Star formation and Stellar Evolution

Mark Thompson, Jan Forbrich

Most surveys of our Milky Way galaxy are focused on the inner third of the Milky Way's disk as this is the most cost effective way to search the volume of the Milky Way for massive star formation. The "outer reaches" of the Milky Way located outside the Sun's orbit have not been well-studied due to the larger areas of sky that need to be mapped and the lower density of star forming regions there. However, this neglects the importance of the low metallicity regions in the Outer Galaxy as potential analogues for star formation in the earlier Universe. In this project you will take advantage of two recent sub-millimetre surveys (SASSy and SASSy-Perseus) to create a definitive atlas of the star formation  in the outer reaches of the Milky Way. You will use archival molecular spectroscopy to determine the distance to these regions, search for signs of embedded star formation using the JVLA telescope and archival data, and determine the star formation rate for these regions. This will create a valuable dataset with which to compare to the ATLASGAL studies of the inner Milky Way (Urquhart et al 2018) and resolved  studies of nearby galaxies (e.g. M31, Forbrich et al 2020). Our eventual aim is to determine a physical relationship between the properties of molecular gas and the resulting massive star formation, which can then be used to interpret unresolved high redshift studies.

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 many 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 by comparing computational simulations to these observational data. Our hydrodynamical simulation code already includes basic physics such as star formation and supernova feedback, and thus it is possible to predict the evolution of elemental abundances in the Milky Way. The student will update the code to include the detailed effects from binary stars and predict the rates of gravitational wave events for future space missions.

Philip Lucas, Zhen Guo, Jan Forbrich

We have recently learned that Young Stellar Objects (YSOs) are the commonest type of highly variable star in the infrared. Much of this is due to episodic accretion: a sudden increase in accretion rate from the circumstellar disc on to the central protostar by up to a factor of 1000. This is caused by an as-yet unknown instability in the disc and it is an active topic in star formation research. These events can cause a YSO to become several magnitudes brighter for a few months or even several decades. In addition, some YSOs have sudden dips in brightness, even fading from view entirely for years.

In some cases due to light being obscured by a part of the disc, perhaps perturbed by a companion star or planet. The goal of this PhD is to investigate these phenomena using near infrared and mid-infrared time series photometry from the Vista Variables in the Via Lactea survey (co-led by Prof Lucas) and the NASA WISE satellite. The project will include spectroscopic follow up with the telescopes in Chile and the student will work as a member of a team at UH and in Chile.

Martin Krause

The magnetic properties of massive stars are now known in quite some detail. About 7% of such stars show strong magnetic fields up to 1 T. The occurrence of strong magnetic fields correlates with likewise strong chemical abundance inhomogeneities on the surface of the stars. The origin of the strong magnetic fields is not known, stellar collisions being one of the leading hypotheses.

The winds of massive stars impact their environment in various ways. Surrounding gas is pressurised and pushed out, sometimes out to 100s of light years. This leads to the formation of superbubbles in the interstellar medium. Superbubbles play interesting roles in compressing other gas possibly leading to more star formation and also by transporting other gas into the halo of a galaxy, thus linking to the galaxy's overall evolution. Our group is strongly involved in observations and modelling of such superbubbles. One direction of this PhD would be to produce the first magnetised superbubble simulations that properly take into account the variety of magnetic field properties of the driving stars, based on existing code setups and know-how present in the team.

The other direction would be to model these magnetised superbubbles in the context of so-called super-massive stars. These so-far hypothetical stars might occur in very dense and massive star clusters and explain chemical peculiarities in such massive star clusters. The super-massive stars are thought to form via collisions, and would hence be expected to be strongly magnetised. If the chemical abundance inhomogeneities on their surface would be similar to the ones of normal magnetic massive stars, chemical inhomogeneities in the wind would arise which could be linked to the inhomogeneities in the star cluster.

In this project, the student would use the numerical tools present in the group to make the first predictions for superbubbles driven by magnetised massive stars. The project could go more in the direction of superbubbles from normal massive magnetised stars or exploring magnetised super-massive star superbubbles, depending on the interests of the student.