Star formation and Stellar Evolution


Anatomy of a spiral arm - the Carina OB stars

Janet Drew, Ralf Napiwotzki

This project builds on the success of a recently-completed PhD project, that has identified the deep OB star population in the Carina Arm (Mohr-Smith et al 2016). OB stars are the youngest, most massive stars in a star-forming Galaxy like the Milky Way (MW) and are critical objects in galactic evolution, thanks to their prodigious luminosities, their winds and eventual explosion as supernovae.  Up until recently, study of the bulk of the MW's population has been hindered because of obscuring dust in the MW disk. This is changing thanks to the well-advanced deep photometric optical surveys, IPHAS and VPHAS+ . A further aid will be the first Gaia-mission astrometry to be released in the last quarter of 2017.

The Carina Arm is rich in massive stars:  the newly-uncovered population of some 6000 OB stars probes its structure from 2 to 10 kpc approximately.  This new sample is large enough to begin answering a series of questions about these important objects, such as: how many in the field are star cluster escapes; what is their relationship with current star formation, and what are the mysterious lower-luminosity hot stars apparently mixed in. The student taking on this project will be working in the astronomy 'big data' world, and will be enthusiastic about acquiring (highly employable) advanced IT skills. Both photometric and spectroscopic optical/infrared data will be examined and modelled.


The ionized Galactic interstellar medium

Janet Drew, Tim Gledhill, Nick Wright (Keele)

This project is about the exploitation of two wide-field digital surveys providing compete coverage of the plane of the Milky Way at optical wavelengths. Together IPHAS (www.iphas.org) and VPHAS+ (www.vphas.eu) are building a massive legacy of high resolution imaging of the ionised interstellar medium in our galaxy as portrayed in H-alpha (656.3 nm), the brightest emission line in the spectrum of hydrogen, which in turn is the most abundant chemical element. On the large scale, H-alpha imagery traces the UV interstellar radiation field, which is strongest in HII regions and otherwise percolates into low opacity regions -- it brings to attention dynamic phenomena provoked by wind-blown bubbles, stellar ejecta, and supernovae. Until now, the possibility to build a complete quantitative view of the ionized Galactic interstellar medium has not existed. A programme of work on the preparation of the northern survey data for scientific exploitation, will be undertaken, including e.g. cross-calibrations to other photometric and spectroscopic data. The student would be involved in developing high-level automated image processing methods and in preparing for and receiving data from a major new spectroscopic survey (WEAVE) beginning on the William Herschel Telescope in late 2018/early 2019 (half way through the project).


Radio surveys of the Milky Way with the next generation of interferometers

Mark Thompson, James Urquhart (Kent)

Radio astronomy is undergoing a renaissance as existing facilities upgrade their instrumentation (e.g. the Jansky Very Large Array, JVLA) or new facilities are built as pathfinders for the Square Kilometre Array (e.g. MeerKAT, ASKAP). The Square Kilometre Array will revolutionise many areas of astronomy from cosmology to strong gravity, as well as driving forward technological developments in data processing and communications.

We are leading two major surveys of the Milky Way Galaxy with SKA pathfinder and precursor telescopes: the MeerGAL survey with MeerKAT and KuGARS on the JVLA. The aim of both of these projects is to uncover high frequency (> 10 GHz) radio emitters which have gone undiscovered in current low frequency radio surveys. As dense plasma becomes optically thick at low frequencies, the characteristic spectrum of dense gas is shifted to higher frequency, which means that current radio surveys have missed out on young hypercompact HII regions, massive stellar winds, ionised jets and young planetary nebulae. This PhD project is aimed at trying to understand the birth of HII regions by discovering and studying hypercompact HII regions within the KuGARS survey. You will begin working on KuGARS pilot data that we have recently obtained from the JVLA, to understand the nature of these elusive objects and to also plan the most effective observing strategies for the full KuGARS survey and the MeerGAL survey (which should commence in 2017-18). This PhD will give you experience of working within the radio domain on some of the most cutting edge radio telescopes, ideally positioning you to take advantage of the SKA when it is commissioned.


Needles in a haystack of galaxies:  Massive Debris Disks in the Herschel ATLAS

Mark Thompson, Daniel Smith, Jason Stevens

The Herschel ATLAS is the largest area survey that the Herschel Space Observatory will undertake. The central aim of the survey is to identify potentially over a quarter of a million high redshift galaxies and understand their formation and evolution. However as a free byproduct Herschel ATLAS will also search over 10,000 nearby stars for the presence of debris disks - remnants of asteroid or Kuiper belts surrounding the stars. This search is much larger than other planned debris disk surveys (DUNES & SUNSS)  by virtue of a completely different selection technique, meaning that Herschel ATLAS will be a powerful complementary and independent test of these studies. So far we have identified over 20 good candidate disks in the first data taken with Herschel. The project will involve identifying further debris disks in the ATLAS survey and studying their properties with follow-up Herschel PACS observations (we recently obtained time to follow up all of our candidates with PACS) and also with other facilities, including ALMA.  You will be working as part of the international Herschel ATLAS team and will gain substantial experience in multi-wavelength astronomy from the far-IR and sub-mm to the infrared and optical.


The unsteady process of star formation

Philip Lucas, Janet Drew, Mark Thompson

This project is based on data from the first large time domain survey to be done in the infrared, the VISTA VVV survey (co-led by Dr Lucas).

Stars and planets are formed via the accretion of matter from a circumstellar disc, which in turn receives matter from a surrounding envelope of gas and dust. This is an unsteady process and huge eruptions are sometimes observed when the accretion rate jumps by a factor of 1000, causing a dramatic increase in the star's luminosity, often accompanied by the ejection of some matter from the system in a fast jet. It is thought that this may be common behaviour for normal stars during formation but this has been hard to prove because there were no large scale infrared variability surveys before VVV.

The VVV survey has recently discovered a large population of eruptive variables, most of them protostars that are hidden from view in visible light. These protostars have diverse outburst durations and spectra. The aim of this project is to empirically develop a unified model that can describe this diversity and help to determine the theoretical explanation for the outbursts, which is not yet clear. This will involve working with members of the international VVV collaboration and using telescopes in Chile for spectroscopy and adaptive optics imaging, as well as archival multiwaveband photometry. It is likely that we will also make many unexpected discoveries in this first exploration of the infrared variable sky. The understanding developed in this work will prepare the student for future work in the growing area of time domain astrophysics with LSST, GAIA and Pan-STARRS.


Galactic and Stellar Archaeology in the Gaia Era

Chiaki Kobayashi, Sean Ryan

Elements heavier than helium are formed in stars, and produced from different astronomical sources (core-collapse supernovae, Type Ia supernovae, asymptotic giant branch, gamma-ray bursts, and neutron-star mergers). Our hydrodynamical simulation code self-consistently includes these enrichment sources as well as other relevant physics such as  star formation and feedback (chemodynamical simulations). The student will study the origin of elements by comparing our simulation results of the Local Group (our Milky Way Galaxy and dwarf satellite galaxies) to the observational data from galactic archaeology surveys such as with the GAIA satellite mission. The student can also explore more detailed physics by modifying the existing code if he/she wishes.


The Orion Radio All-Stars

Jan Forbrich, Mark Thompson, James Dale

Now is an exciting time to return to stellar radio astronomy, with new instrumentation available and the SKA on the horizon. The sensitivity upgrades of both the NRAO Very Large Array (VLA) and the NRAO Very Long Baseline Array (VLBA) have begun to provide us with a much improved perspective on stellar centimetre radio emission. Our ongoing deep VLA and VLBA radio survey of young stellar objects in the famous Orion Nebula Cluster (ONC) has already produced a sevenfold increase in the known radio population over previous studies, and it has revealed intricate radio detail in the proplyds. We can now better disentangle thermal and nonthermal radio emission by assessing spectral indices, polarization, variability, and brightness temperatures (VLBA). With simultaneous radio-X-ray time domain information (Chandra) and VLBA precision astrometry, this project is providing unprecedented constraints on the magnetospheric activity of YSOs across a wide mass range, new insights into the impact of the massive Trapezium stars on their environment, and new astrometric constraints on the ONC itself. In this timely project, we will further explore the radio population in the ONC, with new VLA and VLBA observations. Potential science questions include the physical origins of YSO and proplyd radio emission, the relation of high-energy radio and X-ray emission, protostellar kinematics in the ONC with VLBA astrometry of embedded sources (complementing Gaia), novel imaging algorithms for interferometric wideband continuum data, and more!


Modelling the evolution of realistic model molecular clouds generated by galaxy-scale simulations

James Dale, Chiaki Kobayashi

Most simulations of star formation in molecular clouds begin from artificially-constructed isolated objects, usually formed by imposing a turbulent velocity field on an initially-smooth sphere of gas. Much has been learned from simulations of this kind, but they suffer from several deficiencies: the density and velocity fields are initially unrelated, leading to unrealistically rapid early collapse; the shape of the potential field remains roughly spherical throughout the simulations; the clouds do not accrete new material and feel no external confining pressure.

This project will use state-of-the-art hydrodynamics simulations of realistically-generated model clouds, re-resolved from galactic-scale simulations in which clouds form naturally, and in all shapes and sizes. The first task will be to extract suitable density and velocity fields from the large-scale simulations and increase the resolution sufficiently to allow mass-resolutions of around one solar mass, so that individual stars can be resolved.

The clouds can then be evolved forward in time and allowed to form stars. Once massive stars begin to form, the effects of photoionisation and stellar wind feedback on the realistically-generated clouds, and the ability of feedback to terminate star formation and destroy the clouds can be examined.


Combining feedback from low-mass stars and high-mass stars

James Dale, Mark Thompson

In the later stages of the evolution of star-forming molecular clouds, their structure and dynamics are dominated by feedback from massive stars, Photoionisation and stellar winds blow bubbles and cavities, and drive shockwaves into the remains of the cloud. These are thought to eventually terminate star formation and destroy the cloud long before it is able to convert all of its gas to stars. However, modelling this process has proved difficult and most simulations still produce too many stars, too fast.

However, at the earliest stages of star formation, before any high-mass stars have formed, it is in fact the low-mass stars which are the only sources of feedback on the cloud. Local heating due to the conversion of gravitational potential energy to thermal energy in accretion flows, and deuterium burning in the protostars suppresses fragmentation and star formation at small scales. In addition, jets driven by the young stars inject turbulent energy into the clouds, stabilising them on ~parsec scales, and also likely slowing the star formation rate. These processes control the environment into which the massive stars are eventually born, and in particular slows the rate of star formation in the epoch before there are any massive stars.

The purpose of this project is to upgrade a hydrodynamics code which is already able to model feedback from massive stars, to simulate the heating and jets from low-mass stars, and run a series of simulations of star-forming clouds to understand the influence of these modes of feedback on the global evolution of the clouds.