The Astronomy and Astrophysics Program involves mainly Faculty Astronomers although in 1999 there was also one secondment from the RSAA.
The research covers a wide variety of problems relating to compact stars, in particular white dwarfs and black holes but covers also other areas of astrophysics. The investigations range from purely theoretical studies, to computational modelling and observations using ground based and satellite telescopes covering optical, Ultra-violet and X-ray wavebands. Topics covered include the propagation of radiation through strongly magnetised stellar atmospheres, the computational modelling of white dwarf atmospheres (both magnetic and non-magnetic), the radiation properties of accretion flows onto compact stars in binaries, and radiation and hydrodynamical properties of accretion discs.
Accretion Funnels --
Ferrario has developed further her model for the 3D thermal structure of
magnetically confined accretion flows that occur in the binary systems
known as the Intermediate Polars. Her new calculations of the continuum and
line emission include, in addition to the accretion funnel, also
contributions from the ballistic stream. The sources of heating are the
hard x-rays from the accretion shock, the reprocessed soft-xrays from the
white dwarf surface, and magnetic heating at the coupling region and along
the ballistic stream. The models explain rather nicely the observed
multi-component trailed spectra, and the Doppler tomograms of these systems.
Ferrario and Wickramasinghe have constructed a simplified multi-component model for the line and continuum emission from accretion funnels and shocks in the Intermediate Polars for disced and discless accretion. The idea was to establish the mode of accretion in these systems (disced or discless) by modelling the x-ray and optical power spectra. Their calculations show that as the field increases, the polarised cyclotron emission from the shocks become comparable to the optical radiation from the magnetically confined accretion flows. and the dominant power shifts away from the orbital side0band frequencey to the spin frequency of the white dwarf.
Ferrario and Wickramasinghe in collaboration with Dr Pasi Hakala (Turku University, Finland) have embarked on a project which investigates the use of genetic algorithms for multi-parameter model fitting of astronomical data. The method will be applied to inverse problems involving the analysis of polarised radiation and the determination of magnetic field distributions in magnetised stars.
Li and Wilson continued the investigation of the "angular momentum hypothesis" first introduced by Li in connection with the T-Tauri stars. This hypothesis states that magnetically chanelled material which accretes onto a magnetic star does not carry matter angular momentum. The new calculations look at solutions to the Bernoulli integral and show that the flow must be nearly (poloidal) Alfvenic, but that there must also be a large toroidal magnetic field at the base of the funnel.
White Dwarfs --
Wickramasinghe and Dittman have formulated the Scattering matrix
and the radiation transport equation in the presence of a strong
magnetic field including effects of collisions. A new factorisation
is pesented for the Stokes matrix, and the numerical solution is
carried out by operator splitting techniques. The main features of
Thomson scattering in a one dimensional atmosphere are evaulated,
and the effect of the geometry of the magnetic field on the
emerging polarisation is investigated allowing up to octopole
contributions using a patchwork of plane parallel slabs as a model
for spherical stars.
Vennes, Ferrario and Wickramasinghe reported the discovery of a massive double degenerate star which may be part of the family of typeIa supernova progenitors. Vennes has completed a survey of ultra-soft X-ray sources from the German ROSAT satellite using the MSSSO telescopes (74 inch at Mt Stromlo and the 2.3m at Siding Spring) and the 4m Anglo-Australian telescope. The survey proves the existence of numeraous ultra-massive white dwarfs, some close to the Chandrasekhar mass, and also reveals a link between mass and magnetic field. Vennes also uncovered what is possibly the most massive progenitor (> 8 Solar Masses) of a white dwarfs in the triple system Y Puppis(HR2875). This study is by far the most conclusive evidence that some massive main-sequence stars do form white dwarf stars. Vennes, Ferrario and collaborators initiated in 1999 a study of the strongly magnetic white dwarf EUVEJ0317-855 with the astronomical satellites FUSE and EUVE which should be completed in the course of 2000.
Wickramasinghe and Bailey have investigated the conditions under which an encounter of a high field magnetic white dwarf with a proto-planetary cloud could result in a time integrated photon flux of circularly polarised UV photons in the 200-250nm wavelength region of sufficient magnitude to lead to a significant asymmetry in the photolysis of amino acids. They argue that if magnetic white dwarfs are responsible for enantioselection at a cosmic level, life will need to be a very rare phenomenon with the required conditions occuring in a proto planatary system no more than a few times in the history of our galaxy.
Accretion Discs --
Wickramasinghe and El Khoury have continued their investigations of
the properties of accretion discs around galactic mass black holes.
New models of X-ray irradiated discs incorporating soft-x-ray
opacities have been constructed, and these are being used to
re-investigate the thermal instability of accretion discs in the
Black Hole Soft X-ray transients. Together with Ferrario and
Vennes, they are also currently in the process of constructing
models of the spectral properties of accretion discs and winds,
both for stable and for unstable discs. The idea is to predict the
spectral evolution of a x-ray heated disc during an outburst.
Murray, Armitage, Ferrario and Wickramasinghe investigated the effects of tidally induced asymmetric disc structure on accretion on to the white dwarf in intermediate polars. Using numerical simulation, they showed that it is possible for tidally induced spiral waves to propagate sufficiently far into the disc of an intermediate polar that accretion on to the central white dwarf could be modulated as a result, and argued that accretion from the resulting asymmetric inner disc may contribute to the observed X-ray and optical periodicities in the light curves of these systems.
Murray, deKool and Li numerically investigated the hydrodynamics of accretion disk reversals and related their findings to the observed spin-rate changes in the accreting X-ray pulsar GX 1+4. In this system, which accretes from a slow wind, the accretion disk contains two dynamically distinct regions. In the inner part viscous forces are dominant, and disk evolution occurs on a viscous timescale. In the outer part dynamical mixing of material with opposite angular momentum is more important, and the externally imposed angular momentum reversal timescale governs the flow.
Populations Sythesis and Supernova rates --
Wickramasinghe in collaboration with Tout and Regos (Cambridge)
have embarked on a project aimed at predicting the population of
different types of binaries, and of typeIa supernovae as a function
of metallicity and redshift. The Monte-Carlo simulations are
carried out using binary star evolution codes which incorporate
sophisticated stellar models, and are a significant improvement on
previous similar work. One of the main aims of this project is to
investigate whether Type Ia Supernovae can be used as standard
candles for measuring cosmological parameters, such as the
cosmological constant.
Active Galactic Nuclei --
Bicknell and Wagner studied the x-ray and gamma-ray spectrum of the
BL-Lac blazar Markarian 501 and showed that a Doppler factor of
10-20 was required for the emitting region consistent with the
observed apparent proper motion of approximately 7 times the speed
of light. Bicknell and Groves invetsigated the composition of jets
in AGN and argued that electron/positron jets were feasible in AGN
but that electron/protron jets cannnot be ruled out. Bicknell,
Metchnik and Sutherland carried out supercomputer simulations of
hot spots in radio galaxies and provided an explanation for the
filamentary feature near the hot spot in Pictor A in terms of time
dependent shocks resulting from the pulsing of the jet. Li and
Bicknell advanced a theory for the Galactic Snake invoking
reconnection in a magnetic coiling instability as the energetic
source of relativistic electrons in the Snake.