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Stellar cocoons in surprisingly harsh environment
Jan. 10, 2003
University of Colorado at Boulder astronomers have discovered what they believe to be dozens of potential stellar cocoons within a giant star-forming region that may harbor disks of dust and gas that could one day form planetary systems.
"This is the first large population of so-called 'proplyd' objects to be found outside of the Orion Nebula, the closest region to Earth known to be forming massive stars," said CU-Boulder postdoctoral researcher Nathan Smith of the Center for Astrophysics and Space Astronomy.
The newly discovered proplyds are located in the Carina Nebula, some 7,300 light-years from Earth and are roughly 100 times the diameter of our solar system, he said.
In 2001, CU-Boulder astrophysicists found evidence that particles orbiting in dusty disks around several young sun-like stars in Orion were clumping together and growing.
"These observations may indicate the first phases of planetary growth in Orion's proto-planetary disks," said Professor John Bally of the astrophysical and planetary sciences department.
Stars form when a large interstellar gas cloud contracts, causing them to eventually "turn on" at their centers, said Bally. "In most cases a large, doughnut-shaped disk is left circling the birthing star. Most stars form in clusters in which dozens to hundreds of such clouds produce stars nearly simultaneously."
In star nurseries like the Orion Nebula, stars more than 20 times as massive as the sun can make planetary formation difficult because the ultraviolet light they produce can act like a blowtorch, evaporating the gas and removing the dust from the circumstellar disks of the smaller stars, said Bally. "Massive stars are hazardous to planets forming around nearby, lower-mass stars."
However, said Smith, the Carina Nebula contains about 60 very hot and massive stars --some of the hottest stars in the Milky Way -- each of which is as massive as the one star that powers the Orion Nebula. "This is especially interesting because it will allow us to study early phases of low-mass star formation under more extreme conditions than in Orion."
This suggests planetary disks may be more resilient or more common than previously thought, said Smith, lead author of a study released today at the 201st annual meeting of the American Astronomical Society in Seattle. Co-authors on the paper include Bally, Jacob Theil and Jon Morse of CU's Center for Astrophysics and Space Astronomy.
Detected from the ground using the National Science Foundation's 4-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile, the proplyds in Carina are puzzling for several reasons, said Smith.
For one, they are typically five to 10 times larger than similar objects in Orion. "One explanation for this peculiar situation may be that the Carina proplyds have more massive proto-planetary disks than those in Orion, since the whole region of Carina tends to give birth to more massive stars," he said.
"So far, we have only seen the largest cocoons since Carina is more distant than Orion, but smaller ones may be lurking as well, waiting to be discovered with the Hubble Space Telescope," said Smith.
In addition, some Carina proplyds have relatively large, spherical heads, and at least one appears to have two tails, said Smith. "This is still a mystery that will require additional study."
Located in the Southern Hemisphere sky, the proplyds in the Carina Nebula were unexpected because the conditions are much more extreme than in Orion. Astronomers assume such proplyds should evaporate faster as they are baked in UV radiation produced by the hot, massive stars.
"Most normal stars like our sun form in violent 'starburst' regions along with much more massive stars -- an environment more like the Carina Nebula than Orion," Smith said. "A better understanding of these objects in Carina should tell us more about the prospects for survival of young planetary systems."
University of Colorado at Boulder
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