Young Stars "Undressed" By Massive Stars Nearby

November 25, 1998

Although observations from an Arizona telescope show that several young stars near the Orion Nebula have been stripped of the gas and dust surrounding them by radiation from massive stars nearby, these stars surprisingly produce supersonic jets of gas shooting out from them.

Led by the University of Colorado at Boulder's Bo Reipurth and John Bally, the observation team concluded that ultraviolet radiation from several nearby massive stars destroyed the cocoons of gas and dust surrounding nearby young stars. The destruction has made gaseous jets from the young stars -- billions of miles long and traveling at about 300 miles per second -- visible in their entirety for the first time.

"We were surprised to find that newborn stars, which have been stripped of most of their surrounding envelopes of gas and dust, can still form powerful jets," said Bo Reipurth, a senior research associate at CU's Center for Astrophysics and Space Astronomy. "This provides new insight into the processes that produce stars and planets."

A paper on the subject by Reipurth, CASA Director John Bally, Robert Fesen of Dartmouth College and David Devine of NASA's Goddard Space Flight Center appeared in the Nov. 26 issue of Nature. The research team used ultra wide-field CCD cameras -- electronic detectors extremely sensitive to light -- attached to a 150-inch telescope at the National Optical Astronomy Observatory on Kitt Peak to detect the naked stars.

Stars are born when gigantic interstellar clouds collapse, said Bally. A small portion of the material -- perhaps 10 percent -- congeals to form the groups of young stars in "bursts of star formation" producing hundreds to thousands of stars. Such stellar nurseries often remain hidden from telescopic view by the remaining cloud material.

"Ultraviolet light is the agent that has undressed these young stars by blasting the overlying molecular clouds away," said Bally. "The direct observations of these jets emanating from the stars, which are lit up by massive stars nearby, are helping us understand how stars evolve in regions normally obscured from our view."

"If left to themselves, these stellar infants will gradually grow until they reach their final adult size," said Bally.

This normal course of evolution, however, can be interrupted if one or more massive stars also form, said Reipurth. "Massive stars are like gas-guzzling sports cars," said Reipurth. "They use up their fuel very fast and therefore live for only a few million years. The irony here is that the heavier a star is, the shorter its lifetime."

With this enormous use of fuel, massive stars pour out a tremendous amount of ultraviolet light, said Bally. This can stunt the growth of neighboring "baby stars," which are stripped of their reservoir of gas and dust. "Ultraviolet light is the agent that has undressed these young stars by blasting the surrounding material away," said Bally.

A newborn star grows by accumulating material from its surrounding cocoon. "It appears that only part of the matter falling into these young stars actually adds to their growth, while other matter is ejected in the form of supersonic, gaseous jets," said Reipurth. "We now think such jets are somehow necessary for star birth."

Reipurth likened the process to a carousel. "If the carousel is moving slowly, one can dismount from a horse and walk to the middle. But if it were rotating extremely rapidly, it would fling off everyone riding the carousel. It appears that fledgling stars are selective and use the slower rotating material to augment their masses, and eject the faster moving material through jets.

"The direct observations of these jets emanating from newborn stars lit up by massive stars nearby are helping us understand how stars evolve in regions normally obscured from our view," said Reipurth.

Interestingly, the jets from these recently discovered young stars shoot out only from the side facing away from the massive, nearby stars, said Reipurth. The UV radiation from the massive stars seems to inhibit jet formation on the sides of young stars facing the massive stars.

"We were amazed to find four of these naked jets in a single image," said Bally. "A key question is whether these naked stars will produce solar or planetary systems given the powerful radiation from the nearby massive stars."

Meteorites provide tantalizing evidence that our own sun may have been born near a massive star. "Our solar system may have been part of violent events similar to those unfolding in Orion now, " said Reipurth. "If so, then the formation of the planets in our solar system may have been a rapid event, starting before the gaseous envelope surrounding the newborn sun was blown away."

Star formation is the fundamental process in the universe that acts as a building block for the formation of galaxies, said Reipurth. "On a large scale it provides the key to the evolution of the universe, and on a small scale it provide keys to the formation of planets like our own Earth."

University of Colorado at Boulder

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