vik dhillon: research

research (this page is very out of date!)

what are cvs?

why study cvs?

background theory

my interests

ultracam

what are cvs?

Of the 9000 stars visible to the naked eye from the Earth, well over half consist of two or more bodies locked in gravitationally bound orbits. About half of them, in turn, consist of interacting binary systems where the two component stars are unable to complete their normal evolution without being influenced by the presence of the other. One of the classes of interacting binary are the cataclysmic variables, or CVs, whose members include the novae, dwarf novae and the novalikes. They consist of a white dwarf, conventionally termed the `primary star', and a secondary, which is typically a main-sequence star cooler than the Sun. These variables are characterized by their `cataclysmic' (i.e. violent but non-destructive) eruptions, which are associated with the presence of an accretion disc around the primary star. The image above depicts the five principal components of a typical cataclysmic variable: the primary star (a white dwarf), the secondary star (a red dwarf), the gas stream (formed by the transfer of material from the secondary to the primary), the bright spot (formed by the collision between the gas stream and the edge of the accretion disc), and the accretion disc. The distance between the stellar components is approximately a Solar radius and the orbital period is typically a few hours.

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why study cvs?

Cataclysmic variables provide a unique laboratory for the study of two fundamental astrophysical processes: accretion and binary star evolution. Accretion is the process by which matter is able to overcome the angular momentum barrier which would normally prevent material from spiralling inwards to form compact objects like the Sun, the Earth, and black holes. Cataclysmic variable stars have been central to many developments in the theory of accretion discs. This is because the discs in these systems are nearby (and hence bright), they evolve on very short timescales (hours to weeks) and using techniques akin to medical tomography it is possible to probe structure in the disc with micro-arcsecond angular resolution, resulting in the highest resolution maps currently available in any branch of astrophysics. (Note that the HST only achieves 0.05 arcsecond resolution.) Binary star evolution describes how two widely separated stellar companions may come together and interact, leading to some of the most exotic inhabitants of our Galaxy, such as the black-hole binaries, supernovae and perhaps even the gamma-ray bursters. Cataclysmic variables are a vital link in the evolutionary chain of binary stars, coming immediately after a common-envelope phase and evolving via magnetic braking and gravitational radiation - observations of cataclysmic variables have played a key role in the development of these theories.

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background theory

When two stars come together to form a close binary system, the gravitational fields of the two stars interact and become distorted, as depicted in this image.

By plotting a contour map of the above image, we obtain the Roche gravitational equipotentials. The surfaces close to the centre of each star are spherical. Surfaces further and further from the stellar centres become more and more distorted until we reach the critical surface, which defines two cusped volumes, known as the Roche lobes. These are indicated in this image by the figure-of-eight contour.

In three dimensions, the figure-of-eight potential defines a possible surface for the stellar components, as illustrated in this figure.

When both stars are sufficiently small, each is within its Roche lobe and is relatively undistorted. As the stars evolve, they expand and gradually fill their potential wells. Eventually a point is reached when one star has expanded to such an extent that its surface is defined by the Roche lobe. Further expansion of the star (or, in some close binaries, further contraction of the Roche lobe due to angular momentum loss) results in a flow of material between the stars via the point joining the two Roche lobes - the inner-Lagrangian or L1 point. This process can be likened to a rising water level in this plot of Roche potential as a function of distance between the two stars - when the right-hand well is full, the water will spill into the left-hand well.

The fate of the transferred mass can be modelled by computing particle trajectories, as shown in this plot. As long as the initial velocities of the particles are small, all the trajectories are more or less the same, resulting in a well-defined gas stream between the two stars.

If one of the stars in a close binary is a white dwarf, and the other is a star which is in contact with its Roche lobe, then the system is known as a cataclysmic variable. The transferred material forms a well defined gas stream which eventually settles down into an accretion disc surrounding the white dwarf. The accretion disc is the source of the cataclysmic eruptions often observed in these objects. This movie shows the orbit of an eclipsing cataclysmic variable, speeded up many hundreds of times.

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my interests

My research interests centre on the study of cataclysmic variables, and in particular, their evolution and the study of their accretion discs. I am also interested in astronomical instrumentation, and am currently building ULTRACAM - an ultra-fast, triple-beam CCD camera for studying astrophysics on the fastest timescales - with Tom Marsh. Here are copies of some of my recent papers and talks.


*Click on this image to see my research-level talk on Cataclysmic Variables, which I gave at various UK universities in March/April 2000.*	*Click on this image to see my review talk on Imaging the Secondary Stars in CVs, which I gave at the Astro-tomography workshop in Brussels in July 2000.*	*Click on this image to see my popular-level talk on The Lives of Binary Stars, which I gave to the Sheffield Astronomical Society in November 2000.*	*Click on this image to see my talk on ULTRACAM, which I gave at the ING in January 2001.*	*Click on this image to see my review talk on ULTRACAM, which I gave at the First Galway Workshop on High Time Resolution Astrophysics in April 2001.*

	THE NOVA-LIKE VARIABLES Vik Dhillon Invited review in: Cataclysmic Variables and Related Objects, Proceedings of the 158th Colloquium of the IAU, A Evans, J H Wood eds, Kluwer, Dordrecht, 1996, p. 3 We review optical observations and theoretical models of the non-magnetic nova-like variables (UX UMa, VY Scl and SW Sex stars). A brief discussion of the classification scheme is followed by a statistical overview of the observed properties. The most important features of each of the sub-classes are then reviewed, concluding with a summary of the theoretical models invoked to understand these systems.
	INFRARED OBSERVATIONS OF CVs Vik Dhillon Invited review in: Wild Stars in the Old West: Proceedings of the 13^th North American Workshop on CVs and Related Objects, ASP Conference Series, Vol. 137, Steve Howell, Erik Kuulkers, Chick Woodward eds, 1998, p. 132. We review infrared (1-2.5 micron) observations of cataclysmic variables, a relatively unexplored part of the spectrum in which the dominant sources of emission are the secondary star, the outer regions of the accretion disc and the accretion column in magnetic systems. We describe the advances that have been made in our understanding of cataclysmic variables based on infrared photometry and, more recently, infrared spectroscopy and present spectra of each class of cataclysmic variable - the dwarf novae, novalikes, polars and intermediate polars.
	IMAGING THE SECONDARY STARS IN CATACLYSMIC VARIABLES Vik Dhillon, Chris Watson Invited review in: Astro-tomography: An International Workshop on Indirect Imaging, Brussels, July 2000, Henri Boffin, Danny Steeghs eds, in press. The secondary, Roche-lobe filling stars in cataclysmic variables (CVs) are key to our understanding of the origin, evolution and behaviour of this class of interacting binary. We review the basic properties of the secondary stars in CVs and the observational and analysis methods required to detect them. We then describe the various astro-tomographic techniques which can be used to map the surface intensity distribution of the secondary star, culminating in an detailed explanation of Roche tomography. We conclude with a summary of the most important results obtained to date and future prospects.
	Click on this button for a list of (and links to) some of my other research papers.

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	When two stars come together to form a close binary system, the gravitational fields of the two stars interact and become distorted, as depicted in this image.
	By plotting a contour map of the above image, we obtain the Roche gravitational equipotentials. The surfaces close to the centre of each star are spherical. Surfaces further and further from the stellar centres become more and more distorted until we reach the critical surface, which defines two cusped volumes, known as the Roche lobes. These are indicated in this image by the figure-of-eight contour.
	In three dimensions, the figure-of-eight potential defines a possible surface for the stellar components, as illustrated in this figure.
	When both stars are sufficiently small, each is within its Roche lobe and is relatively undistorted. As the stars evolve, they expand and gradually fill their potential wells. Eventually a point is reached when one star has expanded to such an extent that its surface is defined by the Roche lobe. Further expansion of the star (or, in some close binaries, further contraction of the Roche lobe due to angular momentum loss) results in a flow of material between the stars via the point joining the two Roche lobes - the inner-Lagrangian or L1 point. This process can be likened to a rising water level in this plot of Roche potential as a function of distance between the two stars - when the right-hand well is full, the water will spill into the left-hand well.
	The fate of the transferred mass can be modelled by computing particle trajectories, as shown in this plot. As long as the initial velocities of the particles are small, all the trajectories are more or less the same, resulting in a well-defined gas stream between the two stars.
	If one of the stars in a close binary is a white dwarf, and the other is a star which is in contact with its Roche lobe, then the system is known as a cataclysmic variable. The transferred material forms a well defined gas stream which eventually settles down into an accretion disc surrounding the white dwarf. The accretion disc is the source of the cataclysmic eruptions often observed in these objects. This movie shows the orbit of an eclipsing cataclysmic variable, speeded up many hundreds of times.