A cataclysmic variable transfers mass from a late type star onto a thin accretion disc around the white dwarf. In the cooler outer regions of this disc, the impact of the gas stream causes a local hot-spot
Most of my research involves a class of interacting binary stars called cataclysmic variables, or CVs. These are binaries containing a white dwarf accreting from a companion star near main sequence. We are using indirect imaging techniques such as Doppler tomography to spatially resolve the accretion processes in these stars. My main interest is the dynamics and angular momentum transport regulating the accretion onto the compact object. By extracting spatially resolved images of the accretion discs we can test and enhance our theoretical understanding of accretion. Accretion discs can be found in a wide variety of enviroments, from the small discs around compact stars and protostars, to the massive discs around black holes in the center of galaxies. The efficient conversion of the potential energy of the accreting material makes accretion onto compact objects one of the most potent energy sources.
Accretion in cataclysmic variables is not a steady-state process. Apart from the massive nova outbursts that are expected to occur every 10^4 years in a CV when matter near the white dwarf is ignited, the dwarf nova systems also show smaller outburst with recurrence timescales of weeks to decades. These outburst are though to occur when an instability is triggered within the accretion disc, and the disc makes a global transition from a relatively cool, low mass accretion rate state to a much hotter high mass accretion rate state. This model is now widely accepted as the model for dwarf nova outburst, and is indeed also used as the standard model for outbursts of neutron star and black hole binary systems (X-ray transients) and the discs around young stars (FU Orionis outbursts). Apart from understanding the physical details behind these outburst (still poorly defined), studying dwarf novae and transients allows us to explore the physics of accretion discs in general as well. Amateur astronomers have been invaluable in recent years and are now monitoring dwarf novae on a regular basis. The advent of automated robotic telescopes also allows us to monitor these systems both photometrically as well as through spectroscopy. We will use the Liverpool Robotic Telescope to track the evolution of a small set of dwarf novae on a daily basis. In summer 2000, we also used the 1.25m telescope at Kryonerion Astronomical Station (Greece), to obtain orbit resolved photometry of the dwarf nova EX Draconis for a period of 6 weeks and captured the evolution of two outbursts of this system.
With a typical distance of 100pc and a binary separation of a few solar radii, the angle subtended by a CV on the sky works out to be of the order of microarcseonds. Direct imaging of the stellar components and the accretion flow is clearly out of the question. Doppler tomography exploits the broad emission lines that dominate the spectrum of a typical CV to recover an indirect image of the accretion flow, thus effectively achieving microarcseond resolution. From the observed shapes of the emission line profiles across the binary orbit, the image is recovered using maximum entropy algorithms in a velocity coordinate frame.
- Table of Doppler maps as compiled by Tom Marsh
- 1st Astro-tomography Workshop proceedings
The observed Hydrogen emission resolved using Doppler tomography
reveals an accretion disc dominated by a two-armed spiral pattern
A recent result from Doppler imaging of CVs is the detection of a two armed spiral pattern in the accretion disc of the binary IP Pegasi. Such spirals are thought to be tidally driven waves due to the companion star and lead to co-rotating trailing spirals across the disc. Apart from severly affecting the global geometry of the disc in 3D, such spirals can also transport angular momentum. Tidally generated density waves can be ideally studied in CVs where one can study their properties in real-time since the dynamical and viscous time-scales are conventiently short (minutes to days).
Have a look at my Ph.D. thesis or a recent review for all the details.
Accreting systems typically display large variability on a vast range of timescales from decades to miliseconds. Aperiodic flickering is associated with timescales around the accreting compact object which is seconds for a white dwarf in a CV and as small as mseconds for neutron stars. I am interested in obtaining high time resolution observations of accreting binaries in order to understand the nature of this rapid variability and use it as a probe of the inner regions of the disc and compact object. We have used the 10m Keck-II telescope and the 4.2m WHT to obtain fast time series spectroscopy of some eclipsing CVs. New fast detectors such as SCAM and ULTRACAM will offer the oppurtunity for very high time resolution studies in the optical.
- Emission line oscillations in the dwarf nova V2051 Ophiuchi, MNRAS, 2001, 323, 484
- The mass donor of Scorpius X-1 revealed, ApJ, 2001, in press
- Emission from the secondary star in the old CV WZ Sge
- A first look at cataclysmic variable stars from the 2dF QSO survey
- Spectroscopic evidence for the binary nature of AM CVn , MNRAS, 2001, in press