POSTGRADUATE RESEARCH

Astrophysics and optics

The Earliest Stages of Massive Star Formation
Supervisor: Michael Burton

A PhD scholarship in astronomy is available to work with Professor Michael Burton investing the earliest stages of massive star formation.  The applicant should have an honours degree in Physics, Chemistry, Mathematics, Engineering or Computing Science.

The Department of Astrophysics is one of Australia's leading university research groups in astronomy.  We have a particularly strong program in millimetre-wave astronomy, making use of the world-leading facilities Australia has for these kinds of observations, which enable us to study the emission from molecules in space.  In particular, we make use of the 22m Mopra telescope, located at Coonabarabran in NSW, together with the NANTEN2 telescope on the 5,000m elvation altiplano of Chile.  These facilities provide formidable spectrometers which allows one to study the astrochemistry associated with the formation of stars.

Funded in by part of an ARC Discovery grant, this scholarship will provide $15K per year.  PhD students in the School may simultaneously accept Postgraduate Assistantships, which provide additional guaranteed income of up to $10k per year, in return for agreed teaching duties. The scholarships do not cover tuition fees. Australian and New Zealand students do not pay tuition fees.  The university campus is located in the East of Sydney, close to Coogee Beach.

For further information, please contact Professor Michael Burton, School of Physics, University of New South Wales, Sydney, NSW 2052.  Tel 02-9385-4553, Email m.burton@unsw.edu.au. URL:http://www.phys.unsw.edu.au/STAFF/ACADEMIC/burton.html

The evolution of our Galaxy is driven by the massive stars that reside within it. These are stars greater than ten times the mass of the Sun, and thousands of times brighter.  Their prodigious luminosities drive energy flows, which in turn power the cycle of elements between the stars and the gas, and back to the stars again. Each time the cycle is passed through it is enriched by the products of nucleosynthesis, occurring in the cores of the stars. The crucible of this process is the formation of these massive stars. Yet this remains somewhat of an enigma, for a number of reasons. Massive star formation takes place rapidly.  The nearest examples are distant from us. It takes place in clusters with stars at different stages of formation.  Many different physical processes are taking place simultaneously. This makes it a fascinating challenge to study!

In recent years the advent of sensitive and high angular resolution infrared cameras has made it possible to detect the forming stars.Combined with new millimetre-wave telescopes and interferometers that allow us to observe the rich range of molecules present, we are now able to determine the physical state of the molecular cores where massive stars are born, and to follow their childhood as the star passes through a series of stages being emerging on the Main Sequence.

In particular a time-dependent chemistry is evident, complex organic molecules created inside "hot molecular cores" surrounding an incipient star. The molecules present at any time provide a signpost which points to the stage star formation has reached - if only we could read the signpost. One aspect of this project is to decode the language the signpost is written - by determining what molecules are present in a variety of cores, and how these change as the cores develop towards the formation of a new star. This involves observation using new millimetre-wave telescopes, both in Australia and under construction on the Atacama plateau of northern Chile.

How many stars form in a hot molecular core? Do they follow a universal initial mass function, and does this vary from core to core? What physical characteristics of the core determine the range of stars that form? Such questions can be tackled through deep infrared observations. These will allow us to uncover the newly forming stars while they remain deeply embedded in their natal cores, and to investigate their spectral characteristics to indicate what type of star(s) will emerge from it.

This PhD project will thus involve millimetre-wave and infrared observations of molecular clouds where massive star formation is underway, and the subsequent analysis and interpretation of this data.  It will make use of the single-dish Mopra Telescope and the interferometer of the Australia Telescope to find where the molecular species are.  It will use the new NANTEN2 telescope on the 5,000m Atacama plateau of Chile to determine the excitation of the clouds.  It will also use infrared telescopes in Chile, combined with data from space infrared telescopes, to measure the stellar content and determine what kind of stars are forming.

Turbulent Star Formation: the role of turbulence in regulating star formation in our Galaxy.
Supervisors: Maria Cunningham and Michael Burton

Stars form from within dense cocoons of gas and dust which are hidden to our view at optical wavelengths. The young stars can, however, be seen using infrared and millimetre wave telescopes. A comprehensive theory of star formation is one of the major unsolved problems of astrophysics, with currently only the collapse of an isolated cloud into a single star being reasonably well understood. However most star formation occurs in clusters, so this theory cannot apply to majority of stars that we see.

Recently a new "turbulent model" of star formation has been proposed which has the potential to provide a unified theory for this process. The model, however, remains relatively untested and unconstrained by observation. Testing it will be the prime focus of this PhD project. The idea behind the model is that supersonic turbulence is driven on galactic-scales to smaller scales via an energy cascade. This acts both to encourage collapse of a cloud under gravity, and to also provide support against it. In the theory, turbulence regulates the rate of star formation in the Galaxy, as well as determining the efficiency with which it occurs inside a given molecular cloud. Modelling suggests that density fluctuations in the cloud depend on the strength of the turbulence and the scale on which it is injected, and that this can be examined through observation. In a molecular cloud tracers of low density gas, such as the carbon monoxide (CO) molecule, will have a different distribution to that of high density tracers, molecules like carbon mono-sulphide (CS) and silicon monoxide (SiO), if the turbulence is weak. On the other hand their distribution should be similar if the turbulence is strong. All these molecules, and many more, can be mapped, using the Mopra millimetre wave telescope, together with a new correlator that we have recently installed. We have instigated the "Delta Quadrant Survey" to pursue this investigation, mapping a molecular cloud complex in the southern galactic plane in a variety of molecular lines that emit at mm-wavelengths. These molecular maps will then be compared to the distribution of newly formed stars, as evident through infrared images, to enable us to examine the role of turbulence in driving and regulating the formation of stars.

Exoplanetary Science Group

The Exoplanetary Science Group (headed by Professor Chris Tinney) is  please to be able to offer top-ups of A$6000 per annum for students working within our  group, as well as generous travel support to attend conferences and observe on national and international telescopes. For further information on our group see the web page at http://www.phys.unsw.edu.au/~cgt

In 2010, the projects we are able to offer include:

The Hunt for Free-Floating Planets
Supervisor: Prof Chris Tinney

Over 230 extra-solar planets are now known to orbit nearby stars. Most  of these have been discovered by the Doppler wobble technique, and so  have only been "indirectly" detected via their impact on their host  star. So next-to-none of these planets has actually been seen  directly. In this project you will use the recently developed  technique of methane imaging to search for, and image, "unbound"  planets (ie planets without a host star) and brown dwarfs (ie. stars too low in mass to burn nuclear fuel) in young southern star clusters. The Southern Hemisphere is the ideal location to undertake these studies, as it harbours many of the nearby Galactic star clusters. As a result there is lots of scope for additional students to join the current team working in this area.

Hunting with FourStar: In 2010, the FourStar instrument will go into operation on one of the 6.5m Magellan telescopes in Chile. This instrument will have a massive field of view of 0.18x0.18 degrees, allowing it to targeting nearby (ie. within 150pc) and larger (ie. several square degrees on the sky) star clusters. The University of NSW, together with the Carnegie Institution and MIT, has funded the purchase of methane filters specifically for this new camera. FourStar will make possible large surveys of Southern clusters that have previously only been poorly studied at very low masses, like like IC2602, NGC2461, NGC2547, and NGC6475.

Hunting with Gemini MCAO Imaging
: In 2010 the Australian-built Gemini South Adaptive Optics Imager (GSAOI) will also go into operation on the 8m Gemini telescope in Chile, together with the Canopus Adaptive Optics system. Together these components will enable one of the largest telescopes in the world to deliver diffraction-limited high-resolution images over a wide field (1-2')  on a ground-based 8m telescope. This will enable entirely unprecedented observations of star more distant and denser star clusters. Perhaps the most obvious and most immediate target will be the Orion Nebula Cluster (right). This region is home to a dense, massive star cluster, which HST images have show to not only harbour objects down at brown dwarf masses, but also to host back-illuminated accretion disks (or "proplyds").
Gemini NICI Imaging of AAPS Host Stars
Supervisor: Prof Chris Tinney

Over the last 6-9 months, the Gemini Observatory has been commissioning its Near-Infrared Coronagraphic Imager (NICI).  This dual-channel imager represents the current state-of-the-art for the direct detection of faint young planets and brown dwarfs orbiting nearby stars. Around 20% of the stars currently being monitored by the Anglo-Australian Planet Search (AAPS) team show radial velocity variations. In some cases these are so large that the unseen companion is obviously a brown dwarfs of low-mass star - which category it falls into will depend on the system's orbital inclination to our line of sight. NICI observations can clarify this status, as low-mass stars will be easily detectable, while brown dwarfs will be much fainter (though possibly still detectable). In either case, long-term monitoring will allow all orbital elements for these systems to be determined.

In other cases, the Doppler wobble we see is almost certainly due to a gas-giant planet - most of which will be undetectable by NICI. However, once again if NICI does see something in these systems it will be an exciting result. Either there are further, more distant, planets that Doppler observations have not yet detected. Or the system is much younger than we expected. Either case will be a significant result. The AAPS team, therefore, will be applying for NICI time in 2010 to image all of our Doppler variable target stars, and there is a role for a PhD student to be involved in acquiring, reducing and analysing this data, as well as preparing for any further follow-up observations required.
Understanding Intrinsic Variability in Doppler Planet Host Stars
Supervisor: Prof Chris Tinney

The precisions being achieved by Doppler Wobble exoplanet searches like the AAPS are being continually improved. New planet discoveries are being made at lower and lower masses, corresponding to lower and lower Doppler amplitudes, requiring that planet search teams understand the noise behaviour of their target stars in some detail, because it is the Doppler noise produced by those stars (or rather by their surface inhomgeneities and intrinsic oscillations) that are now a limiting factor.

We therefore need to develop better ways in which to parametrise the surface inhomogeneities of stars, preferably using the spectra we obtain from our Doppler search programs.

One area that has not been explored to date is the use of Doppler Imaging codes. These codes have been developed to tackle a different problem, which is to use many observations of the spectrum from a star, as it rotates, to acquire a tomographic data set, that can then be used to deconvolve back to the 'map' of variations on the stellar surface. Basically these codes track small deviations from the overall spectral line shape that the star would have in the absence of any surface variations. As the star rotates these deviations move across each spectral line, and with enough data on enough spectral lines, you can solve the inverse problem and map the stellar surface.

For Doppler wobble experiments we are interested in the extent to which such surface variations can produce artificial overall radial velocity changes. So while the 30-100 observations made of our Doppler host stars could not possibly be used to obtain a tomographic map, they can be used to estimate the extent to which line variations occur, and to quantify their impact on our over Doppler velocities.

There is scope for a PhD student with a strong data processing and computation background to explore the use of these Doppler Imaging techniques as a means of quantifying the impact of surface variations on Doppler planet detections.

Varying Constants
Supervisor: Prof John Webb

Spectroscopic observations of distant quasars lead to stringent constraints on any change in physics over cosmological timescales. Theoretical motivation for seeking cosmological changes in the laws of Nature come from several new ideas including string theory. Recent advances in telescope and detector technology, and in analytical methods, now permit measurements of unprecedented accuracy, providing sensitive and fundamental tests of Einstein's equivalence principle and potentially opening the doors to new physics.

This project is an experimental one. It requires fastidious analyses of new extremely high quality astrophysical data, from the Keck telescope and from the ESO VLT, to probe the values of certain physical constants billions of years ago.

Introductory articles can be found in: "Inconstant Constants" by John D. Barrow and John K. Webb, Scientific American, June 2005 and "Are the Laws of Nature Changing with Time?", John K, Webb, Physics World, Vol. 16, Part 4, pages 33-38; April 2003, (also available here).

High Redshift UV Background Radiation
Supervisor: Prof John Webb

Knowing the ultra-violet background radiation field at high redshift is fundamental to our understanding of cosmology, galaxy formation and galaxy evolution.

This project involves using new large astrophysical databases to make the most precise measurement of the UV flux to date. The new database should permit the first ever detection of cosmological evolution of the UV flux.

The results will reveal new details of the early history of the universe, provide new information about the formation of the first collapsed objects formed and about the so-called epoch of re-ionization, will constrain the number of massive black holes formed at early times, and will also provide new constraints on the nature of the dark matter.

Extra-Solar Planets
Supervisor: Prof John Webb

One of the most exciting recent developments in astrophysics is the discovery of extra-solar planets. This is a rapidly expanding scientific field, still in its infancy, with the prospect of many discoveries ahead.

At present, the known extrasolar planets have been discovered almost exclusively using the Doppler wobble method, i.e. searching for periodic velocity shifts in the host star due to the orbiting planet. This method has proved very successful but requires large amounts of observing time on large telescopes.

Extrasolar planets can also be found using large field imaging telescopes to search for a periodic decrease in the host star apparent brightness as a planet eclipses it. This requires a favourable alignment of the planetary orbital plane with respect to our line of sight, but wide field imaging provides the necessary statistics to potentially discover large numbers of objects.

A fundamental advantage of discovering planets eclipsing their host stars is the potential for studying any atmospheric absorption features from the planet using the background star as a probe.

A project of this sort is underway in our department using the "Automated Patrol Telescope" at Siding Spring, NSW. In 2006 we shall take delivery of a new CCD camera currently being constructed by the Anglo-Australian Observatory. The new camera will provide roughly a 10-fold efficiency gain for our search and producing a world-class facility for this kind of research.

The PhD project will involve working on all aspects of this challenging and exciting new programme.

Where is the Star Forming Material in the Early Universe?
Supervisors:Steve Curran and Prof John Webb

All baryonic material in the Universe originally arose from cold hydrogen gas, clouds of which collapse to form stars, which in turn form planets, life and all matter comprised of elements heavier than lithium. This occurs in galaxies, which are islands of material in the otherwise sparsely populated cosmos, of which the gas constitutes a major component.

However, from a recent survey of radio galaxies and quasars at redshifts of z > 3, no cold hydrogen gas was detected. This is a surprising result, especially when we expect more of this at these redshifts - look-back times of 12 billion years, when much of it has yet to be consumed by star formation.

Upon a thorough analysis of the optical fluxes of the targets, all of these were found to have ultra-violet luminosities in excess of LUV > 1023 W/Hz. This also applies to the lower redshift (0.1 < z < 1) searches in the literature and may render many of the current explanations regarding the detection of hydrogen in these objects obsolete.

The lack of detections at LUV > 1023 W/Hz, may indicate that the UV photons emitted by the matter accreting onto the central super-massive black hole are ionising much of the gas in the surrounding galaxy and as long as this gas is ionised it cannot form stars. It is therefore important to establish whether the apparent lack of gas at high redshift is a selection effect, where we only know of the most UV bright sources, or due to some other hitherto unknown effect.

Therefore, in addition to assisting with the ongoing high redshift radio astronomical searches, a PhD project would consist of:

1. Using the CLOUDY photo-ionisation package to determine why there is a critical UV luminosity and why this is close to LUV > 1023 W/Hz.

2. Improved optical observations in order to better determine the UV properties of the sources. 3. Calculating the UV luminosities for more "local" (z < 0.1, within a billion light-years) active galaxies in order to determine whether the critical UV luminosity also applies to these.

4. Following-up and spinning-off into other research, based upon these results (for example, this UV threshold discovery itself was entirely unexpected).

As a journal referee recently put it, "the observational result...is remarkable...I expect that this result will spur significant exploration of this new, and curious, phenomenon".

For further reading see,
http://www.sciencedaily.com/releases/2008/11/081124102701.htm
http://www.abc.net.au/science/articles/2008/11/25/2428882.htm

High Redshift Galaxies Obscuring Red Quasars
Supervisors:Steve Curran and Prof John Webb

Absorption of the radiation from quasars by foreground galaxies is a very powerful probe of the distant, and therefore earlier, Universe. In particular, with absorption at radio wavelengths, due to the presence of star-forming gas in an intervening galaxy, we can probe galactic structure and evolution, star formation, as well as potentially measuring the values of the fundamental constants of nature at look-back times of billions of years.

However, such systems are notoriously hard to find, since they tend to be located in the most optically faint galaxies where the dust, conducive to the cool, star-forming gas, blocks the optical light from the background quasar.

With the world's largest steerable radio telescope (The Green Bank Telescope - http://www.gb.nrao.edu/GBT/), we have undertaken "wide-band" spectral scans towards a sample of extremely optically faint, but radio-loud, quasars in the search for the intervening material responsible for the obscuration of the visible light (radio waves travel through dust unhindered).

A preliminary analysis of the data has already detected the 21-centimetre transition of hydrogen at a redshift of z =0.96, indicating the presence of a 100 billion solar mass galaxy at a co-moving distance of 10.5 billion light-years, located along the sight-line to a 19.8 billion light-year distant quasar - see http://www.phys.unsw.edu.au/~sjc/home/J0414+0534.pdf.

The project is to initially analyse the bulk of these data. The scripts have been written and so the analysis would be straightforward if it were not for the radio interference from satellites, mobile phones and solar powered sun-beds, in the West Virginian hills. These data are, however, state-of-the-art and are expected to yield the discovery of new galaxies, too distant to be seen by their own light. Following this, the project will involve the interpretation of the results, as well undertaking new follow-up and spin-off observations on the world's forefront radio telescopes.

 


postgraduate research