Zooming-in on star formation in the Orion Nebula

A team of astronomers is using one of the most advanced ground-based telescopes in the world to “zoom-in” on protostars in the Orion Nebula, revealing in unprecedented detail a variety of phenomena associated with star and planet formation in the presence of extremely massive, luminous stars. These phenomena include high-velocity jets of gas launched from the protostars themselves; evaporation flows driven by the intense radiation of nearby massive stars; and colliding winds that form thin, filamentary sheets of gas. In a report being given today at the American Astronomical Society meeting in Seattle, Washington, Drs. Ralph Shuping and Mark Morris at UCLA and John Bally of the Univ. of Colorado, Boulder, along with Drs. Jennifer Patience (Cal Tech), James Larkin (UCLA), and Bruce Macintosh (Lawrence Livermore National Laboratory) present the most detailed observations yet of gas motions around disks surrounding newborn stars in Orion using the adaptive optics system at the W. M. Keck Observatory.

In the early 1990s, Hubble Space Telescope (HST) produced spectacular images of newborn stars with protoplanetary disks, or “proplyds”, in the Orion Nebula, 1500 light years from the Earth. Astronomers learned that these disks are being evaporated by the intense ultraviolet radiation from nearby stars 10 ? 30 times as massive as our sun and 10,000 times as bright (Fig. 1). Now, with the help of the advanced adaptive optics (AO) system at the W. M. Keck Observatory, situated at nearly 14,000 feet atop Mauna Kea on the island of Hawai’i, astronomers are “zooming-in” on the proplyds to study them in even greater detail.

The team’s images with the Keck/AO system reveal the gas bubbles produced by evaporation of these disks in unprecedented clarity. The gas bubbles are approximately 50 ? 100 Astronomical Units (AU) in radius?slightly larger than our solar system (one AU is the distance between the Earth and Sun)? and the gas and dust are evaporating away at roughly 20 km/s (43,000 mph). These figures agree very nicely with current proplyd models, suggesting that the disks can be evaporated away to almost nothing in a hundred thousand years or less. The formation of gas-giant planets is thought to require a million years or more.

“We’re literally watching these disks evaporate before our eyes as the overwhelming energy of the nearby hot stars bears down on them,” says Professor Morris.

“The ultimate question is, can they form a few planets before they evaporate completely?” adds Dr. Shuping enthusiastically. “Our observations suggest that planets are losing the race ? Unless they are forming much faster than we think, these systems may be devoid of planets.” There are also images of a binary proplyd where the evaporating flows of gas and dust from each disk are crashing into each other roughly half-way between the two objects. Since most stars form in multiple systems, this binary proplyd presents a great opportunity to study in detail how protostars can influence each other during formation.

The team has also confirmed the existence of two high-velocity jets less than 200 AU from their host protostars. These jets are spewing gas into the surrounding region at greater than 50 km/s (> 100,000 mph), 150 times faster than a bullet. Where these jets crash into dense regions of material in the nebula they light up, forming so-called “Herbig-Haro” objects, which can be seen in the HST images. One of the jets observed is among the brightest in the sky, but without AO it is lost in the glare of a nearby bright star.

Astronomers are also finding out that the young stars in Orion have very little “elbow-room”. Images obtained by Drs. Patience and MacIntosh as part of their on-going binary star survey in Orion suggest that “many stars in the sky are born under extremely crowded conditions,” says Professor John Bally. “One of the Trapezium stars has no fewer than five companions, all within a few hundred AU.” For comparison, the next nearest stellar neighbor to our Sun, alpha Centauri, is 4.35 light years (or over 270,000 AU) away.

Observing the proplyds in Orion is like trying to identify the profile of George Washington on a quarter over 20 km (13 mi) away, a feat that is impossible from the ground without adaptive optics. The Earth’s turbulent atmosphere causes images obtained at even the largest groundbased telescopes to be “blurry”. Astronomers have found novel ways to beat this turbulence using AO systems that sense the distortions induced by the atmosphere and correct for the blurring using a small deformable mirror. The result is images with the expected sharpness of the telescope as if the atmosphere were not present. In the near-infrared (just beyond human vision) the AO system on the Keck II 10-meter telescope can produce images sharper than those of HST. “The Keck AO system is nothing short of astonishing,” says Professor John Bally at the University of Colorado. “We can see detailed proplyd features that are totally invisible in the HST images.”

Acknowledgments: This research has been supported by a cooperative agreement through the Universities Space Research Association (USRA) with M. Morris; and also by a NASA Long Term Space Astrophysics grant and Astrobiology grant to J. Bally. Part of this work was performed under the auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory. This work has also been supported in part by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by UC Santa Cruz. J. Bally also acknowledges support from the NASA Astrobiology Institute at the Univ. of Colorado Center for Astrobiology.