Solar, Stellar and Time-domain Physics

Supervisory team: Chiaki Kobayashi, Sean Ryan

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).

Supervisory team: Klaas Wiersema, Dominic Walton, Martin Krause

X-ray binaries are a class of stellar binaries, in which a black hole or neutron star accretes matter from its companion star. This process leads to a bright accretion disk, as well as powerful collimated outflows in the form of narrow jets or wider-angle disk-winds. These outflows slam into the interstellar medium (ISM), forming an expanding shock front: an expanding bubble is inflated. In rare cases, the velocity of the X-ray binary through the Galaxy exceeds the local sound speed in the ISM. In those cases, the bubble gets “stretched out”, taking the shape of a so-called head-tail nebula, with a funnel-like linear “neck’’ terminating in a bubble.

Such Galactic head-tail sources were first predicted from theory, and are believed to be responsible for a significant fraction of the buoyant plasma in the Milky Way. The first actual detection was done accidentally by one of us: in deep imaging data we found a head-tail nebula formed by the neutron star X-ray binary SAX J1712.6-3739. This discovery now allows us to test theoretical models for these jet-ISM interaction sites through detailed observations. We can then use the shape of the nebula to answer questions like: how long have the jets been “on”? How did the X-ray binary get such a high velocity? What does the nebula do to the gas and dust around the X-ray binary? How much hot plasma is produced?

This project is observational in nature, using large amounts of data (radio, X-ray and optical) to study the brightest X-ray binary bubbles in detail; and we will search for additional X-ray binary powered bubbles in the Galaxy. The student will be able to work with data from the largest telescopes (such as VLT, XMM, ATCA, Gemini) and acquire new observations using the biggest and newest facilities (such as the Cerenkov Telescope Array, currently under construction) to study these enigmatic objects.

Supervisory team: Jan Forbrich, Mike Kuhn, Phil Lucas

Young stars have long been known to have highly variable emission. Initially discovered in the optical wavelength regime, this is even true when considering emission in different parts of the electromagnetic spectrum. This is due to both the effect of mass accretion, the process by which stars gather their mass during formation, and due to intense high-energy processes that are already happening in very early evolutionary stages of young stellar objects (YSOs).

In recent years, it has increasingly become possible to study YSOs in the time domain, with timescales ranging from hours to monitoring over many years. The more straightforward availability of time domain data has also enabled several new and targeted multi-wavelength observing campaigns, where variability is studied simultaneously in different wavebands. This includes experiments that are detecting X-ray and radio emission but also experiments that are targeting the interplay of X-ray and mid- to far-infrared emission. The goal is to disentangle and characterize mass accretion and high energy processes in greater detail in the time domain. Furthermore, we may even be able to constrain the impact of high-energy irradiation on protoplanetary disks, affecting the initial conditions of planet formation.

This project will start by taking stock of the available data, including dedicated multi-wavelength projects that we carried out using XMM-Newton and Herschel (X-rays and far-infared) as well as Chandra and Spitzer (X-rays and mid-infrared), accompanied by ongoing work involving X-ray and radio emission. By studying the link between X-ray and infrared emission, we expect to be able to better disentangle mass accretion and high-energy flaring events.

The project will be mainly observational, involving the analysis of data from state-of-the-art facilities, beginning with existing data. Furthermore, this will involve the utilization of statistical tools for time domain analysis. For the interpretation, we will also conduct numerical radiative transfer modeling experiments.

Supervisory Team: Michael Kuhn, Jan Forbrich, Rafael S. de Souza

Project Description: Our understanding of the Milky Way, particularly its spiral arms closest to the Sun, has improved dramatically over the past few years, with astrometric surveys like Gaia and multiwavelength Galactic plane surveys providing a clearer view of our galaxy’s 3D structure. These advances have revealed that several giant filamentary star-forming structures can explain most nearby star formation. However, integrating these giant filaments within the Galaxy's spiral structure remains an open question.

Project Aim: This project focuses on mapping young stars in the Solar neighbourhood, including within several large filamentary structures (e.g., the Sagittarius Spur, the Radcliffe Wave, the Split, and others still needing to be fully mapped). The properties of the young stellar populations, e.g., their ages and velocities, will provide constraints on star-formation history and the astrophysical mechanisms that regulate star-formation rates in these regions. The project will involve developing methodology for estimating stellar properties and modelling stellar clustering and kinematics on multiple spatial scales. Empirical results from the Solar neighbourhood will be compared to theoretical star-formation simulations provided by collaborators to infer theoretical constraints on star-formation processes.

Research Methodology: In this project, you will use multiwavelength astrometric, spectroscopic, and photometric surveys to identify young stars. You will start with published lists of young stars and add to this sample using machine learning and astrophysically motivated selection criteria. Astrometry will be used to investigate kinematics of young stars in clusters and kinematics of young clusters in their orbits around the Galaxy. In particular, the project will use Gaia Data Release 3 (with release 4 expected in 2025), multiple photometric surveys (including the UH-co-led VVV survey), and multiple spectroscopic surveys of young stars from WEAVE and SDSS V (and others that might become available). You will compare spatial distributions of stars to maps of dust and molecular gas in the Galaxy.

This work will build off several previous studies (linked below):

Kinematics in Young Star Clusters and Associations with Gaia DR2

SPICY: The Spitzer/IRAC Candidate YSO Catalog for the Inner Galactic Midplane

Astronomers Find a ‘Break’ in One of the Milky Way’s Spiral Arms

Supervisory Team: Mykola Gordovskyy, Jim Geach

We live in the heliosphere - a volume of space dominated by the Sun. The Sun is active, it changes, constantly producing sunspots, flares and coronal mass ejections. Solar flares - the most energetic explosive events in the solar system - are one of the main features of solar activity. They result in major perturbations in the solar atmosphere, or corona, and heliosphere. Understanding solar activity and solar flares is key to understanding how the Sun and other magnetically-active stars work. In addition, solar flares can explain the behaviour of hot magnetised plasma in laboratory devices such as tokamaks.

High-energy particles - electrons and ions travelling at fractions of the speed of light - dominate the primary energy release in flares. Some of these particles precipitate in the corona, producing bright radio, X-ray and gamma-ray radiations, but some escape into the heliosphere, affecting space weather. This project will use the magnetohydrodynamic-particle modelling framework developed by Dr. Gordovskyy to create models of particle acceleration and transport in individual solar flares matching multi-wavelength and in situ observations from the Solar Orbiter and Parker Solar Probe missions, as well as the LOFAR radio-telescope. The ultimate goal will be to develop a pipeline for nearly-real-time modelling of energetic particles in flares and forecasting their properties in the solar corona and inner heliosphere. Although the project will be primarily theoretical/computational, it will also involve analysis of observational data using state-of-the-art techniques, including machine learning and neural network algorithms.