Dr Cormac R. Purcell

Astronomer @ Macquarie University.

 

Research Interests

My current projects are multi-disciplinary and focused on using machine-learning and remote-sensing techniques to solve problems in astronomy, ecology and archaeology.

My astronomical research centres on cosmic magnetism, traced by polarised radio emission between 0.5 - 11 GHz. At these frequencies the Faraday rotation effect allows us to measure the magnetic field in the gas between stars. This image is from new study of the Giant Radio Arc, a huge filament of magnetic gas near the centre of the Milky Way.

I also develop software and algorithms to automatically analyse data from the ASKAP POSSUM project. This wide-field survey will measure the Faraday rotation of three million extragalactic radio sources, yielding a new understanding of magnetic fields in the Universe.

Below, you can get a flavour for projects I am currently working on, or have led in the past.

Monitoring Sharks and the Marine Environment

The Australian coastline is a priceless ecological resource and worth billions of dollars to Australia's economy each year. However, human-shark conflicts, development and climate change present ongoing threats. The NSW government has prioritised research into shark management and environmental monitoring techniques.

I am currently leading a project to detect and classify shark species with high accuracy in video footage from drones. In collaboration with the NSW Department of Primary Industries (DPI), we are applying astronomical image-processing and machine-learning techniques to footage from shark management drone trials conducted over several years. Early results are very promising and we are expanding the scope of the research into multi-spectral analysis. This project is supported by an MQNS Grant.

Satellite Archaeology

The Tundzha Regional Archaeological Project (TRAP) is a collaborative, multidisciplinary project to investigate long-term environmental change and social evolution in the Tundzha River watershed Bulgaria. In particular, the Kazanlak Valley along the upper Tundzha River contains hundreds of burial mounds, including spectacular Hellenistic ‘royal’ tombs.

Together with my collaborators in the Macquarie Department of Ancient History, I am working to automate the detection and classification of 'true' burial mounds (rather than cairns, or natural features) in multi-band IKONOS satellite data. The project also aims to leverage high-cadence images from Planet Labs to monitor known sites for changes due to human activities.

The Gum Nebula and its Environment

The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of only 450 pc. It has been hypothesised to be an old supernova remnant, fossil HII region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula and its impact on the ISM using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H-alpha data from the Southern H-Alpha Sky Survey Atlas. By analysing rotation measures through the nebula and by fitting a simple model, we are able to measure the geometry and strength of the ordered magnetic field. The fitted compression factor at the edge of the nebula strongly constrains its likely origin for the first time. The nebula is also useful as a probe of the magnetic field on parsec scales and the fitted value of local magnetic pitch-angle represents a significant deviation from the median orientation on kiloparsec scales.

Paper: [ADS] [arXiv]

The CORNISH VLA Project

High-mass stars (> 8 M) form deep inside giant molecular clouds that block all visible light. These young stars ionise their surroundings, creating bubbles of hydrogen that emit radio waves. Long-wavelength radiation can escape the clouds, so mapping the radio-sky is the best way to answer the fundamental question: How many high-mass stars are there in the Milky Way?

The Co-Ordinated Radio 'N' Infrared Survey for High-mass star formation, or CORNISH, is the radio-continuum counterpart of the mid-infrared Spitzer GLIMPSE project. Observations on the Karl Jansky Very Large Array have yielded a high resolution map, revealing ultra-compact H-II regions across the Galaxy. I led the CORNISH team to publish the most complete list of compact 5 GHz radio-emission towards the northern Galactic Plane.

Website: http://cornish.leeds.ac.uk
Paper 1: [ADS] [arXiv] [Citations]
Paper 2: [ADS] [arXiv] [Citations]

HOPS: a survey of the Galactic Plane

The H2O southern Galactic Plane Survey (HOPS) mapped 100 square degrees of the sky, detecting molecules that emit at wavelengths around 2-cm. Observations on the Mopra telescope targeted a 1-degree wide strip of the Galactic Plane, searching for ammonia and bright water masers. These molecules trace the peaks of giant molecular clouds and regions where gas is violently shocked: the cradles of young stars.

I created a software pipeline to process the data into cubes showing the distribution and Doppler shift of dense gas in the Galaxy. I used this information to make a top-down map of the Milky Way's spiral arms by matching Doppler shift to rotational velocity in the Galactic disk.

Website: http://hops.org.au
Paper 1: [ADS] [arXiv] [Citations]
Paper 2: [ADS] [arXiv] [Citations]
Paper 3: [ADS] [arXiv] [Citations]

Triggered Star-formation in NGC 3576

The star-forming region NGC 3576 is a spectacular example of a wave of star-birth sweeping through a giant molecular cloud. At the heart of cloud lies a giant ionised bubble, generated by a cluster of young high-mass stars. Previous observations have shown that the bubble is embedded in clumpy filament of cool dust, prompting the question: Is star-formation being triggered or quenched in the dusty cloud?.

To answer this question I led a project on the Australia Telescope Compact Array (ATCA) and Mopra radio telescopes to map NGC 3576 in a suite of molecules: NH3, CO, HCO+, CS, N2H+ and H2O. The resulting maps of temperature and chemistry reveal that star-formation is underway across the whole filament, likely triggered by the expansion of the central bubble.

Paper: [ADS] [arXiv] [Citations]

Methanol Masers and Hot Molecular Cores

Stars are formed from collapsing clumps of gas and dust at the heart of giant molecular clouds. This process takes at least 100,000 years, so we can only observe a 'snapshot' of stars forming in the Galaxy. Astronomers study the physics of star-formation by finding many examples with a range of ages and then order them on a timeline.

During my PhD I led an investigation of the 'weather' and chemistry towards methanol masers - signposts to young high-mass stars. Using the Mopra radio telescope I measured the abundance of key organic molecules, proving the link between the early hot molecular core phase of high-mass star-formation and 6.67 GHz CH3OH masers.

Paper 1: [ADS] [arXiv] [Citations]
Paper 2: [ADS] [arXiv] [Citations]

Commissioning the Mopra Telescope

The Mopra Observatory is a 22-m radio-telescope operated by the Australia Telescope National Facility and located at the edge of the Warrumbungle Mountains, near Coonabarabran, NSW, Australia. Radio telescopes like Mopra measure the 'brightness temperature' of gas in their beam (field of view) and make maps by scanning back-and-forth across the sky.

In 2004 I was heavily involved in measuring the beam-shape of the dish and calibrating the brightness scale. This work paved the way for Mopra to make spectacular 3D-maps of molecular clouds in the southern Galactic plane: RCW106, Nessie, and many more.

Paper: [ADS] [Citations]