Credit: © DAVID A. HARDY/STFC

Dust generally follows an explosion, but astronomers have found what seems to be the opposite case in RS Ophiuchi. Using the twin telescopes of the Keck Interferometer in a 'null' mode, which suppresses the light of a star by a dark fringe such that faint objects nearby can be resolved, Richard Barry and co-workers studied this recurring nova four days after its latest outburst on 12 February 2006 (Astrophys. J. doi:10.1086/529422; 2008). Previous recorded events occurred in 1898, 1933, 1958, 1967 and 1985.

The exploding star is a white dwarf, whose surface undergoes nuclear explosions. Its companion red giant sheds gaseous matter that the white dwarf incorporates. Whenever the surface temperature of the growing white dwarf reaches some critical value, a thermonuclear explosion ensues and the luminosity increases 600-fold. A thermonuclear explosion is also the engine for a supernova blast, but that involves the core of a massive star and not only its surface.

Although this binary system has been studied using many telescopes at different wavelengths, only the interferometry method provides spatial resolution of the emission components. And in the bright zone surrounding the explosion, there was no dust — but further out, well beyond the blast wave, silicate dust abounds.

To explain all the observations to date, including how the dust got there, requires a new model. In the current model, the interaction between the matter ejected by the white dwarf during the blast and the wind from the red giant creates a shock wave. The observations of Barry et al. suggest that the motion of the binary system through the cool wind causes the shock wave to have a spiral pattern, which leads to density enhancements that enable dust to form from the cooler atoms, in a 'pinwheel' pattern. Of course, this pinwheel structure was destroyed during the latest blast, but it should build up again if the model is correct.