Overview of Research Areas Institute of Astronomy

Name Room Phone email Prof Tim Bedding 554 9351 2680 bedding@physics.usyd.edu.au Dr Hans Bruntt 565 9351 3041 [email protected] Dr Julia Bryant 563 9351 2152 [email protected] Prof Iver Cairns 457 9351 3961 [email protected] Dr Shami Chatterjee 570 9351 5577 [email protected] Dr Scott Croom 561A 9036 5311 [email protected] Prof Bryan Gaensler 556 9351 6053 [email protected] A/Prof Anne Green 564 9351 2727 [email protected] Dr Andrew Hopkins 569 9351 7688 [email protected] Prof Dick Hunstead 567 9351 3871 [email protected] Dr Helen Johnston 563 9351 2152 [email protected] Dr Lucyna Kedziora-Chudczer 323 9351 6080 [email protected] Dr Laszlo Kiss 561 9351 4058 [email protected] Dr Zdenka Kuncic 464 9351 3162 [email protected] A/Prof Geraint Lewis 557 9351 5184 gﬂ@physics.usyd.edu.au Dr Qinghuan Luo 318 9351 2546 [email protected] Prof Don Melrose 454 9351 4234 [email protected] Dr Tara Murphy 565 9351 3041 [email protected] Dr Stephen Ng 570 9351 5577 [email protected] Dr John O’Byrne 568 9351 3184 [email protected] Dr Gordon Robertson 562 9351 2825 [email protected] Prof Elaine Sadler 555 9351 2622 [email protected] Dr Dennis Stello 560A 9036 5108 [email protected] Dr Peter Tuthill 566 9351 3679 [email protected] Dr Mike Wheatland 463 9351 5965 [email protected]

Research in the Institute of Astronomy is grouped in two main areas, Observational Astrophysics and Computational and Theoretical Astrophysics. Observational data are obtained from various facilities in Australia and overseas, as well as observatories in orbit. In addition to the national facilities — the Anglo-Australian Telescope and the Australia Telescope — the School operates its own radio telescope, the Molonglo Observatory Synthesis Telescope (MOST), while the Sydney University Stellar Interferometer (SUSI) is the major element in a broad program of high resolution optical imaging. Research is conducted in many exciting areas over a wide range of wavelengths, including solar and stellar astrophysics, asteroseismology, black-hole binary systems, masers, pulsars, supernovae and their remnants, the interstellar medium and the Galactic centre. Beyond our Galaxy, topics include normal galaxies, the Magellanic Clouds, clusters of galaxies, active galaxies and quasars. Computational and theoretical studies delve into areas of astrophysics that can only be addressed through analytical techniques, computer modelling, or numerical simulation. These include black-hole accretion, interstellar scintillation, planetary and solar emission, pulsar and magnetar radiation mechanisms, gravitational lensing, dark matter, general relativity, and cosmology. Honours projects – Observational Astrophysics 2

Research Projects in the Institute of Astronomy

Observational Astrophysics

Magnets, electrons and supermassive black holes Supervisors: Prof Bryan Gaensler, Dr Ilana Feain (CSIRO ATNF) Contact: Bryan Gaensler, 556, [email protected], 9351 6053 The galaxy Centaurus A hosts the nearest known supermassive black hole to the Milky Way. This black hole is extremely energetic, as evidenced by powerful jets of radio emission that emerge from the galaxy at close to the speed of light, and which extend more than three million light years into intergalactic space. The radio emission from the jets of Centaurus A cover more than 30 square degrees. This means that there are thousands of background radio sources (mainly active galaxies and quasars) that sit behind Centaurus A. The radio emission from many of these sources should be linearly polarised, but this emission will be depolarised as it passes through the small- scale magnetic fields and turbulent electron gas in the radio lobes of Centaurus A. In this project, a student will find and catalogue all the polarised galaxies behind Centaurus A, and will correlate the degree of polarisation of these objects against the foreground structures seen in Centaurus A. These data will then be used to will provide the most detailed and direct study ever made of the magnetic fields and ionised gas ejected from massive black holes. The data required for this Honours project already exist, but there should be opportunities to participate in further radio observations of Centaurus A at the Australia Telescope Compact Array in early 2008.

Intergalactic magnetism in the Phoenix Deep Field Supervisors: Prof Bryan Gaensler, Dr Andrew Hopkins, Dr Ray Norris (CSIRO ATNF) Contact: Bryan Gaensler, 556, [email protected], 9351 6053 Understanding the Universe is impossible without understanding magnetic fields. But in spite of their importance, the origin of magnetic fields is still an open problem. Did significant primordial fields exist before the first stars and galaxies? If not, when and how were magnetic fields subsequently generated? What maintains the present-day magnetic fields of galaxies, stars and planets? Fundamental to all these issues is the search for magnetic fields in intergalactic space. Such a field has not yet been detected, but its role as the likely seed from which all magnetic fields in galaxies and in galaxy clusters have originated places considerable importance on its discovery. In this project, a student will use data from the “Phoenix Deep Field” to search for magnetic fields in the intergalactic medium (IGM). The Phoenix Deep Field is one of the most sensitive radio astronomy observations of the sky ever carried out. While the radio-emitting sources in this area have already been thoroughly analysed, a topic which is totally unex- plored is the linear polarisation from these faint objects. Any intervening magnetic field in the IGM will induce Faraday rotation in this polarised light, and more distant sources should have higher levels of Faraday rotation. This project will involve identification of the polarised sources in the Phoenix Deep Field and the measurement of their Faraday rotation. With these data, we will be able either to finally detect intergalactic magnetism, or put a strong upper limit on its presence.

Gas in the Galaxy: a three-dimensional study of hydrogen in the Milky Way Supervisors: Prof Bryan Gaensler, Dr Naomi McClure-Grifﬁths (CSIRO ATNF) Contact: Bryan Gaensler, 556, [email protected], 9351 6053 The “empty” space between the stars of the Milky Way is anything but. We now know that much of the mass of our Galaxy is distributed in a very low density interstellar medium (ISM), and that this rareﬁed, invisible gas is the fuel from which new stars are continually being formed. Much of the gas in the Milky Way is atomic hydrogen, but curiously, this gas appears to be split up into two separate phases: hot gas, with a temperature of about 6000 kelvin, and cold gas, at a temperature of just 100 kelvin. We generally expect that cold gas should be continually heated up (e.g., via shock waves from exploding stars) and that hot gas should cool down (e.g., on its way to collapsing into dense clouds that form new stars). However, the detailed relation (if any) between the hot and cold phases of the ISM remains unclear. This project will focus on an analysis of a spectacular new set of data on the ISM, made possible via the recently completed International Galactic Plane Survey. Using the radio signal emitted by hydrogen gas at a frequency of 1420 MHz, a student will derive the three-dimensional distributions of both hot and cold gas in the ISM, and will use the similarities and differences between these distributions to better understand the life cycle and ultimate fate of gas in the Milky Way. Honours projects – Observational Astrophysics 3

Understanding the most rapidly rotating stars in the universe Supervisors: Prof Bryan Gaensler, Dr George Hobbs (CSIRO ATNF) Contact: Bryan Gaensler, 556, [email protected], 9351 6053 Pulsars are ultra-dense, rapidly rotating neutron stars. Their extreme rotational stability makes them amongst the most accurate clocks in existence. For the last three years, the Parkes radio telescope has been making a focused study of twenty pulsars that are especially stable and extremely rapid (more than 200 times per second) rotators. The eventual goal of these observations is to see very slight deviations in the clock accuracies produced by distortions in space-time, and thus make the ﬁrst direct detection of gravitational waves (as predicted by Einstein’s Theory of General Relativity). In this project, a student will lay the foundations for this overall experiment by determining the detailed properties of each of the twenty pulsars in the sample. Some of the parameters that need to be determined before we can detect gravitational waves include the exact position, distance and velocity of each pulsar; the presence or absence of planets in orbit around each pulsar; slight intrinsic irregularities in the pulsar spins; and long-term variability in the shape of the pulses from each star. This work will be done in close association with the Parkes Pulsar Timing Array team. There will the opportunity to help carry out some of the on-going observations of these pulsars using the “The Dish” at Parkes. The student will be expected to undertake some of their research work at the Australia Telescope National Facility headquarters in Epping.

X-ray observations of an expanding supernova remnant and its high-speed neutron star Supervisors: Dr Shami Chatterjee and Prof Bryan Gaensler Contact: Shami Chatterjee, 570, [email protected], 9351 5577 Massive stars end their lives in spectacular supernova explosions. The outer layers of the star are blasted off into space, forming a supernova remnant. Meanwhile, the core of the star is left behind as a super-dense, rapidly spinning, neutron star. Asymmetries in the supernova explosion can kick the newborn neutron star at a high velocity (sometimes more than 1000 km per second) in a random direction. In this project, a student will analyse observations of the young supernova remnant Kesteven 73 and its associated neutron star, taken with the Chandra X-ray Observatory. Chandra has observed Kesteven 73 twice, with six years elapsing between the two observations. Careful comparison of these two X-ray images can yield both the motion of the neutron star and the expansion of the supernova remnant. Combining these results will directly yield the time since the supernova, the energy of the initial explosion, and the size of the kick given to the neutron star. Because the neutron star in Kesteven 73 is a hyper-magnetised “magnetar”, these results can be used to understand or limit the role played by the magnetic ﬁeld of a neutron star in the velocity it acquires at birth.

Physical processes and structure in the Galaxy Supervisors: Dr Tara Murphy, A/Prof Anne Green Contact: Tara Murphy, 565, [email protected], 9351 3041 Looking into the plane of our galaxy we can see many interesting objects, from compact objects such as pulsars to diffuse supernova remnants and HII regions. Each tell us different things about the evolution of stars, and the physical processes taking place within our galaxy. The Sydney University Molonglo Synthesis Telescope has produced high quality radio continuum images of the Galactic plane at 843 MHz. To fully utilise these maps it is necessary to identify and characterise the wide variety of sources detected. Some of these sources will be present in existing catalogues, but others will be new ‘undiscovered’ objects. In this project you will use data at other wavelengths (infrared and high radio frequency) to determine the physical processes involved, looking especially for signposts to objects at the extreme ends of the stellar lifecycle — the youngest and the most recently exploded. You will also use multi-resolution techniques such as wavelet analysis to characterise these sources, eventually producing a catalogue. The aim of the project is to understand how Galactic sources inﬂuence each other. The catalogue will be an important resource for future Galactic Plane studies.

Here today, gone tomorrow: the transient radio sky Supervisors: Dr Tara Murphy, A/Prof Anne Green Contact: Tara Murphy, 565, [email protected], 9351 3041 We often think of the sky as relatively unchanging, but as telescopes improve we are finding an increasing number of transient sources (objects whose intensity changes substantially with time). Objects known to be transients at radio wavelengths include Gamma-ray bursts, supernova remnants, pulsars and flare stars. There is also likely to be other transient behaviour in radio sources timescales of weeks to months which has not yet been well-studied, principally because it is so hard to predict the outbursts. In this project you will use the two epochs of the Molonglo Galactic Plane Survey (completed in 1999 and 2007) to find and study transient sources. One focus will be extreme sources that have appeared or disappeared between the observing Honours projects – Observational Astrophysics 4 epochs. We also have two fields that have been monitored on a regular basis to gain an understanding of long term transient behaviour. In addition to searching for new transients, you will investigate long period variability, which may be a new phenomenon.

Astrophotonics: exploring the behaviour of a new device Supervisors: Prof Joss Bland-Hawthorn, Dr John O’Byrne Contact: John O’Byrne, 568, [email protected], 9351 3184 We have been exploring the use of photonic devices, originally developed for telecommunications, to see if there are important applications in astronomy. But telecomm fibres only work in “single mode” whereas astronomical fibres (that are much thicker in order to allow for more light to pass down the fibre) carry many modes because light has many more ways to propagate down the fibre. In order to use exciting single mode devices in conjunction with multimode fibres, we had to invent a remarkable new device called a “photonic lantern.” Here a multimode fibre goes through a taper and splits into a large number of single mode fibres. Just why all of the multimode light finds its way into the single mode fibres is not understood. The photonic lantern has interesting properties that we propose to explore in the lab. Quite apart from exciting new uses in astrophysics, there are possible applications in remote sensing and communications.

The first miniature spectrograph for astronomy Supervisors: Prof Joss Bland-Hawthorn, Dr Gordon Robertson Contact: Gordon Robertson, 562, [email protected], 9351 2825 Astronomical telescopes are getting larger and larger which means that the cost of their instruments (that analyze the light) are getting prohibitively more expensive. In fact, the cost of a single instrument may even reach $100 M. This is a severe problem with no obvious solution. Recently, we suggested a radical new approach to the problem which is to feed thousands of optical fibres into thousands of individual miniature spectrographs rather than one huge spectrograph. The first of these devices was manufactured this year but has yet to be tested in detail. In this project, we will test and play with the device, and investigate ways to improve the design. If we can demonstrate that it has high efficiency and good behaviour, this will revolutionize our approach to building new astronomical instruments.

Measuring the structure of quasar emission regions Supervisor: Dr Scott Croom Contact: Scott Croom, 561A, [email protected], 9036 5311 The standard model describing active galactic nuclei (AGN) has fast moving gas orbiting the central black hole at radii of light days to months. This is the so-called “broad line region” from which the characteristic broad emission lines in quasar spectra are emitted. The detailed structure of these gas clouds is largely unknown. The aim of this project is to use time variations in the spectra of quasars to place constraints on the physical structure of the broad line region. The student will examine a large database of quasar spectra taken over a period of approximately a decade and extract multiple repeat observations to test for variations in the emission lines. Any such line variations found will be compared to the most recent models of the structure of the broad line region.

Do galaxy mergers make quasars? Supervisor: Dr Scott Croom Contact: Scott Croom, 561A, [email protected], 9036 5311 We now understand that most massive galaxies in the Universe contain a super massive black hole at their centre, with masses typically 106 − 109 times that of the Sun. These black holes are built up by rapid accretion of gas. For a brief period of time, during this accretion phase the hot gas radiates at high luminosity, giving rise to quasars. But how does the gas get funneled down to a black hole? The most likely cause is that the disturbances caused by major mergers of galaxies can cause gas to feed a super massive black hole. In this project the student will work with ultra-deep optical and infrared imaging data to ﬁnd faint high redshift galaxies that are close to accreting quasars. These quasars have been selected using the infrared Spitzer Space Telescope, and so even include objects heavily obscured by dust (more likely in the early stage of a merger). The aim is to see whether these environments are conducive to mergers (i.e., they have a relative high density of galaxies). The deep imaging data can also be used to derive galaxy colours, which are used to test for recent starburst activity in the galaxies nearby the quasars.

The proximity effect in quasars Supervisor: Dr Scott Croom Contact: Scott Croom, 561A, [email protected], 9036 5311 The strong UV radiation ﬁeld emitted from quasars is thought to have a signiﬁcant effect on the surrounding inter-galactic medium on scales of up to several megaparsecs. However, measuring this “proximity effect” in detail is hard. One technique that can be used is to take advantage of background, high-redshift quasars. At wavelengths bluer than 121.6 nm Honours projects – Observational Astrophysics 5

(Lyman-α), neutral hydrogen gas in the inter-galactic medium along the line-of-sight to the quasar can absorb the UV radiation from the quasar. The amount of absorption is directly connected to the neutral gas density. If the line of site to a background quasar passes near to another quasar in the foreground, then we should see a reduction in the absorption from neutral hydrogen, as the strong UV ﬂux ionizes the intergalactic medium. The student will work with the largest quasar surveys currently available, including projects currently ongoing at the Anglo-Australian Telescope, to select pairs of quasars which are close together on the sky. You will then examine the spectra of these quasars to search for any evidence of the ionizing radiation ﬁeld emitted from a quasar.

The evolution of galaxy morphologies Supervisor: Dr Andrew Hopkins Contact: Andrew Hopkins, 569, [email protected], 9351 7688 The shapes of galaxies in the nearby universe are dominated by well-deﬁned spiral and elliptical structures, smooth and continuous, with a small contribution from dwarf and irregular systems. At very early times, though, galaxies looked remarkably different, with irregular, amorphous shapes being dominant. At some point these young galaxies developed into the structures we see today, and understanding how this change occurred is a fundamental question in observational astronomy. We can make use of the fact that galaxies appear different at different wavelengths, corresponding to how different underlying physical processes produce or affect the emission, to connect the measured morphologies of galaxies to these underlying processes. We are interested in star-formation in particular as it is the distribution of stars in a galaxy which dominate the morphology we see. In the past decade observational astronomy has undergone a rapid evolution. We now live in an era when many large astronomical surveys provide publicly available archives. This provides a wealth of imaging data with which to quantitatively explore galaxy evolution. This project will make use of two such archives, the Sloan Digital Sky Survey (SDSS, a large survey of relatively nearby galaxies) and the Great Observatories Origins Deep Survey (GOODS, a high-sensitivity survey using the Hubble Space Telescope of very distant galaxies). The aim of this project will be to measure and compare the morphologies of galaxies in these two surveys, to get a direct and quantitative understanding of how galaxy shapes have changed, and to explore the connections with star-formation processes in these systems.

Where are the stars and dust within galaxies? Supervisor: Dr Andrew Hopkins Contact: Andrew Hopkins, 569, [email protected], 9351 7688 Galaxies are made up of billions of stars, and their distribution gives a galaxy its shape. Stars have different colours, depending on their temperature, with very hot stars being blue and cool stars being red. The hot-blue stars are also very massive and short-lived, while the cool-red stars are low-mass and have lifetimes comparable to the age of the universe. Mapping out the colour-distribution across a galaxy gives us insight into the range of stellar populations that are present at different locations within a galaxy, and in turn into the star formation history of these different regions. Dust in galaxies complicates this process by obscuring some of the starlight, but since the bluer light is preferentially more obscured than the redder light, the presence and amount of dust can still be inferred. Exploring where young stars and dust lie within galaxies, and the extent to which they are associated or not for different types of galaxies, is the goal of this project. This is important for understanding the way we measure star formation rates in galaxies, and the process of galaxy evolution. This project will make use of data from the Sloan Digital Sky Survey (SDSS) and an analysis of galaxy images from the SDSS using a tool called pixel-z. Pixel-z provides spatially-distributed measurements of star formation rate, stellar ages, heavy element abundance and dust obscuration throughout a galaxy. These estimates will be analysed for a sample of 45000 SDSS galaxies with varied morphologies, to explore the connection between star formation and dust properties.

Understanding radio galaxies in the early universe Supervisors: Dr Andrew Hopkins, Dr Ray Norris (CSIRO ATNF) Contact: Andrew Hopkins, 569, [email protected], 9351 7688 The ATLAS project (http://www.atnf.csiro.au/research/deep/) is designed to study the evolution of galaxies in the early Universe, using data from the Australia Telescope Compact Array, together with data from the Spitzer Space Telescope and other ground-based observatories. It is one of the largest and most sensitive such studies currently in progress, and has detected about 1600 galaxies. We also have optical and infrared data on most of these, and so can use standard techniques to fit template spectral energy distributions (SEDs) to these data, which should tell us whether the object is powered, for example, by star formation or by a massive black hole in an active galactic nucleus (AGN). But these standard techniques ignore the radio data, which can independently distinguish AGN from star-forming galaxies. So how well do these two techniques agree, and can we synthesise the two techniques to produce a hybrid technique which is even more reliable? This honours project will fit SED’s to the ATLAS galaxies, and compare the results to classifications made from radio data. The project is expected to result in a paper published in an international refereed journal, and there will also be opportunities for the student to participate in observations at the Australia Telescope Compact Array. Honours projects – Observational Astrophysics 6

Identifying and studying bright X-ray sources in the southern sky Supervisors: Profs Elaine Sadler and Richard Hunstead Contact: Elaine Sadler, 555, [email protected], 9351 2622 In this project, you will combine data from recent surveys of the southern sky at X-ray, radio and optical wavelengths to identify and study the astrophysical objects which are the brightest southern X-ray sources. These are expected to include (i) nearby spiral galaxies whose X-ray emission is powered by stars and stellar remnants, (ii) elliptical galaxies and galaxy clusters which are surrounded by enormous haloes of very hot gas, and (iii) quasars and active galactic nuclei, where the X-ray emission arises from processes related to accretion of gas onto a central, supermassive black hole.

Searching for radio supernovae Supervisor: Prof Elaine Sadler Contact: Elaine Sadler, 555, [email protected], 9351 2622 Supernovae which arise from the explosion of massive stars are sometimes extremely strong radio sources which can be detected at distances well beyond our own Galaxy Their radio emission typically peaks a few years after the supernova explodes, and appears to arise from the interaction of the supernova shock with a dense stellar wind shed by the pro- genitor star. Several radio-loud supernovae have been found by chance in nearby spiral galaxies, and it is possible that such objects are common. This project will use data from the University’s Molonglo radio telescope to measure the radio supernova rate in nearby galaxies, and to determine how many of these supernova are obscured by dust which would make them invisible to optical telescopes.

Spectroscopic studies of radio galaxies and quasars Supervisors: Prof Elaine Sadler, Dr Scott Croom, Dr Helen Johnston Contact: Elaine Sadler, 555, [email protected], 9351 2622 Radio galaxies and quasars are believed to be two manifestations of the same physical process: the accretion of gas onto a supermassive black hole at the centre of a galaxy. Optical spectra of these objects can help us learn how they are related, and what events might trigger such an energetic outburst from a galaxy’s centre. We will offer one or more projects in this general area, and would be happy to talk to interested students.

Digging deeper into the early radio universe Supervisors: Prof Dick Hunstead, Dr Ray Norris (CSIRO ATNF) Contact: Dick Hunstead, 567, [email protected], 9351 3871 The ATLAS project (http://www.atnf.csiro.au/research/deep/) is designed to study the evolution of galaxies in the early Universe, using data from the Australia Telescope Compact Array, together with comprehensive data from other ground- based observatories and the Spitzer Space Telescope. It is one of the largest and most sensitive such studies currently in progress. This honours project will re-analyse a subset of ATLAS data to produce deeper images than have so far been achieved, speciﬁcally to probe active galaxies and quasars in the early Universe and reveal extended structures which may have been missed in the previous results. The new image will then be compared with unpublished optical and infrared data to determine the nature of the new galaxies that will be discovered in this project. The project is expected to result in a paper published in an international refereed journal, and there will also be opportunities for the student to participate in observations at the Australia Telescope Compact Array.

Swirling gas around a black hole Supervisors: Dr Helen Johnston, Prof Dick Hunstead Contact: Helen Johnston, 563, [email protected], 9351 2152 A0620−00 is a binary star system consisting of a normal star orbiting around a black hole, one of a class of objects known as low-mass X-ray binaries. The normal star is feeding gas into the black hole via an accretion disk, which produces bright emission lines which we can detect with optical telescopes. A0620−00 last had a major outburst in 1975, at which time it was the brightest X-ray source ever seen. Since then it has been quiescent. It is still producing X-rays at a vastly reduced rate, and these X-rays are illuminating the disk and producing the emission lines we see in our spectra. However, the details of the emission mechanism are not understood. In this project, you will analyse optical spectra from the ANU 2.3m telescope at Siding Spring Observatory to uncover the physical mechanism(s) producing the emission lines in A0620−00 and other low-mass X-ray binaries. There is also a possibility of the honours student observing with the 2.3m telescope to obtain new spectra, which would be reduced and analysed as part of this project. Honours projects – Observational Astrophysics 7

Advanced stellar imaging Supervisor: Dr Peter Tuthill Contact: Peter Tuthill, 566, [email protected], 9351 3679 The question “How would the stars look from close quarters?” must have intrigued humans since prehistory. With recent advances in optics and information processing, we are ﬁnally poised to deliver concrete answers, and a range of startling (and often beautiful) new structures are now being revealed around distant stars. In particular, stars in the process of formation are found to grow from spinning disks of matter, which are also believed to form the nurseries of embryonic planets in the nascent new solar system. With new technologies now in operation at some of the world’s largest observatories such as the Keck or Gemini telescopes, we are trying to probe this fascinating new regime. More information can be found at http://www.physics.usyd.edu.au/∼gekko

Asteroseismology: probing inside stars using stellar oscillations Supervisors: Prof Tim Bedding, Dr Laszlo Kiss, Dr Dennis Stello, Dr Hans Bruntt Contact: Tim Bedding, 554, [email protected], 9351 2680 Asteroseismology involves using the oscillation frequencies of a star to measure its internal properties. Measuring stellar oscillations is a beautiful physics experiment: a star is a gaseous sphere and will oscillate in many different modes when suitably excited. The frequencies of these oscillations depend on the sound speed inside the star, which in turn depends on density, temperature, gas motion and other properties of the stellar interior. This analysis, called asteroseismology, yields information such as composition, age, mixing and internal rotation that cannot be obtained in any other way and is completely analogous to the seismological study of the interior of the Earth. Many stars, including the Sun, are observed to oscillate. Asteroseismology is a new and rapidly developing ﬁeld and there are several possible Honours projects, depending on the preference of the student. These range from using observations of red giants taken over many decades by amateur astronomers, to obtaining high-precision Doppler measurements of sun-like stars with large telescopes such as the AAT and the VLT.

Stellar variability of open cluster stars Supervisors: Dr Dennis Stello, Prof Tim Bedding, Dr Laszlo Kiss, Dr Hans Bruntt Contact: Dennis Stello, 560A, stello physics.usyd.edu.au, 9036 5108 Stars are the building blocks of the Universe and, as such, are essential to understanding many aspects of astrophysics. Our understanding of cosmology, galaxies and planetary formation all depend on stellar evolution. To understand how stars evolve we need to investigate stars in different evolutionary stages. Using the optical variability of stars has shown in many cases to be a very powerful tool to get a better understanding of their properties at different stages of evolution. This variability can either be due to a companion orbiting the host star, such as another star or planet which regularly blocks some of the light from the host as it goes in front of it, or the variability can be due to oscillations or outbursts in the star itself. In both cases we can answer additional questions about the stars we would otherwise not be able to, like: “Is there a planet around the star?”, “How old is it?”, or “How big is the star?”, and many more things. Many stars are born in large clusters as stellar twins, but with slightly different masses. They all have the same age and share the common properties of the former dust cloud from which they were all formed. These common birth conditions enable us to get a much more precise understanding of the stellar evolution. This project combines the power of using stellar variability as a tool and the investigation of stars in a cluster to achieve better insight into stellar evolution. The data for this project come from the most ambitious multi-site observing campaign aimed at a star cluster, which involved simultaneous observations for six weeks from ten telescopes around the world. Honours projects – Computational & Theoretical Astrophysics 8

Computational and Theoretical Astrophysics

Gravitational microlensing of quasars Supervisor: A/Prof Geraint Lewis Contact: Geraint Lewis, 557, gfl@physics.usyd.edu.au, 9351 5184 Gravitational microlensing has proven to be a powerful astrophysical tool, revealing detailed structure within the heart of quasars. In this project, the student will make uses of ‘ray-tracing’ to model the action of gravitational microlensing; caused by individual stars, this can introduce dramatic brightness flares in distant sources. This will be coupled with the recent models for the distribution of emission of radiation in the vicinity of supermassive black holes to predict the expected brightness fluctuations of microlensed quasars, and determine what kind of information can be derived from a quasar light curve.

Linking two universes: view through a wormhole Supervisor: A/Prof Geraint Lewis Contact: Geraint Lewis, 557, gfl@physics.usyd.edu.au, 9351 5184 Wormholes are the staple of science fiction, allowing seamless travel between universes. In this project, you will simulate the trip of a brave adventurer as they fly through a wormhole, calculating their motion and what they will see out of the window. This project, while computationally based, will allow you to play with the mathematics of general relativity.

Putting a spin on things: journey into a Kerr black hole Supervisor: A/Prof Geraint Lewis Contact: Geraint Lewis, 557, gfl@physics.usyd.edu.au, 9351 5184 What would happen if you jumped into a rotating black hole and then fired a rocket pack? How do you use your time wisely to extend the time to your ultimate demise in the singularity? In this project you will find out, using a numerical approach to integrate the equations of motion in a rotating black hole.

Planetary microlensing Supervisor: A/Prof Geraint Lewis Contact: Geraint Lewis, 557, gﬂ@physics.usyd.edu.au, 9351 5184 Gravity can act as a microscope, magnifying our view of the distant universe. Here, you will investigate the use of gravitational lensing in revealing the detailed properties of planets whose light has been boosted in a microlensing “event”. In this project, you will learn the various theoretical techniques employed in the study of the gravitational lensing of light.

Where is the missing dark matter? Supervisor: Prof Joss Bland-Hawthorn Contact: Joss Bland-Hawthorn, [email protected], 9372 4851 It is now well established that most of the matter in the Universe is “dark” and of a form that is completely mysterious. Most of the dark matter appears to reside in large haloes that surround galaxies. Several groups have used supercomputers to simulate how the Universe evolves in the presence of dark matter, and this led to an extraordinary discovery. There should be about a thousand small dark haloes in the vicinity of our Galaxy. So where are they and how would we detect them? In this entirely theoretical project, I propose to explore a new idea. If enough gas falls into these dark haloes, stars form and we end up with a galaxy. But there is no obvious reason why lots of gas should always ﬁnd its way into dark haloes; maybe only trace amounts reside in the majority of them. If this is true, we can calculate the physical state of the low density gas to establish what the likely “dark halo” signature might be. This signature could be detectable in the the spectra of distant quasars. As the light from the quasar passes through the dark halo, the diffuse gas absorbs the light at speciﬁc frequencies. We will search for the possible existence of nearby dark haloes by looking for the unique signature in on-line archives of quasar spectra.

Where are the stars that were born with the Sun? Supervisor: Prof Joss Bland-Hawthorn, A/Prof Geraint Lewis Contact: Geraint Lewis, 557, gﬂ@physics.usyd.edu.au, 9351 5184 Our Sun was born 4.57 billion years ago in a gas cloud that presumably encircled the Galaxy before collapsing to form stars. It is widely believed that the “Solar Family” may have been made up of 10,000 members, most of them dwarf stars like the Sun. In principle, we can identify members of this family from unique chemical signatures (cf. DNA) that were imprinted into the stellar atmospheres at the time of their birth. But where are they now? Since their birth, these stars have orbited the centre of the Galaxy as many as twenty times. In this project, we will build an accurate dynamical model Honours projects – Computational & Theoretical Astrophysics 9 of the Galaxy, and follow the orbits of many stars born at the same site. We will then explore what it would take to ﬁnd these stars with modern astronomical instruments.

Galactic winds: are these caused by black holes or supernovae? Supervisors: Prof Joss Bland-Hawthorn, Dr Zdenka Kuncic Contact: Zdenka Kuncic, 464, [email protected], 9351 3162 Some galaxies exhibit spectacular behaviour in the form of a powerful wind from the central regions. These winds are thought to be produced by the collective behaviour of many thousands of supernovae, although in some cases, a central black hole is also thought to be responsible. An important new survey of these galaxies has begun using the Anglo- Australian Telescope, delivering 3D data on the fast moving gas in these winds. The goal of this project is to extract velocity and mass estimates from the data, and then to derive 3D models of the observations in order to assess just how powerful these winds are, and hence determine whether supernovae or a supermassive black hole is mostly responsible.

Computational modelling of the Sun’s magnetised atmosphere Supervisors: Dr Mike Wheatland and Prof Don Melrose Contact: Mike Wheatland, 463, [email protected], 9351 5965 The Sun’s outer atmosphere, the solar corona, is a magnetised plasma exhibiting dynamic and complex behaviour. Solar flares are dramatic explosions in the corona involving liberation of stored magnetic energy. To better understand flares we need to be able to model the magnetic field in the corona based on available observations. This project involves developing a computational method for 3-D magnetohydrostatic modelling of coronal fields (time-independent modelling involving a balance of magnetic, gravitational, and gas pressure forces) based on a method described by Grad and Rubin in 1958. The Grad-Rubin approach has already been successively developed for nonlinear force-free modelling (time-independent modelling involving only magnetic forces) using a fast Fourier method (Wheatland 2006). In this project the approach will be generalised and applied to available test cases. The project offers scope for theory, large-scale computation, including parallel computation, and visualisation. Wheatland, M.S.: A fast current-field iteration method for calculating nonlinear force-free fields, Solar Physics 238, 29-39 (2006)

Building a shot-noise solar corona Supervisors: Dr Mike Wheatland and Prof Don Melrose Contact: Mike Wheatland, 463, [email protected], 9351 5965 Solar flares are large explosions in which magnetic energy is released in the solar corona, the extended outer atmosphere of the Sun. The solar corona is at a temperature of around two million Kelvin, the high temperature being maintained by an unknown heating mechanism. It is appealing to link solar flares and coronal heating. It is possible that coronal heating is due to the continual occurrence of small, spatially unresolved flares, as argued by Parker and others. Although this idea has been pursued in many ways, the consequences for the observed statistics of coronal X-ray emission do not appear to have been considered in detail. If coronal heating is due to nanoflares, observed time histories of emission should repre- sent random superpositions of basic structures (flare time histories), and hence should have shot-noise statistics. Simple models for shot-noise statistics are available. In this project we will consider the application of these models, and infer properties of the elementary flares from observed X-ray emission. The work will involve analytic calculation as well as numerical simulation and data analysis.

Electron cyclotron maser emission (ECME) Supervisor: Prof Don Melrose Contact: Don Melrose, 454, [email protected], 9351 4234 ECME is the favored mechanism for the Earth’s auroral kilometric radiation (AKR), for Jupiter’s decametric (DAM) radio emission for solar spike bursts and for very bright emission from ﬂare stars, and has even been suggested for AGN. The theory for ECME involves an intrinsically relativistic effect in the cyclotron resonance condition, which moves the predicted frequency of ECME to slightly below the cutoff frequency for waves in a cold plasma, seemingly precluding escape of the radiation from an astrophysical source. Inclusion of the Doppler effect associated with a loss-cone distribution can overcome this effect and shift the resonance to above the cutoff frequency, allowing the radiation to escape. Early measurement of the electron data (for the “inverted V” electrons that generate AKR) generally supported this model, but more detailed data did not conﬁrm this support. The more recent data imply a ring or shell distribution with no cold electrons. The argument now is that it is the absence of cold plasma that allows ECME to escape, despite its frequency being slightly below the cyclotron frequency. This theory plausibly accounts for AKR and DAM, whose source regions do have an extremely low cold plasma density. However, this is not plausibly the case in the applications of ECME, to solar and stellar radio emissions, and to AGN. Honours projects – Computational & Theoretical Astrophysics 10

This project will involve a review of the various theories for ECME, emphasizing the conditions for its escape. The quantitative part of the project will be primarily analytical.

Resonant Comptonization in magnetospheres of strongly magnetized neutron stars Supervisors: Dr Qinghuan Luo and Prof Don Melrose Contact: Qinghuan Luo, 455, [email protected] & [email protected], 9351 2934 The hot surface of a neutron star emits X-rays which can be detected by X-ray telescopes such as Chandra. Observations of X-ray spectra allow us to determine physical conditions on or near the neutron star’s surface. The observed spectra result after radiation transfer through the intervening plasmas in the neutron star’s atmosphere and magnetosphere. Photons emitted from the surface encounter a cyclotron resonance, where the wave frequency is equal to the cyclotron frequency of a particle in its rest frame. This project will study how the cyclotron resonance affects the radiation transfer. The project will involve analytical work—such as application of theory of resonant Compton scattering to radiation transfer—and some numerical calculation, including modelling of X-ray spectra. There are other related projects on radiation processes in pulsar magnetospheres; for details please contact either Don or Qinghuan.

RWH August 18, 2007