First Detection of Spiral Waves in Gas Disc
For the first time, astronomers have detected spiral structure in the disc of gas that surrounds one of the stars in an interacting binary star system. The system they observed is known as IP Pegasi. It consists of a normal star, similar to the Sun but cooler and only half as massive, which is losing material to a white dwarf companion. The material streaming off the normal start forms a thin hot disc around the white dwarf, rather than falling onto it directly.Using a technique called Doppler tomography (similar to a technique used in medical imaging) Dr Emilios Harlaftis, Dr Danny Steeghs and Professor Keith Horne of the University of St Andrews have mapped out detailed gas movements in the disc and deduced its structure. The results reveal a two-armed spiral pattern in the outer part of the disc. No firm detection of this kind has been achieved before. Dr Steeghs presents the results on Wednesday 9th April at the UK's National Astronomy Meeting at the University of Southampton.
IP Pegasi belongs to a class of variable stars known as dwarf novae. From time to time they brighten up. The reason is thought to be a sudden increase in the amount of material being transferred between the stars via the disc. IP Pegasi has an outburst roughly every three months, when its brightness goes up by two magnitudes. The St Andrews researchers observed the star in the first few days of one of these outbursts, when the disc was at its maximum extent.
The disc is smaller than the radius of the Sun, so it is not possible to resolve it directly in any telescope. The technique used involved measuring the velocity of the gas by looking at the Doppler shift in its spectrum. As the stars revolve around each other in their 3.8- hour orbit, the observers got successively different view of the disc. By combining all the information they acquired, they were able to reconstruct a picture of the flow pattern of the gas.
The result shows a two-armed trailing spiral in the outer part of the disc. Such spirals are thought to be created by tidal forces due to the gravitational pull of the normal star. The formation of such spirals had been predicted, but this is the first really positive detection.
Spiral waves of this kind can arise in various kinds of astronomical objects from binary stars to spiral galaxies. Some astronomers have suggested that the formation of spiral shock waves in discs around newly formed stars could be important for triggering the formation of planetesimals, from which larger planets eventually form.
Colour images of the tomograms, revealing the spiral waves, are available on the WWW at the following address:
Dr Jacqueline Mitton, RAS Public Relations Officer
Phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892