Stars & Stellar Systems

Stellar research in Australia covers star formation & stellar evolution, as well as the formation & dynamics of stellar systems and globular clusters.

Asteroseismology:

Asteroseismology research in the Institute of Astronomy (IoA) at the University of Sydney uses measurements of stellar oscillations to determine the structure of stars.

Dense Stellar Systems:

The Monash Centre for Stellar and Planetary Astrophysics is involved in the study of stellar dynamics, and in particular the processes which govern the dynamics of dense stellar systems. This involves understanding how energy and angular momentum are transported via scattering of pairs of stars, but of particular interest is how binary-single, binary-binary, and higher-order interactions affect the evolution of a star cluster. We call this ‘gravitational chemistry’.

Globular Clusters:

Globular clusters are gravitationally bound collections of about a million stars, all with the same age and chemical properties. They are perhaps the oldest stellar systems in the Universe, forming about 1 billion years after the Big Bang. Globular cluster systems follow the star formation history of their host galaxies and as such provide a unique probe of galaxy formation. Their kinematics can be used to trace galaxy dark matter halos at large radii. Proto-globular clusters have been observed to form in merging galaxies. The study of both galactic and extragalactic globular clusters is an area of strength for Australian astronomy, with researchers at Swinburne University, ANU and the AAO.

High Resolution Imaging of Stellar Systems:

Optical interferometry research in the Institute of Astronomy (IoA) at the University of Sydney is directed to measuring basic properties of stellar systems. Long baseline interferometry has been a major research effort in Sydney for 40 years, with the Sydney University Stellar Interferometer (SUSI) now the longest baseline instrument in the world, able to measure basic properties of stars such as: emergent fluxes, effective temperatures, radii and luminosities of single and binary stars; measuring distances and masses of binary stars; distances to variable stars (e.g. Cepheids and Miras); measure the relative sizes of star and emission regions, emergent fluxes and effective temperatures of emission line stars (e.g. Be and Wolf-Rayet stars); stellar rotation; limb darkening; and interstellar extinction. Another approach is masked-aperture interferometry on a conventional large optical telescope. Research has included the MAPPIT project at 3.9-metre AAT and more recent work using the 10-metre Keck telescopes.

Massive Star Formation:

Massive stars are the powerhouses driving the galactic ecology, boasting energetic winds of charged particles, powerful collimated outflows of molecular gas and intense emission of UV radiation. Every element heavier than iron owes its existence to a massive star. Massive stars are the powerhouses driving the galactic ecology, boasting energetic winds of charged particles, powerful collimated outflows of molecular gas and intense emission of UV radiation. The death of such stars in violent supernova explosions catalyses the birth of new generations of stars and enriches the interstellar medium with heavy elements. Comparatively little is known about this pivotal process in the scheme of galactic evolution. The nearest examples of massive star forming regions are kiloparsecs distant from us and require high-resolution telescopes or interferometric techniques to resolve them. Complications are added as unlike their poorer low-mass cousins, massive stars are formed in clusters at the heart of giant molecular clouds(GMCs) that absorb all visible radiation emitted. The UNSW Astrophysics Department has a strong star formation research program combining millimetre, submillimetre and infrared observations to probe regions of massive star formation. Current research programs are centred on the earliest evolutionary stages in the star formation process, before the massive star turns on its stellar winds and sweeps away the nascent cocoon of dust and gas. The group operates the Mopra millimeter wave observatory for three months of the year in conjunction with the ATNF and is currently embarked on a survey of molecular lines associated with star formation.

Molecular Masers:

Masers (Microwave Amplification by Stimulated Emission of Radiation) are the radio equivalent of lasers and arise naturally in a variety of astrophysical environments. The low densities and pressures in interstellar gas clouds means that it takes a long time for thermal equilibrium to develop and so external processes such as radiation from newly formed stars, or shocks can easily produce the conditions required to produce molecular masers. Maser emission of a variety of transitions of the OH, water, methanol and ammonia molecules is frequently observed towards massive star formation regions. We only observer masers from regions where there is a chance velocity coherence along a line of sight, this makes the masers intrinsically very small. This enables masers to be used as probes of the kinematics and physical conditions in regions of star formation at resolutions not attainable through any other technique. Molecular masers are actively studied by researchers at the University of Tasmania, the ATNF and the UNSW Astrophysics Department. A group from the University of Tasmania, Monash University and the ATNF are currently undertaking an extensive three year program of multi-transition observations of OH and methanol masers to better constrain theoretical models of the maser emission and hence the physical conditions in massive star forming regions. The study of molecular masers at the ATNF is a major research area, closely related to massive star formation. Surveys of the masers, particularly of OH, water and methanol, can pinpoint (throughout the Galaxy and unaffected by dust and obscuration) the sites where the earliest stages of massive star formation are taking place. Detailed properties of the individual maser sites yield understanding of the environments that favour the formation of new massive stars.

Star Formation:

The Stars & Planets Group at Swinburne University are engaged in computational studies of star and planet formation, including cloud collapse and the dynamics and evolution of disks around young single and binary stars.

Starspots & Stellar Magnetism:

Stellar research at the University of Southern Queensland includes participation in an international study of stellar magnetic activity. The project, dubbed Zeeman Doppler Imaging or ZDI provides a way to map the starspot features (the stellar equivalent of sunspots) and the magnetic fields of stars. The scientific motivation for ZDI comes from the key role of dynamo-generated magnetic fields in understanding the activity and variability of stars like the Sun.

Stellar Convection:

For a long time, a proper understanding of convection has been one of the greatest challenges for stellar astrophysics. Since convection can strongly influence both the stellar evolution and the emergent stellar spectra, this uncertainty also carries over to other fields of astrophysics and cosmology when relying on stars as probes of the cosmos. Researchers at the Research School of Astronomy and Astrophysics at ANU are performing supercomputer simulations of stellar surface convection in order to study the nature of stellar convection under different regimes. These time-dependent 3D hydrodynamical simulations with a detailed description of the important radiative transfer have proven highly realistic and successfully reproduces observational phenomena. The RSAA group also employs the simulations for studies of spectral line formation and stellar oscillations (asteroseismology) to learn more about cosmic chemical evolution.

Stellar Evolution & Nucleosynthesis:

Knowing the compositions of stars is one of the keys to understanding their structures, how they form and evolve. For many main-sequence stars, the only abundance which is well known is that of iron. Other metal abundances are believed to vary in the same way as iron does, but the validity of this assumption in unknown. This is one of the major areas of work at the Monash Centre for Astrophysics (MoCA). Using large (4- and 8-metre) telescopes such as the AAT and the VLT, we obtain high signal-to-noise spectra for F-, G- and K-type stars, and determine abundances for a range of elements. We are currently involved in abundance studies of Population II (halo) stars, which have low iron abundances, and Population I open-cluster objects similar to the Sun. This work covers both theoretical modelling, following the evolution of stars as well as calculating the nucleosynthesis they exhibit, as well as observational spectroscopy. CSPA at Monash is also involved in understanding the complex structure of AGB stars, and especially determining their role in the production of carbon and other elements. This is tackled through both spectroscopic observations as well as theoretical modelling. These studies are crucial for understanding the chemical history of the Galaxy. The first stars present particular problems, due to their unusual composition. This actually produces more complicated nuclear reactions, and changes the structure so that mixing produces evolution that is unlike “normal” stars. Further, recent high-resolution spectroscopic observations of the ultra-metal-poor stars are providing stimulus for advancing our understanding of these stars. A group of astronomers at the Research School of Astronomy and Astrophysics, ANU, discover and study the physics and nucleosynthesis of the most metal-poor and oldest stars in the Galaxy to obtain fundamental clues to conditions at the earliest times. This provides insight into nucleosynthesis in the Big Bang and in the supernovae and hypernovae which produced the first elements heavier than lithium. Observational aspects of the research are undertaken with ANU’s 2.3m telescope, with AAO’s UK Schmidt and 3.9m telescopes, and with high-resolution spectrographs on the VLT and Subaru 8m telescopes. These efforts played a role in the discovery and analysis of the most metal-deficient, and probably oldest, star (HE0107-5240) currently known. Realistic, state-of-the-art three-dimensional hydrodynamical model atmospheres are being developed to accurately interpret the observations.

Stellar Radial Velocities:

RAVE, the RAdial VElocity survey, is the largest-ever spectroscopic survey for stellar radial velocities. Radial velocities can be combined with proper motions to give a complete description of the movements of stars through space. From such information, the RAVE science team will be able to identify dozens, perhaps hundreds, of streams of stars that represent old satellite galaxies of the Milky Way. RAVE will also measure key chemical abundances. In several ways, data from RAVE is likely to produce great advances in our understanding of how the Galactic disk, bulge and halo all formed and evolved. The survey began in April 2003 on the 1.2-m UK Schmidt Telescope of the Australian Astronomical Observatory. Using the telescope’s 6dF instrument , the RAVE team is collecting up to 600 spectra a night. By 2005, the end of Phase 1 of the project, the team should have collected 100,000 spectra – five times as many as have been measured over the last 125 years. For Phase 2, running until 2010, further observations are planned with a new instrument on the UK Schmidt, and with a northern hemisphere telescope: these should net up to 50 million spectra.

The RSAA Stellar Astrophysics group also study the evolution and pulsation of stars. After the great computational successes of the 1970s in synthesizing this area of astrophysics, the more complex later stages of evolution are now the frontier. During the advanced evolutionary phases, nuclear burning products appear at the stellar surface, nearly all stars pulsate, and a large fraction of the stellar mass is lost via stellar winds. These processes are studied at RSAA by optical and infrared spectroscopy and photometry, by study of microwave maser emissions, and by theoretical evolution and pulsation calculations.

Variable Stars:

The PLANET microlensing collaboration database is being searched for short period variables. Currently over 60 high quality light curves of previously unknown RR Lyrae stars have been obtained. Detailed modelling is expected to lead to a better understanding of the Blazhko effect and stellar pulsation. As partners in the PLANET microlensing followup collaboration, the Univesity of Tasmania group is studying limb darkening in giant stars. PLANET conducts detailed modelling of the light curves of giants generated by binary lens events and high-resolution spectroscopy to determine the atmospheric structure of giant stars. Analysis of the EROS BLG-2000-5 event has provided the best detemination of limb darkening for any star other than the sun. Variable star research in the Institute of Astronomy (IoA) at the University of Sydney covers fields including: stellar pulsation (long-period stars, delta Scuti, Cepheid); eclipsing binaries; observing techniques including photoelectric and CCD photometry and high-resolution optical spectroscopy; period searching methods (Fourier, Clean, wavelet); and non-linear time-series analysis (chaotic pulsations).

White Dwarfs & Cataclysmic Variables:

The Mathematical Sciences Institute (MSI) group at the ANU has one of the worlds leading groups on the study of Magnetism in White Dwarfs. The group pioneered the modelling of polarised radiative transfer in magnetised white dwarf atmospheres and have been actively involved in the interpretation of polarisation data of white dwarfs with magnetic fields ranging from weak (of the order of kiloGauss) to super-strong (1 billion Gauss). Through their studies, they have established the presence of complex non-dipolar field structures in a large subset of these stars with evidence in some cases for spot-type field enhancements. Their studies may also have unravelled the first Earth-like planet around a white dwarf, and evidence for white dwarfs with masses close to the Chandrasekhar limit, probably due to a white dwarf-white dwarf mergers. They are currently involved in modelling the Zeeman spectrum of helium in superstrong fields the atomic calculations for which have become available only recently in an attempt to explain the bizarre spectra of a subset of magnetic white dwarfs which have remained a puzzle for over a quarter of a century. The MSI Astrophysics Groupare one of the worlds leading groups on the study of Magnetic Cataclysmic Variables. They have been responsible for introducing a new field of optical stellar spectroscopy – known as cyclotron spectroscopy – for studying the magnetic fields and field structure of magnetic white dwarfs in close interacting binary systems. They are using a combination of Zeeman and Cyclotron spectroscopy to study the properties of these systems. The Group also AM CVn systems, very close white dwarf – white dwarf (or helium star) binary systems which show only helium in their spectra. Understanding the spectra of these systems which originate from tidally interacting compact helium accretion discs is proving to be a significant challenge. The AM CVn systems are expected to be some of the first systems that will be detected by gravitational wave astronomy.