A distorted view of the Southern Crab.

A distorted view of the Southern Crab

The Crab nebula is one of the more famous astronomical sights visible from the Northern Hemisphere. But it takes a well-developed imagination to see the shape of a crab in its envelope of glowing gases. That designation better suits the crab-like appearance of a recently discovered nebula, He 2-104, in the Southern Hemisphere constellation Centaurus. He 2-104's crustacean shape is evident in the accompanying photograph of the object as seen in the light emitted by nitrogen ions.

But why does the "Southern Crab" have this unusual shape? New evidence suggests this nebula represents an important but comparatively brief stage in the evolution of certain pairs of interacting stars.

Julie H. Lutz of Washington State University in Pullman and her colleagues argue in the November PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC that the Southern Crab may represent a link between symbiotic starts and planetary nebulas -- astronomical entities usually put into separate categories. Astronomers picture symbiotic stars as binary systems in which interactions between a cool red giant and a small, not star excite gas to produce light of particular wavelengths. Planetary nebulas result from the ejection of gaseous material as a star makes the transition from the red-giant to the white-dwarf stage of stellar evolution.

"Our results, as well as the studies of others, show that He 2-104 is a complicated object," Luz and her colleagues report. The system appears to consist of a pulsating red giant with a small, hot companion. Dust obscuring the red giant makes it visible only in the infrared.

The hot companion is either dust-enshrouded or too faint to show up in photographs. A disk of material representing mass lost from the red giant surrounds the entire system. That material is visible as a glowing band of gas and dust that constitutes the crab's body. Some of the material ejected by the red giant has also accumulated as a smaller, high-density disk ringing just the hot star. The "crab legs" are the visible traces left by high-velocity winds shooting out of the hot star, as deflected and channeled by the two gas and dust disks, Lutz and her co-workers propose.

"This model for He 2-104 is neither that of a symbiotic star nor that of a planetary nebula," they say. "We believe that it is an object in transition between the two stages of evolution." In this scenario, the red giant would continue to lose mass, causing the outer disk to thicken further. Eventually, the red giant would shed its outer layers to produce a true planetary nebula. At the same time, the two stars in the system would move closer together. The resulting nebula would probably have a disctinctive butterfly shape.

"This is what we expected if He 2-104 represents a way for some symbiotic starts to make the transition to the planetary nebula phase," the researchers write. In support of their argument, they cite the discovery of a small number of butterfly-shaped planetary nebulas, which may represent just such an endpoint. To check the model further, astronomers must now search for traces of extended structures surrounding other, known dustenshrouded symbiotic stars and make more measurements of the velocities of material in various regions surrounding such objects.