decay of radioactive Al-26 consistent with the observations by the INTEGRAL instrument [4]. Further, we’ll apply the insights gained from these models to draw conclusions about the rapid neutron capture process, one of the most important nucleosynthesis process for the formation of the heaviest elements.
[1] Lugaro, Ott, Kereszturi, 2018 PrPNP 102, 1L [2] Côté et al., 2021 Science 371, 945
[3] Wallner et al., 2021 Science 372, 742W
[4] Kretschmer et al., 2013 A&A 559, A99
William M. Baker (Univ. Cambridge)
The Fundamental Metallicity Relation (FMR) describes how gas-phase metallicity is observed to primarily depend on stellar mass, whilst having a secondary inverse dependence on star formation rate. However, the metallicity may depend (both theoretically and observationally) on several other galactic quantities, which are directly or indirectly connected with the mass, SFR and metallicity. Therefore, identifying the quantities that are directly driving the metallicity from those that related to the metallicity via secondary correlations with other quantities is difficult, but it is also important to infer from these relations the galaxy evolutionary processes that they might trace. Determining galactic properties that are primary drivers of the galaxy metallicity provides a test to both observations and simulations. In this presentation I will analyse spatially resolved observational data obtained from the MaNGA and ALMaQUEST surveys and apply partial correlation coefficients and random forest regression to untangle the intrinsic dependencies of the resolved metallicity. I will show that, based on these analyses, the local metallicity depends not only on local properties (e.g. local stellar mass surface density, galactocentric radius) but also has a fundamental dependence on global properties (total stellar mass, global SFR). I will quantify these intrinsic correlations and discuss plausible physical mechanisms that may be driving them.
Lorenzo Roberti (CSFK - Konkoly Obs.)
Neutron-capture processes made most of the abundances of heavy elements in the Solar System, however they cannot produce a number of rare proton-rich stable isotopes (p-nuclei) lying on the left side of the valley of stability. The γ-process, i.e., a chain of photodisintegrations starting on heavy nuclei, is recognized and generally accepted as a feasible process for the synthesis of p-nuclei in core-collapse supernovae (CCSNe). However, this scenario still leaves some puzzling discrepancies between theory and observations. We aim to explore in more detail the γ-process production from massive stars, using different sets of CCSNe models and the latest nuclear reaction rates. Here we show our preliminary analysis, by identifying the γ-process sites and focusing on progenitors of CCSNe that experience a C-O shell merger just before the collapse of the Fe core.
Vishnu Varma (Univ. Keele)
Core-collapse supernovae (CCSNe) are some of the brightest, most energetic events in the
universe. In order to model these phenomena accurately, we need to have a diverse range of physics such as neutrino transport and neutrino interactions, general-relativistic gravity, detailed equations of state (EoS) of dense matter, magnetohydrodynamic (MHD) and detailed progenitor models. The advent of powerful supercomputers and the development of numerical methods has allowed us to simulate these explosions in great detail, and in recent years, in full 3D geometry. However, aside from the modelling of rare hypernova progenitors (where rapid rotation and very strong magnetic fields are imposed), magnetic fields haven’t been explored extensively for the neutrino-driven explosion mechanism.
In this talk, I will focus primarily on the inclusion of magnetic fields in modelling these events. I will present the first multidimensional simulation of magnetic fields in the final phases of oxygen shell burning, which provides a first step in understanding what realistic magnetic field strengths and geometries should be in CCSNe progenitors. I will then explore some of the first 3D simulations of magnetic neutrino-driven supernovae, and explain how this inclusion can impact the dynamics of the supernova as well as their remnants.
Janet Bowey (Univ. Cardiff)
The line of sight towards blazar PKS 1830–211 passes through a face-on lensing spiral galaxy at z=0.886. The absorber produces submm and radio absorption bands due to molecular species indicative of conditions similar to those of Milky-Way (MW) molecular clouds and HII regions associated with massive young stellar objects (YSOs). I decipher the carriers of solid-state infrared absorption features near 4.3, 4.6, 6.0 and 6.9 micron and compare the results with those for MW sightlines through the Taurus molecular cloud towards Elias 16, and massive YSOs in high-mass star forming regions (S140 IRS 1,
Mon R2 IRS 3, AFGL 989). I show that overlapping 6.9 micron components, due to carbonate dust and CH3OH ice, can be distinguished during feature fitting and that the derived
proportions are consistent with CH3OH:H2O ratios obtained from 3.0--4.0 micron spectra. Other dust components and associated features include H2O (6.0 micron) and CO2 (4.3 micron) ices and an organic residue OCN- (4.6 micron). The ratio of CH3OH ice to H2O ice in the PKS 1830--211 absorber (20--40 %) is 3--8 times the value in quescent MW molecular clouds and similar to the highest values obtained in massive MW YSOs; it is also consistent
with the unique detection of gas-phase submm CH3OH absorption in this galaxy-absorber. Carbonate mass-densities towards PKS 1830--211 and Elias 16 are similar to those detected in the circumstellar environment of the dust-forming post-AGB star Sakurai's Object. No
CH3OH was detected in Mon R2 IRS 3, but this source may contain 20-micron-sized SiC grains which contribute to an absorption plateau near 6.4 micron. Carbonate dust was not detected towards AFGL 989.
Ryan Alexander (Univ. Hull)
Ultra faint dwarf galaxies are observed to contain no traces of gas within them, providing insight into the nature of star formation and chemical evolution during the early Universe through their small scale structure and high dark matter concentrations.
Understanding the chemical abundance dispersion in such hostile environments could shed light on the stellar properties and chemical abundances of these stars. We present the inhomogeneous chemical evolution model i- GEtool for two ultra faint dwarf galaxies within the orbit of the Large Magellanic Cloud - Reticulum II and Carina II. Using several stellar yields, we compute the chemical abundances of generated stars, each with a mass, metallicity and age. For both models, we predict a star formation period with many stars being enriched from type Ia and core collapse supernovae. We recreate the chemical abundance of these galaxies for several - and odd-z elements in Carina II and Reticulum II. We compute the average mass loading factor for our models ans find relatively high values compared to those predicted in massive galaxies. A synthetic color-magnitude diagram is created comparing the stars within Reticulum II with our model, including a prediction with the Car II model. We found that more gas was removed due to supernova ejecta in our Carina II model than Reticulum II, suggesting that other factors were necessary to remove the gas from these ultra faint dwarfs.
Francesca De Angeli (Univ. Cambridge)
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.
David Hendriks (Surrey)
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.
Federico Rizzuti (Keele)
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.
Kate Womack (Hull)
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.
Sophie Abrahams (York/Los Alamos)
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.
Chloe Bosomworth (Hertfordshire)
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.
Rob Yates (Hertfordshire)
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.
Borbála Cseh (Konkoly Observatory)
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.
Umberto Battino (Hull)
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.
Josh Wilson (York)
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.
Erin Higgins (Armagh)
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.
Stephen Smartt