DETECTION OF SPIRAL WAVES IN STELLAR ACCRETION DISC

INT+IDS

For the first time, astronomers detected spiral structure in the disc of gas that surrounds one of the stars in an interacting binary star system. The observed system is known as IP Pegasi. IP Pegasi is an eclipsing dwarf nova, a subclass of cataclysmic variables, consisting of a K5 secondary star of 0.5 solar masses losing mass to a white dwarf of 1.0 solar masses in a 3.8-hour orbit. At semi-regular intervals of about three months the system brightens by 2 magnitudes as the mass transfer through the accretion disc suddenly increases.

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 views of the disc. By using a technique called "tomography", they were able to reconstruct a picture of the flow pattern of the gas.

The results showed 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 positive detection.

This discovery was made thanks to observations carried out at the INT using Service time. The Service programme at the ING telescopes is well suited for undertaking a quick look at new cataclysmic variables or providing complementary emission-line information on old ones. But the programme's main advantage is that it offers the observers the opportunity of some flexibility over the predetermined schedule to cover unexpected events such as nova outbursts. Indeed the astronomers observed IP Pegasi while it was on the rise to outburst with the Intermediate Dispersion Spectrograph (IDS) on the INT, which resulted in the discovery of spiral structure in the binary's accretion disc.
 
 

A hot disk of gas surrounding a compact white dwarf star in the constellation of Pegasus has recently been revealed to be imprinted with this dramatic pattern. The white dwarf is part of the interacting binary star system IP Pegasi and the disk of gas is an accretion disk formed of material lost from a companion star and falling toward the white dwarf. The disk itself is smaller than the Sun's diameter, so the spiral pattern can not be imaged directly by telescopes. Instead, the spiralling disk of gas is mapped over a series of observations using a spectroscopic technique known as Doppler "tomography". The left panel above shows a tomogram, the directly measured gas velocity map for the system. The relative brightness corresponds to the intensity of light emitted by Hydrogen gas moving at the indicated velocity. The position at the center of this panel represents the velocity of the binary system's center of mass. In the middle panel, a simple model velocity field consistent with the measurements is shown. At the right, the calculated position map of the IP Pegasi accretion disk reveals a striking two armed trailing spiral pattern [ JPG | BMP ].

References
 

• E T Harlaftis and D Steeghs, 1997, "Spiral Waves in a Solar-size Accretion Disc", Spectrum Newsletter, 13, 4.
• D Steeghs et al, 1997, "Spiral structure in the accretion disc of the binary IP Pegasi", MNRAS, 290, L28.