Galactic Student Research Projects
Here are some possible Ph.D. research projects in Galactic Astrophysics, and related fields.
This list is meant to be representative and exact project descriptions will be agreed between you and your potential supervisor.
- Spinning Dust in Circumstellar Envelopes
- Post-AGB Stars
- SiO Masers in Late-Type Stars
- The Role of Supernovae in Dust Production
- Population Flow Tracing in Astrophysical Molecules
- The Three-Dimensional Natural Maser
- Spectral Surveys
- Low Mass Star Formation and the Local Molecular Clouds
- High Mass Star Formation
- The SCUBA-2 "All-Sky" Survey
- The molecular circumstellar material in planetary nebulae
- Small Scale Structure in the Local Interstellar Medium
- e-MERLIN and e-VLBI observations of transients
- Numerical hydrodynamical simulations of stellar outflows
- Searching for coalesced supermassive black holes using e-MERLIN astrometry
Spinning Dust in Circumstellar Envelopes
Supervisor: Albert Zijlstra
In recent years evidence has been found for a new component to the Galactic radio emission. It is seen in regions with little or no ionization, ruling out free-free emission as the origin. It is now believed to be due to small spinning dust particles. Dust forms in circumstellar nebulae, before being distributed over the Galaxy. In this project, the radio spectrum of circumstellar nebulae will be analyzed to find evidence for the excess emission, and determine the type of dust from which it originates
Supervisor: Albert Zijlstra
Stars like the Sun end their lifes with a phase of spectacular mass loss, when as much as 80 percent of their mass is ejected into space within a hundred thousand years. The ejecta rejuvenate the interstellar matter, and provide the dust and other raw materials for the next generation of star and planet formation. The cause of this superwind is not wel understood. Several projects involving this phase of evolution are offered. These include high-angular-resolution studies of the ejecta, the formation and compostion of the dust in the ejecta, interaction with the interstellar medium, and using the IPHAS survey of the Galaxy to look for real-time stellar evolution.
SiO Masers in Late-Type Stars
Supervisor: Malcolm Gray
Time-series data of SiO maser rings in Mira variables, taken with the VLBA and EVN interferometers, reveal one of the most spectacular dynamic environments in Astronomy. The structure of a shell changes substantially over a fraction of the pulsation period of the host star (pulsation periods are typically between 150 and 450 days). Successive shells of matter are injected into the pulsating circumstellar envelope by sub-photospheric shock-waves, and are eventually swept out into a dust- and molecule-rich stellar wind. The aim of the project is to produce a new computational model of the time-evolution of the SiO masers. This needs to incorporate new molecular data (rate-coefficients for SiO in collision with atomic and molecular hydrogen) and data from an improved hydrodynamic model, which can be made specific to individually chosen stars. Possible developments include the incorporation of polarization. The model should be capable of predicting the time-varying mass-loss rates in the inner envelope of the host star.
The Role of Supernovae in Dust Production
Supervisor: Malcolm Gray
Supernovae produce violent shock-waves and very high-temperature gas, so it is likely that they destroy copious amounts of interstellar dust. However, the presence of grains with peculiar isotopic composition, and the presence of dust in the early Universe, that pre-dates a significant supply from AGB-star winds, suggest that supernovae are also significant producers of interstellar dust. Recent observations of cold dust in the Cas~A and Tycho remnants remain controversial. The aim of the project is to model the evolution of grain material in a supernova remnant to determine whether supernovae can be net producers of dust.
The project will involve the construction of a Monte-Carlo model of grain collisions with both gas particles and other grains. In this way, populations of grains can be established with 'consistent histories', that include size, speed and composition distributions. The model will be applied to both grains that condense from the cool ejecta, and to grains that previously existed in the ISM, and are subsequently processed by the supernova. The model will also be able to handle the transfer of grains from one environment to another, for example from a background of hot gas, to one of cool gas, or from a metal-enriched ejecta environment to unenriched interstellar gas. The study will attempt to find out whether supernovae are net destroyers or creators of dust, both at the current epoch and in more metal-poor environments, such as the early Universe.
Population Flow Tracing in Astrophysical Molecules
Supervisor: Malcolm Gray
Numerical solutions to the combined problem of radiative transfer and population kinetics provide mutually consistent sets of radiation energy densities and energy-level populations. Useful though such solutions are, a lot of information is usually lost in the details of the numerical method used, so one cannot identify, for example, which of the inputs to the model are particularly important in generating result, and which are not. I have developed a traceback method which, in some cases at least, can restore this lost information, leading to a simplified network of flow routes which are chiefly responsible for the numerical results obtained. The important point is that this simplified set is directly related to the model inputs: line-strengths and collisional rate-coefficients. The Ph.D. project is to apply the traceback method to the following cases: additional molecules, including water and methanol; variation with spatial position; saturation effects in masers and the stability of traced solutions in parameter space.
The Three-Dimensional Natural Maser
Supervisor: Malcolm Gray
Observations suggest that real astrophysical masers are tightly beamed at the source, having opening solid angles of 0.01 steradian or less - in some cases probably as small as 10(-5) steradian. However, the origin of this beaming is not always clear. Magnetic fields can control beaming in molecules like OH, which exhibit a strong Zeeman effect, but this cannot be extended to other species, like water and SiO.
This project aims to investigate, in detail, the beaming effects of magnetic fields (in OH), and combined geometry plus velocity-geometry, involving the alignment of velocity-coherent pockets of masing gas, for both bulk and turbulent velocity fields. In addition, it should be possible to attack the more challenging problem of natural beaming, where the geometry of the masing gas, its own self-saturation, and the isotropy of the background radiation, control the beaming.
Supervisor: Gary Fuller
Understanding how stars and planets form is one of the key goals of astrophysics. During star and planet formation molecular gas evolves through a range of density, temperature, and velocity structure: from low density, pressure confined molecular cloud material, the gas evolves to cold high density cores which collapse to form stars. As these young stars grow in mass and develop winds which drive massive outflows, the molecular gas out of which they formed is heated, shocked and eventually dispersed. As the molecular gas evolves so does its chemical composition. The various trace species can probe not only the current kinematics and structure of the material but also its past history. Spectral surveys provide the only way to systematically and comprehensively study the the properties of this gas.
The JCMT Spectral Legacy Survey (SLS) has recently been awarded time to use HARP-B with ACSIS to undertake a spectro-imaging survey of the different evolutionary stages and environments in the star formation process. The goal of the SLS is to trace the variations in the molecular inventory of, and physical conditions in, a sample prototypical young objects. The detailed, systematic studies of template sources provided by the SLS will provide a baseline to guide future observations of star formation not only in our Galaxy, but also in other galaxies. The SLS will also provide the basis for analyzing and interpreting such molecular line observations in terms of the basic astrophysics of the underlying objects.
PhD students interested in spectral surveys will be involved in the SLS or one of several related projects. A thesis project in this area will involve in the data collection, including undertaking observations at various radio and millimetre telescope, data analysis and modelling of the sources. More information about the SLS can be found here.
Low Mass Star Formation and the Local Molecular Clouds
Supervisor: Gary Fuller
The molecular clouds closest the sun are found in a large scale structure known as Gould's Belt. These clouds are predominantly forming low mass stars. Their proximity make these clouds the targets of choice for studying how gas evolves through 23 orders of magnitude in density from low density turbulent gas to a hydrogen burning pre-main sequence star. These clouds in the Gould's Belt are the target of a large scale survey with SCUBA-2 on JCMT. SCUBA-2 will map the emission of the cool dust in these clouds tracing both the formation and evolution of the dense cores and the young protostars which they form. The survey will be sufficiently sensitive to detect objects well in to the brown dwarf sub-stellar regime. These objects are too low mass to initiate nuclear fusion in their cores and so be true stars.
Opportunities exist for PhD students to work on both the SCUBA-2 survey and the associated molecular line observations to be carried out with HARP-B on JCMT as well as related follow-up observations. More details of the JCMT Gould's Belt survey can be found here.
High Mass Star Formation
Supervisor: Gary Fuller
High mass stars, although less common than their lower mass counterparts, dominate the structure and evolution of the ISM in galaxies. The formation and early evolution of high mass stars also influences the formation of lower mass stars as once formed the high mass objects rapidly destroy their natal molecular clouds, shutting off further star formation. The high mass stars may also terminate the accretion of already formed low mass protostars as ionizing radiation and energetic winds destroy any remaining envelopes around low mass protostars.
Our detailed understanding of how high mass protostars form and evolve has been seriously hampered by the lack of systematic surveys to identify young high mass at various stages of evolution. Opportunities exist for PhD students to work on a number of surveys of high mass star formation and related follow-up observations. These surveys include the JCMT Plane Survey, the Methanol Maser Multi-Beam Survey and HiGal, a survey of the Galactic Plane with the Herschel satellite. These surveys will produce galaxy-wide catalogues of high mass star forming regions which will be used to study the physics of high mass star formation and the effects young high mass stars have on their environment.
The SCUBA-2 "All-Sky Survey
Supervisor: Gary Fuller
The "SCUBA-2 "All-Sky" Survey, or SASSy has opportunities for PhD student projects. SASSy is a JCMT Legacy Survey project designed to exploit the rapid mapping capability of SCUBA-2 to ultimately map the entire sky visible from the JCMT at an angular resolution of 14" at 850microns. The benefits of such a wide-field survey are many, ranging from a complete census of infrared dark clouds (IRDCs), massive cores which may be precursors to the formation of high mass stars, to the potential discovery of some of the most luminous high-redshift galaxies in the Universe.
The molecular circumstellar material in planetary nebulae
Supervisor: Gary Fuller
The evolution of solar-like stars is terminated by a catastrophic mass loss event on the Asymptotic Giant Branch (AGB). During this so-called `superwind', between 50% and 80% of the stellar mass is lost, at very mass-loss rates. The hot stellar remnant eventually ionizes its ejected shell: the ionized gas is visible as a planetary nebula (PN). Many young PN show dark absorption lanes due to the presence of a disk or torus of dense gas and dust around the central stellar remnant. The origin and properties of these disks/torii are poorly understood. This project will involve using single dish radio and millimetre telescopes and interferometers to study these disks/torii to determine their properties and structure to investigate their origin and the ultimate fate of the material. These objects also make interesting test cases for understanding the properties of molecular material in extreme environments since the material in these structures is exposed to intense radiation fields from the hot central sources.
Small Scale Structure in the Local Interstellar Medium
Supervisor: Andrew Markwick
The same spatial resolution and sensitivity increases which allow new facilities (e.g.the Atacama Large Millimetre Array, ALMA) to be able to study the interstellar medium (ISM) in external galaxies will of course enable new observations to be made of our own ISM at spatial resolutions never obtained before. These observations will allow us to study the initial stages of star formation in unprecedented detail, for example.
This project will build the next-generation in astrochemical models, which can deal with the stochastic nature of physical and chemical processes on small spatial scales, and apply it statistically to current (lower) resolution observations.
e-MERLIN and e-VLBI observations of transients
Supervisors: Ralph Spencer, Tim O'Brien, Richard Davis, Myfanwy Lloyd, Phil Diamond, Rob Beswick
Radio sources which undergo rapid outbursts form perhaps the most intriguing set of objects in the Universe, including such things as X-ray binary stars, Novae, SuperNovae, RRATs, Red Dwarf flare stars, Brown dwarfs and even Giant planets. They are best studied by target of opportunity multi-wavelength observations, and indeed a number of programs using LOFAR, e-MERLIN and e-VLBI are being set up. Staff at JBCA are interested in all of the above objects.
One project in particular is on X-ray Binary Transients (Spencer). These are objects situated in our galaxy at distances up to within ~10 kpc. These systems are powered by accretion in a binary system onto a neutron star or black hole. Accretion can be characterised by the X-ray state. Two states are particularly relevant and cover a general picture of the accretion process: the low/hard state (LS) and the high/soft state (HS).In the LS the X-ray energy spectrum is dominated by a power-law component. This state is characterized by the presence of low intensity, but very powerful, persistent and compact (~ 50 AU) jets emitting synchrotron radiation at radio frequencies. In the HS the thermal black-body component becomes evident in the X-ray spectrum and the radio jets are quenched (i.e. suppressed), but intermittent relativistic ejections usually accompany the state transition to HS. The black hole XRBs spend most of the time in quiescence and undergo sudden and bright months-long X-ray outbursts with typical recurrence periods of years. Superluminal radio jets have been spatially resolved in both neutron star and black candidate objects. Our aims would be to
- investigate the disc-jet coupling in XRBs via the radio/ X-ray correlations.
- gain a better understanding of the radio / X-ray behaviour during an outburst.
- study the dynamics of potentially resolved jets; catching a jet-producing XRB transient in outburst and observing it at relatively close spaced epochs.
Another project will focus on nova explosions (O'Brien). Novae are interacting binary systems in which the transfer of matter from a main-sequence dwarf or red giant onto a companion white dwarf results in a thermonuclear explosion. Radio observations can provide the best estimates of fundamental parameters as distance, ejected mass and kinetic energy. A recent excellent example is our study of the nova RS Ophiuchi in 2006 where we used MERLIN/VLBI to observe the early radio light curve, image the expanding shock wave from only a few weeks after the explosion, and determine that the expanding remnant was bipolar or jet-like.
However radio studies suffer from poor sampling of the population. e-MERLIN's hugely increased sensitivity will allow us to detect all galactic novae and resolve the majority through the important phases of evolution. In addition multi-frequency synthesis and spectral coverage will provide excellent image fidelity. In future e-MERLIN and e-VLBI studies we aim to address the following questions:
- Do novae eject matter in the form of jets and how does this link to evidence for jets in other CV's and X-ray binaries?
- Are the ejecta subsequently shaped by the binary companion in a common envelope phase, by an external medium (e.g. companion wind) or by magnetic fields?
- Observational estimates of the ejected mass are greater by an order of magnitude than numerical simulations of the explosions predict - which is wrong?
- How does the development of X-ray emission from hot gas (observed in some classical and recurrent novae with Swift), relate to the way in which matter is ejected and interacts with itself and any external medium?
- How does clumping in the ejected matter develop and how is it related to dust formation?
Numerical hydrodynamical simulations of stellar outflows
Supervisors: Tim O'Brien, Myfanwy Lloyd, Albert Zijlstra
We are interested in the way in matter is ejected from stellar systems such as novae (binaries comprising a main sequence or red giant star and a white dwarf) and planetary nebulae (end-points of low-moderate mass stars, some of which may be binaries). We carry out a range of observational programmes across the electromagnetic spectrum from radio to X-rays and we combine these with numerical studies of the outflowing material. These typically employ hydrodynamical codes devloped by ourselves based on gridded Eulerian schemes using Godunov's method. Most recently a student has developed a 3-dimensional code in spherical polar coordinates which is ripe for exploitation. A number of projects could be addressed, for example
- Studies of the ejection of material from the recurrent nova RS Ophiuchi. We have a huge amount of observational data with which to test the models. Of particular interest is whether the ejection can be jet-like and how this relates to any asphericity in the red giant wind.
- The shaping of planetary nebulae. A long-standing problem is how planetary nebulae are shaped into elongated, bipolar or point-symmetric structures. A common suggestion which requires further attention has been that the progenitor is a binary and that this somehow imposes a preferred axis on the ejection. However an alternative that could be considered is collimation by a magnetic field.
Searching for coalesced supermassive black holes using e-MERLIN astrometry
Supervisors: Ian Browne, Simon Garrington and Ben Stappers
In the hierarchical model of galaxy formation smaller galaxies merge to produce bigger ones. The super-massive (around 109 solar masses) which reside at the centres of these galaxies may eventually coalesce and depending on their spins and relative masses , the resultant single BH can receive a remarkably large "kick" as a reaction to the asymmetric emission of the gravitational radiation. We can search for such kicked back holes by looking for galaxies where the active nucleus, a revealed by radio emission, is displaced from the centroid of the stellar light. This requires very accurate positional measurements (astrometry). This project would start with a technical phase in developing and testing the astrometric capabilities of the new e-MERLIN array, a network of 7 large radio telescopes across the UK, operated at the University of Manchester's Jodrell Bank Observatory. The recent upgrade to e-MERLIN (new optical fibre links, receivers and correlator) make it a potentially very powerful astrometric instrument, especially for the study of faint active nuclei at the centres of many galaxies. A second phase will be to carry out a survey to measure potential displacements between the radio active nuclei and the stellar population of their host galaxies. Other follow-ups from the technical phase include pulsar astrometry and the cross-identification of objects found in deep surveys of the Universe made at widely disparate wavelengths.