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PhD Projects: Galactic Astrophysics

Star formation and magnetic fields in the interstellar medium

The process of star formation is one of the most important unsolved problems in modern astrophysics.  Understanding the origin of the mass distribution of new stars (the Initial Mass Function), and the cause of the very low efficiency with which these stars form, is crucial to questions as diverse as how galaxies evolve, and how solar systems such as our own are born.  One of the key unknowns in the star formation process is the role and dynamic importance of interstellar magnetic fields in mediating the gravitational collapse of star-forming regions.  We are at the beginning of a revolution in our ability to deduce the properties of these interstellar magnetic fields, driven by instrumentation such as the Atacama Large Millimeter Array (ALMA), the James Clerk Maxwell Telescope (JCMT) and the Planck satellite, which are capable of measuring polarised dust emission on size scales ranging from the spiral arms of the Milky Way through to the discs around young stars. This project will use far-infrared and submillimetre polarimetric instrumentation, primarily the JCMT and ALMA, to observe dust polarisation, and so magnetic field properties, in a variety of star-forming regions within the Milky Way.  These observations will be used to investigate the existence or otherwise of distinct weak- and strong-field modes of star formation, and so the role of magnetic fields in determining star formation efficiency and the mass distribution of new stars.

Contact: Dr. Kate Pattle (k.pattle AT ucl.ac.uk)

 

* Stellar genealogy: chemo-dynamical evolution of the Milky Way from million star spectroscopic datasets

The key to uncovering the history, evolution and formation of the Milky Way is locked up in the chemical abundances of stars. The specific chemical make-up of a star acts like a fingerprint revealing the star's birth environment. These fingerprints can be used to find stars of common origin e.g. from a heavily-disrupted dwarf galaxy that was accreted by the Milky Way long ago, or from a specific star forming region within the Milky Way itself. The corresponding locations and motions of stars give us a picture of the current state of the Galaxy with little indication of how the Milky Way reached this state. However, when combined with the stellar fossil record, we can begin forming a timeline for the formation of our Galaxy.

Much like our own DNA, the chemical make-up of stars also tells us about the prior generations of stars. Essentially every star is formed from material pre-processed in previous stellar generations. These nucleosynthetic processes are some of the most extreme astrophysical environments known, including supernovae and neutron star mergers, and produced the diversity of elements in the periodic table. Different pathways produce different chemical patterns so a given star's ancestry can be pieced together from its distinctive DNA. When taken together, the chemical patterns of stars are a combination of their ancestry and their environment so offer great potential in revealing the stellar processes that enrich a galaxy as well as the specific evolutionary history of our own Galaxy.

With multi-dimensional chemical abundance data available (e.g. GALAH, APOGEE) and on the horizon (WEAVE, DESI, 4-MOST) for millions of stars in the Milky Way, there is now huge potential in tackling these problems. The project would involve the development of chemical evolution modelling codes and their application to these million+ spectroscopic Milky Way datasets. This would involve a combination of analytic, numerical and data-intensive approaches, and requires a study and understanding of a range of astrophysical processes from the nucleosynthetic pathways for different elements to the motion of stars and gas within a galaxy. See Sanders, Belokurov & Man (www.arxiv.org/abs/2106.11324) for a recent related study. 

Contact: Dr. Jason Sanders (jason.sanders AT ucl.ac.uk)

 

* Probing the early stages of star formation

The process(es) and efficiencies of the of the earliest stages of star formation, and the means of their diagnosis, are all highly uncertain.  At UCL we have developed STARCHEM, a sophisticated combined model of the physics of infall, coupled to comprehensive descriptions of the astrochemical processes that occur in the gas. This model has been very successful in describing the physical and chemical properties of many star-forming regions but is built around the 'standard' (somewhat simplistic) paradigm of spherically symmetric, continuous, infall.  However, recent high resolution observations of star-forming complexes are revealing a web of gravitationally bound - potentially star-forming - cores, connected by filaments along which matter is apparently accreted. In addition there is a 'luminosity problem' in low-mass protostars, in which the wide range of luminosities at any particular evolutionary epoch is apparently incompatible with any single, continuous, mode of accretion. This suggests that these objects are subject to 'episodic accretion' leading to short, but intense, luminosity outbursts.

In this project the student would adapt and extend existing models to include the effects of both filamentary accretion and episodicaccretion. The resulting models would make observationally verifiable predictions that can be used to diagnose the physicalthe physical nature of the early stages of accretion and help to unravel the long-standing mysteries of the star-formation process.

Contact: Prof Jonathan Rawlings (jcr AT star.ucl.ac.uk)

 

* The astrochemical legacy of molecular clouds to protoplanetary disks

Recent observations of protoplanetary disks (PPDs) are revealing the presence of a wide range of complex organic molecules. These molecules feed into the planet forming process and may even be the seeds of the larger molecules that are the biochemical precursors of life.

What is not clear is how and when these molecules are formed. It is highly probable that they are a legacy of the molecular cloud from which the nascent PPD forms. As such the molecular cloud chemistry may be crucial in determining the chemical complexity of planetary systems. This idea has yet to be tested comprehensively, and this project will aim to establish the astrochemical link between molecular clouds and PPDs.

Our STARCHEM model has recently been updated to model the layered structures of interstellar ices. The composition of these ices has a profound effect on the chemical evolution of star-forming regions and they may act as the 'carriers' of molecular complexity as a molecular cloud evolves. We would start by gaining an understanding of how and when simple molecules (such as CO) 'freeze out' onto dust grains, and then go on to consider how larger molecules are formed and transported into PPDs. The key result from these studies will be a clear understanding of how the chemical composition of a PPD is determined by the physical evolution of its parent molecular cloud.

Contact: Prof Jonathan Rawlings (jcr AT star.ucl.ac.uk)

 

Multi-waveband studies of massive stars and their environments

Although massive stars (M>10Msun) are relatively rare, their significance for astrophysics in general is enormous. They dominate the optical spectra of active star-forming galaxies, both directly at optical–UV wavelengths, and indirectly at IR and radio wavelengths (due to reprocessing by dust and gas). Ultimately, as supernovae, and with the subsequent production of relativistic remnants (neutron stars and black holes), they generate a substantial X-ray flux which dominates this emission in normal galaxies. Massive-star winds and supernova eruptions provide substantial inputs of mechanical energy and chemically enriched material into the wider galactic environment, acting in concert in young, massive ‘super star clusters’.

Remarkably, many of the physical processes governing the evolution of massive stars are still highly uncertain. Unlike solar-mass stars, the evolution of massive stars is profoundly influenced by the amount of mass they lose throughout their lives, which is currently uncertain at the order-of-magnitude level. The problems raised by this dependency are illustrated by the diversity of core-collapse SNe and formation of appropriate massive black hole binaries implied by the first gravitational wave sources detected.

The objective of this research programme is to exploit multi-waveband line spectroscopy and continuum observations of massive in the Galaxy, LMC and SMC to advance our understanding of fundamental drivers in the evolution of massive stars; these include mass-loss through clumped and structured stellar winds, the incidence of binarity, and metallicity. By also focusing on massive we will additionally address the nebulae/ejecta/winds from these stars and how they are being profoundly affected by the cluster in which they're embedded, and most likely by the cluster wind. This in turn is highly significant to our understanding of the environments into which supernovae explode.

The PhD project will be primarily based on HST UV, near-IR and optical spectroscopy, plus data from the UCL-led e-MERLIN COBRaS Legacy radio data and sub-mm continuum of massive clusters secured with ALMA and ATCA. Ultimately we seek to study the mass-loss and environment of massive stars in all they key evolutionary phases, including blue and red supergiants, yellow hypergiants, luminous blue variables and Wolf-Rayet stars.

Contact: Prof. Raman Prinja (rkp AT star.ucl.ac.uk)

 

* Drinking evolved stars and binaries from the Gaia firehose

In 2022, the ongoing Gaia mission released its first full data release that includes not only a deeper 3D motion picture for a significant fraction of the Milky Way, but also catalogs overflowing with binary and variable stars.  It is not an exaggeration to say that a virtual fountain of discoveries awaits any research student interested in binary stars and stellar evolution.  At UCL, students have recently led a number of scientific breakthroughs using Gaia; for example, a new phase of stellar evolution in which white dwarfs develop active chromospheres like our Sun, and a previously unknown population of ancient stars through unexpected  arbon signatures in high velocity red dwarfs.  The data-driven and observational project will focus on improving our understanding of white dwarfs and dwarf carbon stars, variability, magnetism, binary fractions, origins and evolution.  Many major discoveries are just over the horizon thanks to Gaia.

Contact: Prof Jay Farihi (jfarihi AT star.ucl.ac.uk)