Space dust annoys astronomers just as much as the household variety when it interferes with their observations of distant stars. And yet space dust also poses one of the great mysteries of astronomy.
“We not only do not know what the stuff is, but we do not know where it is made or how it gets into space,” said Donald York, the Horace B. Horton Professor in Astronomy & Astrophysics at the University of Chicago.
But now York, the University of Toledo’s Adolf Witt and their collaborators have observed a double-star system that displays all the characteristics that astronomers suspect are associated with dust production. The Astrophysical Journal will publish a paper reporting their discovery in March.
The double star system, designated HD 44179, sits within what astronomers call the Red Rectangle, an interstellar cloud of gas and dust (nebula) located approximately 2,300 light years from Earth.
One of the double stars is of a type that astronomers regard as a likely source of dust. These stars, unlike the sun, have already burned all the hydrogen in their cores. Labeled post-AGB (post-asymptotic giant branch) stars, these objects collapsed after burning their initial hydrogen, until they could generate enough heat to burn a new fuel, helium.
Dust in the solar wind
During this transition, which takes place over tens of thousands of years, these stars lose an outer layer of their atmosphere. Dust may form in this cooling layer, in which radiation pressure coming from the star’s interior pushes out the dust away from the star, along with a fair amount of gas.
In double-star systems, a disk of material from the post-AGB star may form around the second smaller, more slowly evolving star. “When disks form in astronomy, they often form jets that blow part of the material out of the original system, distributing the material in space,” York explained.
This seems to be the phenomenon that Witt’s team observed in the Red Rectangle, probably the best example so far discovered. The discovery has wide-ranging implications, because dust is critical to scientific theories about how stars form.
“If a cloud of gas and dust collapses under its own gravity, it immediately gets hotter and starts to evaporate,” York said. Something, possibly dust, must immediately cool the cloud to prevent it from reheating.
The giant star sitting in the Red Rectangle is among those that are far too hot to allow dust condensation within their atmospheres. And yet a giant ring of dusty gas encircles it.
Witt’s team made approximately 15 hours of observations on the double star over a seven-year period with the 3.5-meter telescope at Apache Point Observatory in New Mexico. “Our observations have shown that it is most likely the gravitational or tidal interaction between our Red Rectangle giant star and a close sun-like companion star that causes material to leave the envelope of the giant,” said Witt, an emeritus distinguished university professor of astronomy.