Weekly Science Update

Wednesday, December 20, 2006
Massive Jets in Aging Stars
Once a normal star like the Sun has burned its hydrogen fuel into helium, and then in turn converted its helium into carbon and oxygen, it begins to show its age. Already very swollen and red, shells of residual hydrogen and helium gas around the star's core begin to shrink, heat up, and burn briefly in a series of nuclear pulses. These pulses of energy ultimately result in ejecting the star's outer layers. Ultraviolet light from the hot star then illuminates these layers, producing a planetary nebula -- so they were dubbed in the nineteenth century because, like real planets, the shells of gas are seen around stars. Besides being beautiful to look at, planetary nebulae hold clues to how stars age, and how they recycle material into space. That enriched material, with its carbon and oxygen, may someday end up forming real planets around a later generation of stars.

No one understands in any detail how the processes of ejection actually work. At some point, for example, the generally spherically symmetric shell motions change into highly collimated, very fast, bipolar outflows. Writing in this week's Astrophysical Journal, SAO astronomers Ken Young and Nimesh Patel, along with three of their colleagues, describe new observations with the Submillimeter Array (SMA) that help to clarify what is going on. The star IRAS 22036+5306 is known to be at an early stage of producing a planetary nebula. As a so-called pre-planetary nebula, it was identified by the team (using Hubble Space Telescope pictures) as harboring such bipolar jets. The SMA results identified in the shell a very fast moving jet with a velocity of about 220 kilometers per second, and containing as much material as about ten thousand Earths. This kind of jet cannot be powered by the pressure of intense light. The authors speculate that somewhere, still undetected around the star, is a small circumstellar disk accreting material. That orbiting disk could generate along its axes the powerful flows that drive the protoplanetary nebula outward. Other SMA measurements indicate that the central star itself must be more massive than four solar masses. The new research helps to identify and quantify some of the key features of the transition processes in play as an old star ejects its outer layers into the cosmos.


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