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: Philip Lucas, Michael Kuhn

In the past 15 years there have been more and more observations of fine gas filaments associated with star formation in nearby molecular clouds. These include “streamers” that connect cloud cores to the envelopes and accretion discs around newborn stars (protostars). The importance of these structures for star formation is not yet clear. Recently, we unexpectedly discovered a large set of giant stars near the Galactic Centre that undergo years-long occultations or eclipses by fine gaseous structures in the foreground, specifically in the Central Molecular Zone (CMZ) of the Milky Way. This phenomenon offers a new probe of fine structures in molecular clouds. The aim of the project is to determine the origin of these “Dipping Giant” events by building a large catalogue of the events from near infrared VVV survey data and then cross-matching with catalogues of dense cores and YSOs that are becoming available from ALMA and the James Webb Space Telescope. There would also be an opportunity to broaden the research to investigate non-periodic variability in VVV stars more generally and follow up unusual sources.

Supervisory Team: Klaas Wiersema, Martin Krause

The universe hosts extreme acceleration sites (for example the so-called “Pevatrons") where particles can be accelerated to incredibly high energies, orders of magnitude higher than is possible with accelerators on Earth. These particles produce high energy radiation, which we can detect using satellites (e.g. Fermi) and groundbased Cerenkov telescopes. Several sensitive high-energy photon observatories are about to start operations, such as  the Cerenkov Telescope Array (CTA), which will give us a more complete census of high-energy particle acceleration in the nearby Universe.

Some of the most extreme accelerator sites are relativistic jet sources, such as blazars: accreting supermassive black holes that produce highly relativistic jets pointing right at us. Electron acceleration in these jets produces most of the detected high energy photons, but in some circumstances protons may also get accelerated to highly relativistic velocities. When those protons interact with photons, a flux of high energy muon neutrinos is produced. These muon neutrinos can be detected by gigantic detectors like Icecube at the South Pole (Nobel prize 2026) and KM3Net in the deep ocean. Icecube detects a few dozen candidate  astrophysical high-energy muon neutrinos (with energies of TeV - PeV) per year, with uncertainties in their localisation of a degree or two. So far only a very small number can be confidently associated with a blazar in outburst.  Other origins for some of these neutrinos have been proposed (e.g tidal disruption events) but statistical evidence for association of detected neutrinos with (jet) sources is still rather weak. This is in part because most jet sources (blazars in particular) capable of producing large rates of muon neutrinos are extremely variable, hindering our ability to infer the presence of accelerated protons through fits of simultaneous multiwavelength observations. Additionally, a proper statistical approach requires a good observational  understanding of the rate and physics of unrelated transients and variables that may pop up as chance coincidences in the error circles of neutrino alerts.

The focus of this project is optical followup of high energy transient sources capable of emitting neutrinos, as well as followup of neutrino alerts themselves. This involves rapid response followup observations of neutrino alerts issued by Icecube and KM3Net, acquiring multi-wavelength lightcurves and optical polarimetry of candidate sources, and fitting the data using new models. The student will analyse the automatic followup data of the GOTO observatory (homogeneous followup of a large number of high energy neutrino alerts from Icecube for several years), and use the lessons learned from that to define and expand the followup strategy for KM3Net neutrinos and, in the future, low energy neutrinos detected by Hyper-K.

Supervisory Team: Mykola Gordovskyy, Mike Kuhn, Jan Forbrich

Solar flares are the most powerful explosions in the Solar System. On the Sun and other stars, they are one of the primary manifestations of magnetic activity. Flares release enormous amounts of energy within a very short time and accelerate electrons and ions to near-relativistic speeds. These high-energy particles carry up to 90% of the energy released during a flare. They generate intense X-ray, gamma-ray, and radio emissions across a wide range of wavelengths, providing unique diagnostics of the underlying physical processes. Some of these particles escape from the solar or stellar atmosphere, driving space weather. This is why solar flares are also of practical importance: energetic particles from flares can affect satellites, communication systems, navigation technologies, and power infrastructure on Earth. Understanding how energetic particles are accelerated and transported therefore represents one of the key challenges in modern astrophysics, with both fundamental scientific and practical significance.

In this PhD project, you will investigate the physics of particle acceleration and transport in solar and stellar flares and explore their impact on the space environment around stars. The project offers opportunities to work at the forefront of astrophysics using a combination of theory, numerical modelling, observations, and data science techniques.

Depending on your interests, you may focus on:

• Theoretical modelling of energy release, particle acceleration, and thermal and non-thermal emission in solar or stellar flares;
• Analysis of observational data related to high-energy particles from ground- and space-based observatories;
• Application of advanced data analysis methods, including machine learning, to large observational datasets.