Solar, Stellar and Time-domain Physics

Mykola Gordovskyy, James Geach

Solar flares, the most powerful explosions in the solar system, are one of the main manifestations of solar activity. They are produced by sudden dissipation of magnetic field energy stored in the solar atmosphere, or corona. Up to 90% of the energy released in flares is carried by high-energy particles – electrons and ions travelling at a fraction of the speed of light. These particles produce bright X- and gamma-ray radiations, as well as radio-emission from decametre to centimetre wavelengths. Furthermore, some of these particles escape from the corona and cause major perturbations in the interplanetary space, or heliosphere, and, often, in the near-Earth environment. Hence, understanding how solar energetic particles are accelerated and transported is not only an important problem of astrophysics, but also of practical importance.

In this PhD project you will explore how solar energetic particles are accelerated in flares, propagate in the solar corona, escape into the heliosphere, and affect space weather. You will be able to choose whether to focus more on theoretical aspects and computational modelling of energetic particles in the solar corona, or on observational aspects and the use of advanced data processing technics, such as machine learning.

Example projects are:

• Hybrid fluid-particle modelling of hot magnetised plasmas in the solar corona;
• Computational modelling of energetic particles in the solar corona, and their escape into the interplanetary space;
• Sizes and shapes of low-frequency radio-sources produced by energetic electrons in the solar corona.

Chiaki Kobayashi, Sean Ryan

Project description: Just after the Big Bang, only very light elements (H, He, Li, Be, and B) can be produced, and heavier elements are all formed in stars and ejected by supernovae. Many elements (from carbon to uranium) have been observed in millions of stars in the Milky Way with spectroscopic ‘galactic archaeology' surveys, and some of the production sites are also observed as gravitational wave events such as the neutron star merger in 2017. The student will study the origin of elements in the Universe by comparing computational simulations of galaxies to these observational data, then predict the gravitational wave events for future missions in space (LISA) and on Moon. Our hydrodynamical simulation code already includes basic physics such as star formation and supernova feedback, and thus it is possible to compare with the observed elemental abundances in the Milky Way and its satellite galaxies. The student will update the code (written in c) to include the detailed effects from binary stars (for the first time in the world), and use local LINUX cluster and national supercomputer facility (DiRAC).

Rafael S. de Souza, Mike Kuhn

This thesis will focus on developing new methods for the analysis and uncertainty quantification of astronomical variability. The focus will be given to multiwavelength time-series data. The project will combine principled statistical models with cutting-edge machine learning methods. Including but not limited to Bayesian structural time series, functional data analysis, Fourier and Wavelet analysis, and Gated Recurrent Unit.

Particular emphasis will be given to analysing the variability of Young Stellar Objects, but the methods developed herein will be widely applicable in other contexts in preparation for the LSST. YSO variability can be caused by various time-dependent phenomena associated with star formation, including accretion rates, geometric changes in the circumstellar disks, and reconnections within the stellar magnetosphere.

However, accurately characterising their light curves is challenging due to peculiarities of Astronomical data not commonly encountered in other fields, including non-regularly spaced data, heteroscedastic measurement errors, and spatial correlation due to telescope cadence. The project will use Zwicky Transient Facility (ZTF) and the VVV survey data. The project will aim to develop a data-driven classification scheme for YSO variability that can provide constraints for astrophysical models of YSOs.

The student will have the opportunity to collaborate with colleagues in France, under the umbrella of Fink, but also from the USA, Brazil, and the international Cosmostatistics Initiative collaboration. This project will give the student a series of skills in data processing, time-series analysis, and scalability.

Jan Forbrich, Elias Brinks

Thermal centimetric bremsstrahlung continuum emission from individual HII regions is one of the most direct and extinction-free tracers of the star formation rate (SFR) in galaxies. The VLA upgrade now enables large-scale studies of such emission in nearby galaxies. After a targeted survey of the known Giant Molecular Associations in M31, we have conducted an unbiased C-band (4-8 GHz) survey of the entire M31 galaxy, using the Karl G. Jansky Very Large Array. With a synthesized beam size of 13pc, this experiment will probe all radio sources on sub-cloud  scales and help identify not only candidate HII regions but also supernova remnants (SNRs) throughout the galaxy  at unprecedented sensitivity. SNRs in particular may no longer be associated with GMAs while tracing previous  generations of star formation. These data will be analyzed in conjunction with our continuing SMA and optical spectroscopic surveys, targeting star-forming regions in M31 with the goal of studying star formation scaling  relations (and more!). In this project, you will learn about radio interferometry using a key facility, and you will explore the radio sources of an entire external galaxy (a famous one at that) in a rich multi-wavelength context.