ANU Astrophysical Theory Centre


THE ANU ASTROPHYSICAL THEORY CENTRE

1. STAFF

Head and Professor

D. T. Wickramasinghe, B. A.(Cantab), M. A. (Cantab), Ph. D. (Cantab), F.R.A.S.

Senior Fellow

G. Bicknell, M. Sc, Ph. D (Syd)

Senior Lecturer

L. Ferrario, Laurea in Astronomia (Bologna), Ph. D. (ANU)

Senior ARC Research Associate

S. Vennes, Ph. D. [Montreal]

Research Fellow/Lecturer

M. de Kool, Ph. D. (University of Amsterdam)

P. Francis, Ph. D. (Cantab)

J. Li, Ph. D. (Sussex)

R. Sutherland, Ph. D.(ANU)

ARC Research Associate

J. Murray, Ph. D. (Monash)

Postdoctoral Fellow

Z. Kuncic, M. Sc. (Syd), Ph. D.(Cantab)

ARC International Fellows

Professor R. Wehrse, Institute Theoretische Astrophysik, Heidelberg

Dr. Stephan Wagner, Heidelberg

Honorary Fellow

Emeritus Professor D. W. N. Stibbs, M.Sc. (Syd), D. Phil. (Oxon), F.R.A.S., F.R.S.E.

Associate Members

Professor M. Dopita, MSSSO

Dr. D. J. Faulkner, MSSSO

Professor K. C. Freeman, MSSSO

Dr. A. Kalnajs, MSSSO

Dr. D. McClelland, Department of Physics

Dr. P. Wood, MSSSO


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Visiting Fellows

Dr. D. Buckley, SAAO

Professor G. Rudiger, Potsdam University

Prof. L. Titarchuck, NASA

Dr. C. Tout, Cambridge, UK.

Dr. W. van Breugel

Professor R. Webbink, Illinois, USA

Dr. R. Whitehurst, DSTO, Melbourne

Graduate Students

Michelle Buxton

Tavis Hamer

Roberto Soria

Mike Burgess

Walid El-Khoury

Lisa Kewley

Honours Students

Daniel Shaw

Holly Syms

Greg Charles

Greg Wilson

Chairs of ATC Committees

Visitor Program Committee: Lilia Ferrario

Colloquium Committee: Jianke Li

Outreach Committee: Geoff Bicknell

Curriculum Committee: Dayal Wickramasinghe

Library Committee: Walter Stibbs

Computing Committee: Ralph Sutherland

2. OPERATION OF THE CENTRE

2.1 GENERAL

The Centre reached its full complement consistent with its current level of funding in late 1997. Dr. Stephane Vennes from the Centre for EUV Astrophysics, Berkeley joined the Centre as a Senior ARC Research Associate early in the year. Dr. James Murray, who was a post doctoral fellow at the Canadian Centre for Theoretical Astrophysics, arrived late in the year to take up his position as an ARC Research Associate. Dr. Martijn de Kool from the Joint Institute for Laboratory Astrophysics


ANU Astrophysical Theory Centre


took up an appointment as Research Fellow. The Centre also welcomed Dr. Paul Francis from the University of Melbourne, the first joint appointment between the Department of Physics and MSSSO linked to the Centre. Dr. Zdenka Kuncic completed one year at the Centre and left on a Fellowship to the Dominion Astrophysical Observatory.

With the arrival of new staff, the research interests of the Centre broadened to encompass new areas such as numerical fluid dynamics, high energy astrophysics, radiation processes in supernovae, binary evolution and population synthesis. The Centre now has a strong group in compact stars, binaries, radiative transfer, interstellar physics, astrophysical magnetohydrodynamics and active galactic nuclei and one can fairly say that a critical mass has been achieved in these core areas of astrophysics which form the basis for many astrophysical investigations. I am pleased to record that there is regular interaction between staff with exchange and cross-fertilisation of ideas, and that the morale and research ethos in the Centre is high. I am confident that with time, the Centre will develop new directions building on the complementary research interests of its current staff.

My own contention is that a Theory Centre should have a centralised location, as is presently the case, and that this is essential for the exchange of ideas and for engendering intellectual intercourse. The strong collaborations that are occurring between members of the Centre, as is evidenced by the research output of the Centre, clearly demonstrate that the current organisational structure is working well and should be maintained. Normally this would not be an issue. Theory Centres should operate as "think tanks", and this is the principle under which other similar organisations operate. The circumstance that some staff allocated to the Centre have substantive appointments in other Departments is not unusual. These staff have offices in the School of Mathematical Sciences where the ATC is located, and have generally succeeded in maintaining a strong presence at the ATC.

Some of the organisational difficulties in running a multi-site centre which I alluded to in my last report have in a sense disappeared with the setting up of a Management Committee under the chairmanship of Professor Ken Freeman. The Committee has representatives from the different sites, meets regularly, and has acted as an important sounding board for new ideas on the management of the Centre. The responsibility for the day to day operation of the major activities of the Centre has been delegated to sub-committees, and this appears to be working quite well. I would like to record my thanks to Professor Freeman for undertaking the onerous task of Chairing the Management Committee, and to the Chairs of the various ATC subcommittees for assisting in the smooth running of the Centre.

The route of expansion of the Centre in the immediate future is through ARC Fellowships and Grants, and I am very pleased to record the success of one of our staff members, Dr. Stephane Vennes, in winning a prestigious Queen Elizabeth II


MSSSO Annual Report 1997


Research Fellowship in the 1997 competitive round of ARC Fellowships. It is important that all members of staff who have appointments to the Centre from Mathematics, Physics or MSSSO should have the opportunity to compete for ARC funds. It may be possible to achieve this by re-classifying the Centre as a University Centre. This is a matter that I am currently pursuing.

As already noted in last year's annual report, the Large ARC grant scheme is not geared towards promoting the development of new areas of research which are clearly outside the range of expertise of the applicants. I would like to reiterate that an area which falls into this category, and which requires development as a matter of urgency, is Theoretical or Physical Cosmology. The strength of MSSSO in observational cosmology is unquestionable, and it is clear that any plans for the future expansion of the Centre should involve the development of Theoretical Cosmology as a matter of high priority.

The Honours Program in Astronomy and Astrophysics has continued to attract students of high quality. It is my pleasure to record that one of our astrophysics students, Greg Wilson, was awarded a University Medal in 1997. Finally, it is my pleasure to record my thanks to Dr. Don Faulkner, who retired from MSSSO at the end of 1997, for his continued support of the ATC throughout its formative years. I am particularly indebted to him for the important role that he played in the teaching of courses in the Honours Program in Astronomy and Astrophysics which I was instrumental in setting up jointly with MSSSO and Physics three years ago.

2.3 Visitors to the ATC

The ATC Visitor Program was only partially funded by internal funds in 1997, and as a result there was a marked drop in the number of years compared to previous years. However, this was offset somewhat by long term visits by Professor Rainer Wehrse and Dr. Stephan Wagner, both on ARC International Visiting Fellowships.

Professor Wehrse brought to the Centre expertise in 3-D radiative transfer. During his visit he interacted strongly with a number of members of the Centre and was instrumental in originating several new research projects in diverse areas such as the 3-D structure of accretion funnels (with Dr. Ferrario), advective discs (with Wickramasinghe) and accretion disc structure (with Ph. D. student Walid El-Khoury).

Professor Ron Webbink from the University of Illinois visited the Centre partially funded by a grant from NASA. He worked with Ferrario, Burgess and Wickramasinghe on problems associated with binary evolution.


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Dr. Chris Tout from the Institute of Astronomy Cambridge visited the Centre and completed a paper on Black Hole binary evolution with Regos and Wickramasinghe.

The visit of Professor Lev Titarchuk from NASA was funded jointly by the ATC and RCfTA, and resulted in the initiation of collaborative work with Wickramasinghe on the radiation properties of accreting black holes.

Professor Gunter Rudinger of the University of Potsdam visited the Centre on a grant obtained by Jianke Li and worked with him and Wickramasinghe on a project on turbulence in discs.

Dr. Wil Van Breugel visited the ATC in 1997 during a sabbatical which also involved visits to the Australia Telescope National Facility and the Anglo-Australian Observatory. His visit to the ATC stimulated theoretical work on the high redshift radio galaxy 4C 41.17 and further work (in progress) on the radio galaxy 3C338 Van Breugel's sabbatical was partially funded by the ATC and by the ATNF and AAO.

Dr. Stefan Wagner began a one year's visit in December, 1997 as an ARC International Fellow following a successful application coordinated by Dr. Geoff Bicknell. Wagner is an expert in extragalactic astronomy - Intraday Variability of quasars and high energy emission (X-Ray - TeV gamma rays) from blazars and is currently working with Bicknell on these topics. He also has other X-Ray data on the nearest active galaxy Centaurus A and optical emission line data on a sample of Seyfert galaxies and it is anticipated that later in his visit he will work with other ATC academics on these topics. His visit has been very popular with other Australian scientists and in a recent visit to institutions in Sydney gave three lectures on various aspects of his work.

2.4 THE HONOURS AND GRADUATE PROGRAMS

The BSc (Hons) Program in Astronomy and Astrophysics run jointly by MSSSO and the Departments of Mathematics and of Physics and Theoretical Physics, continued to attract students from both Departments. Lecturers from ATC and MSSSO participated in the Program with courses offered in " Black Holes, White Dwarfs and Neutron Stars", "Stellar Structure and Pulsations", "Magnetohydrodynamics", "Physics of the Interstellar Medium" and " Galaxies and Cosmology".

During June and July, Professor Stibbs conducted a series of Seminars on Data Analysis for First-Year Research Students in the ATC and the Mount Stromlo Observatory.


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The Honours year Bok scholarship was awarded to Greg Wilson who went on to receive a First Class Honours degree in Mathematics in the Astrophysics Program, and a University Medal.

The following students are presently enrolled in a Ph. D. degree, and are supervised by staff members of the ATC, either as principal supervisor or as advisor.

Mike Burgess (Wickramasinghe, Wu [RCfTA] and Wood [MSSSO])

Michelle Buxton (Vennes, Ferrario and Wickramasinghe)

Walid El-Khoury (Wickramsinghe, Roberts [SMS] and Ferrario)

Tavis Hamer (Bicknell)

Lisa Kewley (Dopita [MSSSO], Heisler [MSSSO] and Sutherland )

Roberto Soria (Wickramasinghe, Li and Bessell [MSSSO])

Witting, M. (Francis jointly with Webster [Melbourne])

2.5 ANUATC WORKSHOPS

The ANUATC hosted two well attended workshops in 1997. The first was on 'Compact Stars and Accretion Discs' organised by Jianke Li, Stephane Vennes and Lilia Ferrario, and coincided with visits to the Centre by Professor Ron Webbink from the University of Illinois, Professor Rainer Wehrse from the University of Heidelberg and Dr. Chris Tout from the University of Cambridge. The workshop, which was held between August 11-12th 1997, attracted over 25 participants from around the country, and included members of the Special Research Centre for Theoretical Astrophysics, University of Sydney led by Professor Don Melrose, Professor Dick Manchester from the ATNF, Professor Ravi Sood from ADFA, Dr. John Greenhill from the University of Tasmania and Dr. Rosemary Mardling from Monash University. Dr. Helen Johnston from AAO and Dr. Kurt Liffman from AFDL also attended the workshop. The topics covered ranged from Binary Evolution and Pulsars to black holes. The decision by the LOC to have half of the sessions at Mount Stromlo Observatory and the other half on campus at the ATC turned out to be quite popular, and was in keeping with the spirit of the ATC. While at Stromlo, the participants had the opportunity to visit the newly built MSO Exploratory. The workshop was funded by an ARC grant.

The second workshop was on 'Active Galactic Nuclei' and was organised by Bicknell, Francis, Heisler (MSSSO) and Sutherland on Dec. 4 and 5. The venue was chosen by the LOC to be MSSSO. The workshop was timed to precede the ESO/Australia workshop, 'Looking Deep in the Southern Sky', held in Sydney on Dec. 10-12. With the organisational assistance of Dr. Rafaella Morganti (ATNF) we were able to attract a number of speakers from the ESO/Australia meeting to attend the AGN workshop. The meeting also attracted about 18 participants from other Australian institutions. 27 papers were presented including 10 from ANU scientists and visitors working in this


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area. This workshop provides a low cost way for Australian scientists working in the area of AGN to meet and discuss their research, and it was unanimously agreed by the attendees that this meeting should be a regular annual event.

3. RESEARCH ACTIVITIES

Advective Discs around Black Holes in AGN and Soft-X-ray Transients

Shaviv, Wickramasinghe and Wehrse have investigated the self consistency of advective disc models in relation to recent proposals that such models may be applicable to low mass transfer rate accretion discs around black holes in AGN, our galactic centre, and Soft X-ray Transients. Advection dominated accretion flows have usually been discussed in the (1+1) D slim disc approximation, where vertically averaged equations are introduced for pressure, temperature and surface density, and it is explicitly assumed that the vertical energy transport decouples from the radial structure equations. Shaviv, Wickramasinghe and Wehrse have shown that this assumption, which can be justified in the Shakura - Sunyaev thin disc models, could lead to gross errors in the slim disc advective models when cooling is inefficient. They have shown that the coupling with the vertical structure is non- trivial, and may in fact be the determining factor in the hydrodynamics of the accretion flow in advective discs. Their results question the validity of the canonical model for advective discs proposed by Narayan, and may have far reaching consequences for current models of low mass transfer rate accretion discs around black holes in Soft X-ray transients and in galactic nuclei.

Measuring the mass of the Black Hole in GRO 1655-40

The single most important characteristic which distinguishes neutron star (NS) and black hole (BH) X-ray binaries is the mass of the compact object associated with the X-ray source. If the mass is greater than 3 solar masses, the collapsed star is likely to be a black hole. Soria and Wickramasinghe, in collaboration with Hunstead and Wu of the University of Sydney, have measured the motion of the compact object in GRO1655-40 which was discovered nearly three years ago as an enigmatic X-ray and gamma ray source exhibiting superluminal motion. The measurement was made using a unique set of observations obtained with the 2.3m ANU telescope during an outburst of GRO1655-40 in 1996 and has yielded a firm lower limit of 5.4 solar masses and a probable mass of 7 solar masses for the compact star in this system. GRO1655-40 thus becomes one of the best established candidates for a black hole in a Low Mass X-ray Binary in our galaxy.

The Evolutionary Status of GRO1655-40

Regos, Tout and Wickramasinghe have investigated the unusual evolutionary state of the Soft X-ray Black Hole transient GRO 1655-40 which seems to challenge the theory of stellar and binary evolution. A crucial problem is that both the observationally estimated mean mass transfer rate, and the upper limit to the mass transfer rate that


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would allow accretion disc limit cycles is much smaller than the mass transfer rate expected for a 2-2.5 solar mass donor. One of the proposed models is that the donor star is crossing the Hertsprung gap, and filling its Roche lobe, but the calculations of Regos et al. have shown that the expected mass transfer rate is about 1000 times greater than the estimated upper limit based on observations. The proposal that the star is in the Pennington Hyatus is also rejected on the grounds that the star will shrink into its Roche lobe during this phase. The best model appears to be one in which the star is still on the main sequence in which convective overshooting or some equivalent process occurs at the core-envelope boundary.

Disc Instability in Black Hole Transients

El-Khoury and Wickramasinghe have investigated the thermal stability of X-ray irradiated accretion discs in the low and high mass transfer rate regimes by calculating the vertical structure using a bi-gray approximation. The effects of X-ray heating depends on the hardness of the incident X-ray spectrum, and the magnitude of the x-ray flux relative to the viscously generated flux. For heating by soft x-rays a temperature inversion occurs in the upper atmosphere of the disc when X-ray heating dominates, while for heating by hard X-rays, no significant inversion occurs. In general the S curves (mass transfer rate versus surface density relationship) become increasingly distorted as the irradiation flux increases affecting the stability of the disc. The black hole soft X-ray transients tend to hug the X-ray irradiated instability line, thus providing strong support to the idea that the outbursts in BH-SXTs are triggered by thermal instability in the disc.

Neutron Star Accretion Processes

Sutherland has been investigating the coupled radiation/dynamics problem of instability in highly sub-Eddington accretion in isolated old neutron stars. Together with collaborators, he has shown that if accretion occurs on old isolated neutron stars, which have spun down sufficiently to start accreting interstellar gas, we can use an iterative scheme involving the MAPPINGS III code to couple the accretion dynamics with non-equilibrium atomic heating and ionization. We have found, for both blackbody and more realistic ionizing spectra, that steady accretion is not possible, even for accretion luminosities as small a 10-7 of the Eddington luminosity of the system. This new extremely low luminosity limit is found for the first time because we include a more complete set radiation cross-sections and atomic processes than has been hitherto possible.

The Thermal Structure of Accretion Funnels

Accretion funnels occur whenever a central magnetised star interacts with an accretion disc and channels matter along magnetic field lines onto the central object. Thus funnels occur in proto planetary systems such as the T Tauri stars, the AM Herculis type variables and the Intermediate polars. The infalling material forms stand off shocks on the stellar surface, which radiates high energy photons and heats the accretion


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funnel. Ferrario and Wehrse have carried out the first self consistent calculations of the thermal structure of accretion funnels heated externally by soft X-rays. The calculations involve 3-D radiative transfer, and take full account of the 3-D structure of the accretion funnels, and of the coupling of the radiation field at different angles due to scattering. The temperature of the 3-D configurations are determined self-consistently by an iterative scheme which allows both for transfer of the external radiation field and for the trapping of radiation within the funnel. Their research shows that the continuum radiation from the funnel is an important contributor to the unpolarised flux which is known to dilute the cyclotron radiation that arises from the accretion shocks in these systems, and that the funnels are the dominant contributor to line emission.

Magnetic Braking in Magnetic CVs

Li & Wickramasinghe carried out a systematic investigation of magnetic braking under an external magnetic field thus generalising their earlier work. Such a braking mechanism directly applies to magnetic CVs where the primary stars contain strong magnetic fields. In addition to their own importance, the magnetic systems become a unique laboratory for studying details of a basic magnetic braking process. The previous work on the magnetic braking of magnetic CVs, on the basis of aligned dipoles, indicates that the primary field reduces magnetic braking strongly and even curtails the braking. New calculations considering inclined dipoles show that the primary dipole inclination has a strong effect on the braking rate and that magnetic braking is in general dependent on the primary dipole inclination. But as constrained by the observed inclinations, the magnetic system loses about one order of magnitude less angular momentum than their non-magnetic counterpart, qualitatively similar to our previous work, but the 'cut-off' in braking is less abrupt. The new results establish that (1) modelling of a magnetic system must consider the inclination effect (2) AM Her stars have less braking and should be less luminous. Some preliminary results were reported in the 13th North American Workshop held in Wyoming in July 1997.

Magnetic Braking of the Sun

The joint ESA-NASA {Ulysses} mission, launched in 1990 aimed at the first-ever exploration of the heliosphere far from the plane of the ecliptic, has fundamentally changed our view the Sun's 3-D environment. The first measurements were taken near the solar minimum, when the solar magnetic field is closest to being a dipole, and these have given us a unique opportunity to test a theoretical dipole magnetic braking model for the Sun. Li used the current observations from a 3-D solar wind and constrains the Mestel & Spruit model for the Sun. It is found that the Alfven surface at high latitudes is 16 solar radii larger than estimated from the observations in {Helios} mission for the ecliptic plane. It is also found that a dipole braking model is generally consistent with the Ulysses observations provided a linear relation between the dead zone extent and the dipole field strength is satisfied. The Ulysses data implies an angular momentum lose rate of the current Sun which corresponds to a characteristic


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spin-down time of 36 Giga years. The braking rate is a factor 4 smaller than previously anticipated. The detailed braking rate for the present Sun yields crucial information on the unresolved fundamental problem of whether the Sun was a fast rotator or slow rotator.

Singularity in a Relativistic Pulsar Wind

The nature of a singularity in a cool, axisymmetrical, relativistic, steady pulsar wind is further investigated, in line with the recent counter-argument of Ardavan (1995 or A95) that the pure Alfven singularity' is as important as the Alfven singularity if one defines the singularity via a quadratic form rather than by using the ratio adopted by Li & Melrose (1994). The pure Alfvenic point in Ardavan's analysis coincides with the proposed characteristic point through which a continuous wind will meet the star at infinity. However, Li finds that the critical point as implied in the quadratic form is in fact the intermediate point, but not the pure Alfvenic point as argued in A95. Thus the analysis of A95 does not indicate any significance of the pure Alfvenic point. It is also demonstrates that the intermediate point which appears in the quadratic form of A95 is not genuine. The result maintains the early view, and once again shows that there is only one genuine singularity in the standard pulsar wind.

Planets around White Dwarfs

The fate of a planetary system like our own, as the parent star expands through the red giant phase and becomes a white dwarf has been a topic of some discussion. While it appears certain that the outer giant planets will survive this phase, for an Earth-like inner planet, only the conducting core is likely to remain intact. Li , Ferrario and Wickramasinghe argue that a planetary core in orbit around a white dwarf may reveal its presence through its interaction with the magnetosphere of the white dwarf. As the planet moves through the magnetosphere, electrical currents will be generated, which will heat the atmosphere of the white dwarf near its magnetic poles leading to the ejection of plumes of hot gas which may be detected in the optical as H alpha emission. Ohmic dissipation will result in the slow decay of the planetary orbit, and such a planet will merge with the white dwarf in less than a Hubble time, unless the initial orbital separation is greater than 10 solar radii. We propose that the peculiar emission line white dwarf GD356 may be a system in the process of such a merger.

Torque Reversals in Disc Accreting Pulsars

Wickramasinghe and Li continued their research on the spin-up and spin-down properties of neutron stars in X-ray binaries motivated by recent BASTE observations. Their idea that a disc induced magnetospheric instability may drive the observed oscillations in the torque operating on the neutron star was presented at the ANUATC workshop "Compact stars and accretion discs".


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Binary Evolution

Dr. de Kool has started working on a project which uses the properties of the binary stars found in the EUV selected white dwarf sample obtained by Dr. Vennes to study the formation mechanisms of very close binaries such as cataclysmic variables, double white dwarfs and AMCVn stars. The results of this study could have important consequences for the expected number of nearby gravitational wave sources.

Discs in AM CVN Stars

El-Khoury and Wickramasinghe have continued their investigations of the structure and spectrum of helium rich accretion discs around white dwarfs. Such discs are believed to occur in AM Cvn systems, which are very short period binary systems with orbital periods of tens of minutes consisting of two evolved stars. A computer code has been written which calculates the vertical structure of accretion discs allowing for gray radiative transfer. The first results of these calculations were presented at the 10th North American Conference on CVs in July 97.

Radiation Induced Feedback Mass Transfer

Burgess and Wickramasinghe in collaboration with Ron Webbink of the University of Illinois have continued to investigate irradiation driven feed back mass transfer in close binary systems. This process is believed to occur in a variety of systems ranging from black hole x-ray binaries, low mass x-ray binaries with neutron star companions, and cataclysmic variables with white dwarf companions. The basic process involves the heating of the mass losing star by the accretion energy released as radiation during mass transfer. The first stage of this project, which involves the calculation of the effects of irradiation on the structure of the secondary star, and of the accretion heating of the white dwarf, has been completed. It is expected that results of the full feedback problem, which will isolate among other things, the parameter range in which stable mass transfer can occur under irradiation, and the amplitude of expected light variations, will become available by the middle of the year.

Evolved Stars

S. Vennes is involved in stellar atmosphere research and is pursuing both observational and theoretical investigations of the properties of evolved stars. Questions relating to binary evolution and the effect of interaction between the binary components are now deemed critical in shaping the stellar population in the solar neighbourhood. Of particular interest are object recently discovered in ultraviolet and X-ray all-sky surveys. Investigations are carried with the help of both ground-based (Lick Observatory, Mount Stromlo and Siding Spring Observatories) and space-borne observatories (EUVE, HST, ASTRO-SPAS/ORFEUS).

To this aim, he has initiated a systematic program of exploration of the Southern soft X-ray sky (0.1-0.4 keV). Spectroscopic observations at the Mount Stromlo 74 inch telescope of a sample of X-ray sources drawn from the German Roentgen Satellite


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(ROSAT) all-sky survey have uncovered 12 new young white dwarfs (age < 10 million years) and some peculiar active galactic nuclei (AGN). The recently formed white dwarfs are of particular interest in helping understand the evolution of stars like our Sun.

In 1997, in collaboration with astrophysicists from the Space Sciences Laboratory of the University of California Berkeley (Dupuis, Chayer, Hurwitz), S. Vennes has completed a study of the temperature, mass, and chemical composition of two extremely young white dwarf stars using the ORFEUS-II telescope equipped with a high-dispersion spectrograph. The telescope was taken to orbit part of the ASTRO-SPAS mission flown aboard the Columbia Space Shuttle in November 1996.The two white dwarfs show evidence of recent changes in their surface chemical composition that are interpreted with chemical diffusion models.

In collaboration with J. R. Thorstensen (Dartmouth College) and E.F. Polomski (U. of Florida), S. Vennes has completed a series of far ultraviolet spectroscopic observations of white dwarfs in close binaries with late-type stars. The observations were obtained with the Goddard High Resolution Spectrometer (GHRS) and the newly commissioned Space Telescope Imaging Spectrograph (STIS) and allow measurements of the component masses and white dwarf gravitational redshift with unprecedented accuracy.

In collaboration with Wickramasinghe, Ferrario and graduate student M. Buxton, S. Vennes has embarked on a study of black-hole binaries. The team hope to determine stellar masses in newly identified black hole candidates and refine masses in know systems adopting improved observational techniques and binary modeling. Infrared and optical observations are being carried out at Mount Stromlo and Siding Spring Observatories.

Smooth Particle Hydrodynamic Simulations of Astrophysical Systems

Murray is involved in the following projects involving smooth particle hydrodynamics: He is collaborating with

· Bond and Wadsley of the Canadian Institute for Theoretical Astrophysics to develop and test a Lagrangian based magnetohydrodynamics code, and to model the evolution of magnetic fields in the early universe.

· Armitage of the Canadian Institute for Theoretical Astrophysics in numerically investigating tidal instability in accretion discs in close binaries. We have investigated the possibility of exciting a tilt or warp in such discs.

· Wickramasinghe and Lilia Ferrario to numerically model accretion discs around black holes. Various radiative regimes are being explored.

· Wickramasinghe, Lilia Ferrario and Phil Armitage to investigate the observational consequences of tidal instability in accretion discs around weakly magnetised central objects.


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· Pichon of Observatoire de Strasbourg, France, to investigate the nonlinear stability of galactic discs.

· Holman of Smithsonian Astrophysical Observatory, USA and Omar Lopez-Cruz of INAOE, Mexico in a search for Kuiper belt objects from Southern lattitudes.

Astrophysical Shockwaves

Sutherland continued to develop a the astrophysical plasma simulation code, MAPPINGS III, which was used as a foundation of a new grid of shock excitation models for narrowline and extended emission line regions in active galaxies. Major enhancements in 1997 now permit very high resolution time-dependent modelling of X-ray spectra, paving the way for a series of AXAF observing proposals.

MAPPINGS III still retains and improves on spectral modelling capabilities in other areas, such as optical, IR and UV bands. With this capability, MAPPINGS III is now in a unique position to be the analysis workhorse for truly high resolution multiband studies of many astrophysical phenomena.

Line emission from SN1987A

De Kool, together with McCray and Hongwei Li from the University of Colorado, has been modelling the late (200-1200 days after the explosion) line emission from the envelope of SN1987A, with emphasis on the temperature evolution. Theoretical light curves for the most important emission lines were obtained that are in good agreement with the observations, showing that our understanding of the processes going on in the unusual environment of a relatively cold, gamma ray heated gas now stands on a good footing. It is also remarkable that most emission line strengths are reproduced without adjusting the abundances derived from the circumstellar matter.

Together with Borkowski of North Carolina State University and McCray, de Kool also constructed a model for the very late (approx. 10 years) infrared continuum and line emission from the envelope. It appears from this study that the strength of the FeII 26 µ line, observable with ISO, will provide us with an accurate measurement of the total amount of Ti produced in the explosion.

Broad Absorption Line Quasars

In collaboration with Arav (Caltech), de Kool has been working on the interpretation of new, high quality HST spectra of broad absorption line quasars. Specifically, a new method was developed to deconvolve the observed broad, blue shifted absorption lines, which are usually caused by multiplets, into a column density distribution as a function of velocity. This is essential to be able to compare the column density of different ions at a given velocity, which in turn is necessary to calculate the ionization state of the outflowing gas as a function of velocity. Since the new HST spectra cover a very wide wavelength range (580-1600 Angstrom in the rest frame of the QSO), lines of many ions are available. Therefore these spectra will allow us to determine the ionization state to a degree of accuracy that has not been achievable previously.


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Induced Compton Scattering in Active Galactic Nuclei

"Induced Compton Scattering" (ICS) is a non-linear effect which is important when the photon density, measured by the brightness temperature of the radiation, is large. ICS redistributes high brightness temperature photons to lower energies where they are absorbed and can be significant even when the Thomson Optical Depth is much less than unity. It has been speculated for some time that this process may be important in active galactic nuclei (AGN) which are known to contain sources of high brightness temperature radiation. Kuncic, Bicknell and Dopita (MSSSO) have provided the first example of a class of AGN in which Induced Compton Scattering may well be important, namely the AGN known as Gigahertz Peak Spectrum (GPS) radio sources. In their model, an expanding shell of ionized gas driven outwards by the expanding radio source, scatters the brightness temperature radiation from the radio source. A very appealing feature of their model, is that it correctly predicts the slope of the low frequency part of the spectrum. Observations have shown that in the low frequency region the flux densities of most GPS sources are proportional to frequency.

Jet-induced Narrow Line Regions in Seyfert Galaxies

It has been accepted for a long time, that the Narrow Line Regions (NLRs) in Seyfert Galaxies, typically located up to a kiloparsec from the nucleus, are photoionized by UV and X-ray radiation from the accretion disk surrounding the black hole in the nucleus. However, ground-based observations and observations from the Hubble Space Telescope, indicating significant interaction between the jets in Seyfert galaxies and the gas in the NLR have led to increasing scepticism of this view. Bicknell, Dopita (MSSSO), Tsvetanov (Johns Hopkins University) and Sutherland have proposed a model wherein the radio-emitting jets emitted from the vicinity of the black hole interact with gas clouds in the NLR and produce optical line emission via shock waves. This mechanism is only possible if the jets are much weaker radio emitters, for their energy flux than the jets in radio galaxies. This has led these authors to suggest that Seyfert jets are predominantly composed of thermal material (i.e. gas in which most of the particles are nonrelativistic) so that most of the power of the jets is hidden in these particles. This proposition is supported by the detection of thermal X-ray emitting gas in the lobes of a number of Seyferts with large scale outflows. In the well known Seyfert 2 galaxy, NGC 1068, the model predicts an initial mass flux in the jets of order 0.02 solar masses per years. This establishes an interesting link with the hard X-ray observations of Seyfert galaxies which reveal coronae with densities which are consistent with such a mass flux. Another consequence of this model is that the initial velocities of Seyfert jets are at most only mildly relativistic, in comparison to jets in radio galaxies and quasars which are highly relativistic.


ANU Astrophysical Theory Centre


High Redshift Radio Galaxies

High redshift radio galaxies are one of the most significant discoveries by radio astronomers in the past decade. The radio galaxy 4C41.17 with a redshift of 3.8 is one of the most distant examples of this class and has been the target of ground-based (VLA and Keck) and HST observations by Wil van Breugel and his colleagues. During van Breugel's visit to the ATC, Bicknell, Sutherland and Dopita worked with him on the interpretation of the most recent Keck, HST and VLA data which shows evidence for shock excited emission lines and star formation induced by the interaction between one of the radio jets and a dense cloud in the interstellar medium of the parent galaxy. The analysis of the shock excited emission lines is similar, in part, to the analysis of the jet-cloud interaction in NGC 1068. In this case however, the analysis is based upon the rest-frame ultra violet line of [CIV]. The shock diagnostics lead to an estimate of the cloud density of 600 cm-3 and this is easily large enough that shock-driven star formation (i.e. gravitational collapse induced by shock compression) can occur. Analysis of the energy and momentum budget of the jet shows that it has an energy flux of approximately 10-47 ergs/s making 4C41.17 one of the most energetic active galaxies. Possibly the most significant aspect of this work is that it establishes a physical basis (jet-cloud interactions) for the alignment of radio and optical images (the so-called "alignment effect") in high redshift radio galaxies.

Red Quasars

Francis is carrying out a study of the nature of red quasars: optical and radio follow-up of the Parkes radio survey, in collaboration with Webster, Masci and Whiting (Melbourne), Drinkwater (UNSW) and Peterson (MSSSO). They are finding that red quasars are heterogeneous - some are dusty and some are probably red due to unusual synchrotron emission.

Merging Galaxies

Francis and collaborators are studying a rich galaxy cluster at redshift z=2.38, using ground-based telescopes and HST. In collaboration with Woodgate (NASA GSFC) and Danks (Hughes STX). They seem to have discovered a merger between two elliptical galaxies which is not triggering detectable star formation (support for hierarchical elliptical formation?). They also have evidence for a baryon problem - there appear to be too many baryons in this cluster to be explained by conventional models.

Star formation in the Solar Neighbourhood

Sutherland has recently undertaken a very wide field (35 degree FOV) narrow band survey of the Orion-Eridanus bubble, which is being combined with other wide-field radio and IRAS surveys to throw new light on the dynamics and structure of the complex.


MSSSO Annual Report 1997


4. GRANTS AND AWARDS

Dr. Vennes (5 year QEII Fellowship) $100,000 p.a.

Professor Wickramasinghe (Large ARC) $51,000

Dr. Bicknell (DIST International Exchange Fellowship) $4,500

Dr. Ferrario (Small ARC Grant) $17,000

Dr. Li (Faculties New Starters Grant) $4,500

Drs Li, Ferrario and Wickramasinghe{Small ARC Grant} $15,000

Dr. Bicknell

(ARC International Fellowship for Dr. Wagner) $94,400

Professor Wickramasinghe

(ARC International Fellowship for Professor Wehrse) $36,000

5. CONFERENCES, OVERSEAS VISITS AND SEMINARS

Bicknell attended the conference on Relativistic Jets in AGN, Krakow, Poland, May 27-30, and presented an invited review. He attended the Ringberg Workshop on M87, September 15-19 and preseneted an invited review with Beggelman: He also attended the 21st International Shock Wave Symposium, Great Keppel Island, Australia, and presented a keynote address jointly with R. Sutherland.

Wickramasinghe and Ferrario attended the North American Workshop on Cataclysmic Variables, June 15-19, Jackson Hole, USA., and presented invited review papers.

Li, Ferrario, Wickramasinghe, Bicknell and Vennes presented papers at the Workshop on Compact Stars and Accretion Discs held in Canberra on August 11-12.

6. EXTERNAL COLLABORATORS

Members of the ANUATC are heavily involved in collaborations with astronomers from outside the ANU. A list of these astronomers is given below.

N. Achilleos, University College, London

J. Bailey, Anglo-Australian Observatory, Sydney

P. Barrett, NASA

M.C. Begelman, JILA, University of Colorado

D. Buckley, South African Astronomical Observatory

A. Celotti, IAS Trieste, Italy

A. Konigl, University of Chicago

J. Li, High Altitude Observatory, Boulder, Colorado

J. Liebert, Steward Observatory, University of Arizona

B. Lloyd, University of Western Sydney

P. Jones, University of Western Sydney


ANU Astrophysical Theory Centre


M.L. Norman, NCSA, University of Illinois

M. Rees, University of Cambridge

G. Rudiger, Potsdam

G. Schmidt, University of Arizona

L. Titarchuk, NASA

C. Tout, Cambridge, UK

Z. Tsvetanov, Baltimore

J. Wang, University of Maryland

J. F. C. Wardle, Brandeis University

B. Warner, University of Cape Town

R. Webbink, University of Illinois

R. Wehrse, University of Heidelberg

K. Wu, Research Centre for Theoretical Astrophysics, Sydney

7. PROFESSIONAL SERVICE AND ACTIVITIES

Dr. Bicknell was a member of the MSSSO Scholarships Committee, SOC for Ringberg Worskhop on M87, SOC for 1998, ASA AGM, SOC for 1998 Texas Symposium (Paris, France).

Dr. Ferrario was a member of the Graduate Program in Astronomy & Astrophysics, elected representative of the Department of Mathematics in the Faculty of Science, and elected representative of the Faculty of Science in the Faculty of Arts, member of the organising committee of the workshop on "Compact Star and Accretion Discs" held in Canberra Aug 11-12, Chair, ANUATC International Visitors Program, member of the astrophysics curriculum committee.

Dr. Li was Chair of the organising committee of the workshop on "Compact Star and Accretion Discs" held in Canberra Aug 11-12, Chair of the lunch time colloquium series.

Dr. Sutherland is a member of the Committee of the Observatories, MSSSO and Organising Committee for the annual ANUATC/MSSSO AGN workshop. He is also a project scientist for the Antarctic ADIMM site testing program.

Professor Stibbs has continued to service and supervise the ATC Library in addition to his duties as Honorary Librarian to the School of Mathematical Sciences, a post to which he was appointed in recognition of his outstanding work on the restoration of the School Library in the Kurt Mahler Room.

Professor Wickramasinghe was Head of the Department of Mathematics, Chairman of the Faculties Grants Committee and a member of the Advisory Board of The Research Centre for Theoretical Astrophysics, University of Sydney.


MSSSO Annual Report 1997


8. PUBLICATIONS

8.1 Refereed Journals and Books

Celotti, A., Kuncic, Z., Rees M. J and Wardle, J. F. C., MNRAS, 293, 288, "Thermal Material in Relativistic Jets"

Dupuis, J., & Vennes, S., ApJ 475, L131," The Ultraviolet White Dwarf EUVE J1746-706"

Ferrario, L., Wickramasinghe, D.T., Liebert, J., Schmidt, G.D., and Bieging, J.H., MNRAS, 289, 105, "The Magnetic Field and Emission-Line Spectrum of the Remarkable White Dwarf GD~356''.

Ferrario, L., Vennes, S., Wickramasinghe, D.T., Bailey, J.A., & Christian, D.J., MNRAS, 292, 205, "EUVE J0317855: A Rapidly Rotating, High-field Magnetic White Dwarf"

Francis, P. J., Woodgate, B. E. and Danks, A. C., ApJ Letters, 482, 25, "A Group of Red, Ly-alpha Emitting, High Redshift Galaxies ".

Li, J., & Wickramasinghe D., T., MNRAS, 286 L25, "On the fast parameter and inner disc radius".

Polomski, E.F., Vennes, S., et al., ApJ 486, 179, "Faint Sources in the EUVE Survey. I "Identification of White Dwarfs, Active Late Type Stars, and Galactic Nuclei"

Provencal, J.L., Winget, D.E., Nather, R.E., Robinson, E.L., Clemens, J.C., Bradley, P.A., Claver, C.F., Kleinman, S.J., Grauer, A.D., Hine, B.P., Ferrario, L., O'Donoghue, D., Warner, B., Vauclair, G., Chevreton, M., Kepler, S.O., Wood, M.A., and Henry, G.W., Ap. J., 480, 383, "Whole Earth Telescope Observations of the Interacting Binary PG1346+082''.

Soria, R., Li. J., & Wickramasinghe D.T., ApJ, 487, 769, "Magnetised Wind Driven Advective Accretion Disc"

Szkody, P., Vennes, S., et al., ApJ, 487, 916, "Extreme Ultraviolet Photometry and Spectroscopy of BL Hyi"

Vennes, S., Berghofer, T., & Christian, D., ApJ, 491, L85, "A White Dwarf Companion to the B5 V Star HR2875 (y Puppis)"


ANU Astrophysical Theory Centre


Vennes, S., Korpela, E., & Bowyer, S., AJ 114, 1567, "Faint Sources in the EUVE Survey. II Identification of Two White Dwarfs and Four Late-Type Active Stars"

Vennes, S., Dupuis, J., Bowyer, S., & Pradhan, A.K. , ApJ 482, L73, "The Accreting White Dwarf in the Close Binary EUVE J1016-053"

Vennes, S., Thejll, P.A., Galvan-Genova, R., & Dupuis, J., ApJ 480, 714, "Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey II Mass Distribution, Space Density and Population Age".

Vennes, S., Christian, D.J., Mathioudakis, M., & Doyle, J.G., A&A 318, L9, "An Active K0IV-V Star and a Hot White Dwarf (EUVE J0702+129) in a Wide Binary"

Wickramasinghe, D.T., Bicknell, G. V. and Ferrario, L., "Accretion Phenomena and Related Outflows", IAU Symposium No 163, ASP Conf. Ser., Vol 121.

8.2 Conference Proceedings

Bicknell, A. Koekemoer, M.A. Dopita and C.P. O'Dea, Activity in Ellipticals, Proceedings of the Second Stromlo Symposium, ed M. Arnaboldi, G. Da Costa and P. Saha, (San Francisco: Astronomical Society of the Pacific), 432

Bicknell, M.A. Dopita and C.P. O'Dea, Interaction of Jets with the Interstellar Medium in GPS, CSS and CSO Sources, in Proceedings of IAU Colloquium 163: Accretion Phenomena and Associated Outflows, ed. D.T. Wickramasinghe, G.V. Bicknell and L. Ferrario (San Francisco: Astronomical Society of the Pacific), 672

Bicknell G.V. and Dopita M.A. Relativistic Jets in FR1 and FR2 Radio Galaxies and Seyfert Galaxies

in Proceedings of Relativistic Jets in AGN, ed. Ostrowski, M., Sikora M., Madejski, G. and Begelman M.C., Poligrafia Inspektoratu Towarzystwa Salezjanskiego, Krakow, 1

Ferrario, L., "Properties of Magnetic Accretion Curtains: Magnetic CVs and T Tauri Stars", in Accretion Phenomena & Related Outflows, eds. Wickramasinghe, D.T., Ferrario, L., Bicknell, G.V., 1997, A.S.P. Conf. Ser. , Vol. 121, p. 403.

Li, J. and Wickramasinghe, D. T.,'Accretion onto Magnetised Stars: Slow rotators and Propellers", in Accretion Phenomena and related outflows, IAU 163, eds. , Wickramasinghe, D.T., Ferrario, L., Bicknell, G.V, 1997, ASPA Conference Series Vol 121


MSSSO Annual Report 1997


Woodgate, B.E., Francis, P. J., and Danks, A. C., "High Redshift Galaxies Already Old?", in "The Ultraviolet Universe at Low and High Redshift: Probing the Process of Galaxy Evolution", ed. W. H. Waller, M. N. Fanelli, J. E. Hollis, A. C. Danks (Woodbury, AIP), p 297

8.3 Refereed Journals (in press)

Koekemoer A.M. and Bicknell G.V., Dynamics and Excitation of Radio Galaxy Emission-Line Regions-I PKS 2356-61, Ap.J. Letts

Kuncic Z., Bicknell G.V. and Dopita M.A.," Induced Compton Scattering in Gigahertz Peak Spectrum Radio Sources", Ap. J. Letts

Li, J.," Compact stars and Accretion Discs", PASA

Li, J. & Wickramasinghe D.T., "On spin-up and spin-down Torque Reversals in Disc Accreting Pulsars"., MNRAS

Murray, J.R.,"Simulations of Superhumps and Superoutbursts", 1997, M.N.R.A.S.

Regos, E., Tout, C., and Wickramasighe, D. T., " The unusual Evolutionary Status of GRO1655-40", Ap. J.

Soria, R., Wickramasinghe, D. T., Hunstead, R. and Wu, K., "Measuring the Black Hole Mass in GRO1655-40 ", Ap. J. Letts.

Sutherland R. S. "Accurate Free-Free Gaunt Factors for Astrophysical Plasmas", MNRAS

Willes, A. J., Bale, S. D. and Kuncic, Z., "Lower ionospheric Z- mode Maser", J. Geophys. Res.

8.4 Refereed Journal (submitted)

Armitage, P.J & Murray, J.R, "Simulations of spiral structure in the accretion disc of IP Pegasi during outburst", 1997, MNRAS.

El-Khoury, W. and Wickramasighe, D. T., "S-curves of x-ray heated accretion discs and the soft x-ray Transients", 1997, MNRAS.

Ferrario, L. and Wehrse, R., 1997, "The Temperature Charcteristics of Soft X-ray Heated accretion Funnels in Magnetic Binaries", 1997, PASA.


ANU Astrophysical Theory Centre


Li, J., Ferrario L. & Wickramasinghe D.T., "Planets around white dwarfs", 1997, Nature.

Li, J. & Wickramasinghe D.T., "Torque reversals in disc accreting pulsars", 1997, PASP.

Li, J. & Wickramasinghe D.T., "Magnetic braking in magnetic binary stars", 1997, MNRAS.

Li, J., "Magnetic Braking of the Present Sun", 1997, MNRAS.

Li, J., "On singularities of a cool relativistic pulsar wind", 1997, PASA.

Murray, J.R & Armitage, P.J, "Accretion discs in cataclysmic variables: Tidal instabilities and superhumps'', 1997, MNRAS.

Regos, E., Tout, C., and Wickramasighe, D. T., "The Unusual Evolutionary Status of GRO1655-40", 1997, Ap. J.

Shaviv, G., Wickramasighe, D. T. and Wehrse, R., "Self-similarity in Advection Dominated Discs", 1997, Ap. J. Lett.

Vennes, S., Christian, D. J. & Thorstenson, J. R., "Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions", 1997, Ap. J.

Vennes, S., Dupuis, J., Chayer, P., Polomski, E.F., Van Dixon, W., & Hurwitz, M., "The Complete Spectral Energy Distribution and the Atmospheric Properties of the Helium-Rich White Dwarf MCT 05012858", 1997, Ap. J. Lett.

8.5 Conference Proceedings (in press)

Bally J., Theil D. and Sutherland R.S. 1998, in press Proceedings of the Ringburg Workshop The Orion Region, 1997, "Orion's Superbubble"

Bicknell G.V., Kuncic Z., Sutherland R.S., Dopita M.A. and O'Dea C.P.,1998, " Shock Excitation of Emission Lines in Active Galactic Nuclei", in Proceedings of the 18th Texas Symposium on Relativistic Astrophysics, ed. Olinto A., Frieman J. and Schramm D., World Scientific Press.

Bicknell G.V. and Sutherland R.S., 1998, "Astrophysical Shock Waves", in Proceedings of the 21st International Symposium on Shock Waves, ed. Paull A., Boyce, T., McIntyre T., Dannehy, P., Mee, D., Kurtz, J., Sandeman, J. and Tanno, H.


MSSSO Annual Report 1997


Bicknell G.V. and Begelman M.C., 1998, "Theory of the M87 Jet", in Proceedings of the Ringberg Workshop on M87, ed. Harris D.E., Meisenheimer K. and Roeser H.-J., Springer-Verlag

El-Khoury, W. and Wickramasinghe. D. T.,1998, "Theoretical Spectra of Helium Accretion Discs", paper presented at the North American Workshop on Cataclysmic Variables}, June 15-19, Jackson Hole, USA.

El-Khoury, W. and Wickramasinghe. D. T.,1998, "Theoretical S-curves For Irradiated Accretion Discs" , paper presented at the 8th Annual October Astrophysics Conference in Maryland; Accretion Processes in Astrophysical Systems: some like it hot.

Ferrario, L., and Wehrse, R.,1998, "3-D Thermal Structure of Magnetically Channelled Flows'', paper presented at the North American Workshop on Cataclysmic Variables}, June 15-19, Jackson Hole, USA.

Ferrario, L., Wickramasinghe, D.T., Liebert, J., Schmidt, G.D., and Bieging, J.H., 1998, "Constraints on GD356 as a Magnetic Binary'', paper presented at the North American Workshop on Cataclysmic Variables, June 15-19, Jackson Hole, USA.

Ferrario, L. and Wehrse, R.,1998, "The Thermal Characteristics of Soft X-ray Heated Accretion Funnels in Magnetic Binaries'', paper presented at the Compact Stars & Accretion Disks Workshop, Aug. 11-12, Canberra.

Soria, R., Wickramasinghe, D. T., Hunstead, R. and Wu, K., 1998, "Optical Spectroscopy of the Black Hole Binary GRO1655-40 ", paper presented at the North American Workshop on Cataclysmic Variables}, June 15-19, Jackson Hole, USA.

Soria, R., Wickramasinghe, D. T., Hunstead, R. and Wu, K.,1998, "GRO1655-40: Measuring the Mass of the Black Hole from the Optical Emission Lines" paper presented at the 8th Annual October Astrophysics Conference in Maryland; Accretion Processes in Astrophysical Systems: some like it hot.

Wang, J. C. L. and Sutherland, R. S. 1998, in Proc. 8th Annual Maryland Astrophysics Conference: "Accretion Processes in Astrophysical Systems: Some Like it Hot", S. S. Holt & T. R. Kallman, eds., (NY:AIP), in press " Low Luminosity Spherical Accretion onto Neutron Stars: Self-Consistent Models "


ANU Astrophysical Theory Centre


Wickramasinghe D.T. & Li J., 1998, "Magnetic Braking in CVs", paper presented at the Compact Stars & Accretion Disks Workshop, Aug. 11-12, Canberra.

Wickramasinghe D. T. & Li J., 1998, "New results on Magnetic Braking in CVs" paper presented at the North American Workshop on Cataclysmic Variables}, June 15-19, Jackson Hole, USA.