Solar, Stellar and Time-domain Astrophysics

Supervisory Team: Martin Krause, Rob Yates

Stellar collisions are rare in the Milky Way, and even when galaxies merge, the individual stars do usually not get very close to each other. This may be different in particularly dense, possibly young stellar regions. The nearby Orion star-forming region hosts the so-called Becklin-Neugebauer (BN)/Kleinman-Low (KL) explosive outflow, which may have been generated by a stellar merger some 500 years ago. Also, massive binary stars are known with short orbits that may lead to eventual mergers and high magnetic field strength in some massive stars may point to this.

This project targets primarily the merging of very massive stars with smaller stars, a process that has been proposed to produce much more massive stars than commonly known. Stellar evolution models show that such stars would have strong winds which would be severely depleted in oxygen and enriched in sodium. It is well known that a large fraction of low-mass stars in globular clusters has this chemical fingerprint that would be expected if they formed from gas ejected by such particularly massive stars. Continuous advances in stellar spectroscopy have recently also found globular clusters where some stars are additionally enhanced other interesting elements. One suspicion that now needs to be theoretically clarified is if the collisions themselves could synthesise new chemical elements, which would be an important confirmation of the above scenario.

To this end, we have obtained stellar evolutionary models of very massive stars. The student in this project would run 3D hydrodynamics simulations of stellar mergers, and determine transient observational signatures of the actual mergers as well as the produced nucleosynthesis fingerprint from the simulated data. The aim is to make predictions to be compared to observations such that the hypothesis of globular cluster formation involving very massive stars formed via mergers can be either corroborated or otherwise.

Supervisory Team: Klaas Wiersema, James Collett, William Cooper

The last few years have seen an exponential growth in the number of orbiting satellites and debris objects, mostly because of the build-up of mega-constellations of communication satellites (e.g. Starlink), anti-satellite weapon tests and accidental collisions. This has greatly increased the importance of accurate monitoring of orbiting objects, not just in low-Earth orbits but also in geostationary orbits and even in the cis-lunar domain. Astronomers have the best equipment: large telescopes with very sensitive instruments, suited for highly detailed studies of these objects using novel techniques that are not yet employed by commercial space domain awareness (SDA) companies. This allows us to turn these objects from a nuisance into useful physics and astronomy tools, to study things like: the physics of reflection and absorption of light by objects in the near-Earth environment; the properties of dust particles in the upper atmosphere; the magnetic field properties far from Earth; the interaction of radiation and high-energy particles with solid surfaces, and the long-term orbital and spin evolution of (irregularly shaped) debris under solar radiation pressure.

In this project, you will obtain and use high-cadence data of light reflected by orbiting satellites and debris, using large astronomical telescopes with specialist astronomical instrumentation. This project will particularly focus on high-speed optical linear polarisation measurements, using data obtained through several large surveys currently led by the project supervisor. Through these polarisation data, combined with simultaneous multi-colour light curves, we will measure the orientation of reflecting elements on satellites (e.g. the solar panels, antennae, etc) and the spin-state evolution of inactive (dead) satellites and pieces of debris. We will use these data to characterise the effects of sunlight on the long-term evolution of tumbling debris, and compare these measurements with those of near-Earth asteroids. In addition, you will use the light reflected by satellites to measure properties of dust particles in the upper atmosphere, which are valuable input into exoplanet atmosphere models. Lastly, you will be able to take advantage of new developments in sensor technology and employ them in the SDA domain, such as novel polarisation detectors (originally developed for the self-driving car industry) and new widefield astronomical facilities such as the Digital Telescope (a 52-telescope prototype will be built soon on La Palma).

Supervisory Team: Mike Kuhn, Jan Forbrich, Phil Lucas

There is growing evidence that planet formation in discs around young stars is shaped by the external environment, especially ultraviolet (UV) radiation from nearby high-mass stars. While previous studies have focused on relatively quiescent regions, giant star-forming complexes with multiple high-mass stars produced a large fraction of the stars in the Galaxy. In these environments, far-ultraviolet (FUV) photons can drive external photoevaporation, stripping mass from disc exteriors, truncating discs, and potentially limiting planet growth. Understanding how extreme radiation fields influence discs is therefore central to assessing whether planet formation is ubiquitous or strongly regulated by external conditions.

We have obtained ~400 high signal-to-noise spectra that, for the first time, reveal detailed compositional features in discs around low-mass stars exposed to these radiation fields. This study targets Trumpler 14, a million-year-old star cluster in the Carina Nebula complex. The O2 supergiant system HD 93129, near the cluster centre, irradiates the young stars and their protoplanetary discs in some of the strongest FUV fields in our Galaxy—substantially exceeding those in massive star-forming regions such as the Orion Nebula. A preliminary inspection of JWST imaging and spectra shows clear signatures of discs around low-mass stars undergoing photoevaporation.

In this project, you will characterise and catalogue spectral features from the JWST/NIRSpec MSA observations (0.97–1.82 and 2.87–5.14 microns). This includes measuring diagnostics of infalling and outflowing gas, dust grains in discs, and molecules (e.g., CO, CO₂, H₂O), applying physical emission models, and developing astrophysical scenarios to explain the origin of these features. You will use the large statistical sample to search for correlations between star and disc properties. Possible directions for your project could include identifying new near-infrared indicators of disc photoevaporation, explaining the diversity of observed disc properties, investigating the connection between disc properties and accretion rates, or comparing the observed spectra with theoretical models of disc photoevaporation and truncation. The overall goal will be to determine whether strong FUV irradiation suppresses or fundamentally alters planet formation.

The JWST General Observer programme is GO 5791 (PI M. Kuhn). You will collaborate with an international team as part of the Extreme Ultraviolet Environments (XUE) project.

Supervisory Team: Jan Forbrich, Mykola Gordovskyy, Mike Kuhn

Within the past year, aurorae - or northern lights - have captured public attention in the UK. Aurorae occur when particles from the Sun, known as solar wind, interact with the Earth's magnetic field. As the Sun reaches a peak in its activity cycle, researchers are also exploring other effects of space weather, such as the impact of solar flares - massive bursts of energy from the Sun - on satellites and infrastructure on Earth. While today's space weather poses challenges, it is far milder than the conditions that existed when the Sun was young and planets were forming. Recent advances in radio telescope technology allow us to investigate these extreme environments by observing young stars and protostars, as examples similar to the young Sun. By searching for flares at different radio wavelengths, including simultaneously to disentangle emission mechanisms, and linking stellar activity to the process of mass accretion, we are also uncovering how stars grow and gain their mass. In this project you will analyse novel observations of high-energy flares and potentially even coronal mass ejections from protostars, primarily using the first-ever simultaneous ALMA and VLA observations, our ALMA high-priority project targeting the rich Orion Nebula Cluster (PI: Forbrich). In a collaboration of stellar and solar astronomers, we will draw connections between young stars and the Sun's activity, offering a window into our Sun's early history and the effects of these high-energy processes on the forming solar system. Our findings will not only shed light on the origins of our own star but also help us understand how extreme space weather shapes other planetary systems. This project is part of a larger project called The Orion Radio All-Stars, which encompasses extensive VLA, VLBA, ALMA, and Chandra observations of the Orion Nebula Cluster.

Supervisory Team:  Chiaki Kobayashi, Federico Sestito, Sophie Koudmani

Soon after the Big Bang, only very light elements (H, He, Li, Be, and B) can be produced, and carbon and heavier elements (up to uranium) are all formed in stars and ejected by supernovae. Elemental abundances observed in millions of stars in the Milky Way are fossils in the study of ‘Galactic archaeology’, recording the history of our Milky Way Galaxy. When and where did stars form and satellite galaxies merge? How were the Milky Way’s substructures (bulge, disks, halo) made? The student will study the evolution of the Milky Way using computational simulations including chemical enrichment from various types of stars and supernovae. The student will combine our state-of-the-art chemical enrichment routine in a hydrodynamical code AREPO, predict the spatial distribution of elements, and compare with high-resolution spectroscopic observations of stars in the Milky Way (from APOGEE, HERMES-GALAH, WEAVE, 4MOST, and Subaru PFS). This also leads to understanding the origin of elements (such as gold!) in the Universe.

Supervisory Team: Mike Kuhn, Phil Lucas, Jan Forbrich

Most stars form in giant molecular cloud complexes that produce hundreds to thousands of stars. Nevertheless, once a star-formation episode ends, most stars disperse and only a small fraction remain gravitationally bound in clusters. In this project, you will investigate young, embedded clusters that are still assembling with ongoing star formation, to determine what sets their initial properties.

The project will start with a multiwavelength examination of the young cluster in W40, the youngest nearby massive star-forming region. You will analyse deep Chandra X-ray observations of the cluster and use several near-infrared spectroscopic data sets to characterise individual sources and compile a definitive membership catalogue. Using this catalogue, you will examine the spatial distribution of members and apply dynamical models of cluster formation. For example, the most massive star lies near the centre and is surrounded by an extremely dense group of young, low-mass stars. You will test whether this configuration could result from dynamical core collapse or required in-situ formation. Proper motions from Gaia will be used to probe the cluster's internal kinematics, assessing whether the system is dynamically hot, cold, or in equilibrium. In particular, the fourth Gaia data release (planned for December 2026) is expected to yield a factor-of-2 increase in proper-motion precision, significantly improving measurements for W40 and enabling the first Gaia-based kinematic analysis of an embedded cluster.

The project will then proceed with comparative studies of other young, embedded clusters, especially RCW 36, RCW 38, and DR 21.