Spiral Waves in an Accretion Disc (II)
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 spiraling 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. These pictures are
courtesy D. Steeghs, E. Harlaftis, and K. Horne (Astronomy Group, Univ.
St.Andrews).
Javier Méndez
jma@ing.iac.es