Radio telescope images second dust disk.

For nearly a decade, Beta Pictoris has reigned as the only mature, main-sequence star with an encircling disk of dust - perhaps much like the disk from which our solar system evolved 4.6 billion years ago - that astronomers have clearly imaged. Now, astronomers have used a French radio telescope to map the dust encircling nearby Fomalhaut, the second star proved conclusively to have such a disk.

Fomalhaut, Beta Pictoris, and our own sun are main-sequence stars because they fuse hydrogen into helium to generate energy. The presence of dust around such middle-aged stars may signify that the process of solar-system formation has proceeded at least to the point where comets and asteroids have come into existence, explains planetary astronomer Alan Stern of the Southwest Research Institute in San Antonio, Texas.

Stern and colleagues David A. Weintraub and Michel C. Festou announced their observations of Fomalhaut in a March 30 circular of the International Astronomical Union. The disk appears to extend at least 500 astronomical units (AU) across - 500 times the average distance between the Earth and the sun - although Stern expects this number to grow with more extensive observations. In comparison, the Beta Pictoris disk reaches 2,300 AU across and is significantly more massive. Fomalhaut, however, lies 21 light-years closer to Earth than Beta Pictoris does, which may prove advantageous in upcoming efforts to map the star's disk in more detail, says Stern.

The new observation also marks the first time astronomers have imaged a disk around a mature, main-sequence star using the radio energy given off by dust particles. The dust in Fomalhaut's disk soaks up energy from the star and then emits a wan thermal signal at a wavelength of 1.3 millimeters, whereas the star itself is almost undetectable in this region of the spectrum. Consequently, the radio trace of the dust stands out comparatively brightly.

In contrast, astronomers had to use special instruments to mask Beta Pictoris' face in order to detect the scant amount of visible light scattered off dust particles in that star's disk (SN:6/20/92, p.413). "You've got a more effective tool [for mapping dust disks] if you can see the stuff in its own thermal emissions," comments Weintraub, an astronomer at Vanderbilt University in Nashville.

Astronomer Dana E. Backman of Franklin and Marshall College in Lancaster, Pa., explains that the new radio survey of Fomalhaut has confirmed in a concrete manner observations made a decade ago with the Infrared Astronomical Satellite (IRAS). Then, astronomers discovered tantalizing evidence that dusty rings may encircle a number of nearby stars, but IRAS proved inadequate for mapping them in detail.

"You now have substantially better information about the second well-studied example of what I think is a ubiquitous phenomenon," says Backman, whose studies of nearby stars have convinced him that many, if not most, main-sequence stars have disks. "This study of Fomalhaut is just one more step [toward] showing that."