Destruction of Proto-Planetary Disks in Orion's Trapezium Explained

Theoretical models, coupled with NASA Hubble Space Telescope observations of the Trapezium cluster in the Orion nebula, implies that there are certain hostile environments in star-forming regions that may inhibit planet formation.

The findings, presented by an international collaboration of astronomers today at the meeting of the American Astronomical Society in Toronto, Canada, explain the destruction of circumstellar disks in Orion's Trapezium, a star cluster at the very center of the nebula.

The report is being presented by Doug Johnstone, a Natural Sciences and Engineering Research Council (NSERC) Post-Doctoral Fellow at the Canadian Institute for Theoretical Astrophysics, University of Toronto. ``For the first time we have a complete evolutionary picture for the stunning objects observed in the Trapezium," says Johnstone.

Their work provides an innovative technique for analyzing circumstellar disks, determining disk masses, and constraining the gestation period for planet embryos around stars in dense clusters, say researchers. ``This may help produce a consensus within the star and planet formation community on standard disk properties," says Johnstone.

The team's results show the disks of dust and gas, which can be several billion kilometers across, are initially similar to the disk which is believed to have formed the planets in our own Solar system, but quickly evaporate in the glare of bright massive neighboring stars in the Trapezium. Radiation from these stars photo-ionizes, or heats and disperses, the cold gas. Within 1 million years the disk is eroded, a time scale shorter than the 1 to 10 million years it would take for planets to form according to current models.

``The theory of disk destruction predicts most efficient destruction at large distances from the embedded, central star. Near the center of the disk, perhaps even at the same distance as the Earth is from the Sun, the remnant disk might survive long enough to form planet embryos," says Johnstone. ``Without a more detailed understanding of planet formation it is not possible to predict the future of these disks, but standard models based on our own Solar system suggest that giant planets like Jupiter and Saturn, at comparable distances from their central star,
would be ruled out."

Using the Planetary Camera on the Hubble Space Telescope (HST), Johnstone's collaborators John Bally and Dave Devine of the University of Colorado, and Ralph Sutherland of the Australian National University observed the Trapezium, a young, million-year-old star-forming region just below the belt of the constellation Orion.

Located nearby, only fifteen hundred light years away, the Trapezium region is the closest star formation site containing both Sun-like stars and stars much more massive than the Sun. While ground-based observations have hinted at extended structures surrounding the Sun-like stars, HST has produced images with incredible detail revealing that these stars are embedded in circumstellar disks and surrounded by diffuse hot ionized gas.

The idea that these young stars are embedded in evaporating disks was first proposed by Ed Churchwell of the University of Wisconsin, based on ground based radio-wave observations. Recognizing that these disks might be the birth sites for planets, C. Robert O'Dell of Rice University in Houston, Texas confirmed the disk hypothesis using HST, and named the objects ``proplyds" as an acronym for proto-planetary disk.

``However, until our work there was no satisfactory model detailing the origin of the diffuse cloud of hot gas observed around each of these Sun-like stars," says Johnstone. Johnstone, along with David Hollenbach and Herbert Stoerzer of NASA Ames Research Center in California, have developed theoretical models describing the destructive effect of high energy radiation on disks, combining the results with Hubble observations by John Bally's team to produce a coherent evolutionary picture.

They report that the disk surface is initially heated to temperatures in excess of 1000 Celsius by the impinging radiation, evaporating the surface layer much like steam evaporates from the surface of boiling water. As this material flows away from the central star and disk, higher energy photons ionize the gas, heating it to temperatures reaching 10000 degrees Celsius and in the process producing the nebulous glow seen in the images. ``We are witnessing the destructive event through the illumination of the evaporated material" according to Johnstone.

Evaporation of the circumstellar disk erodes approximately three moon masses of material per year according to the theoretical model, a number which is verified by the HST observations. The exact evaporation rate from the circumstellar disk is directly related to the size of the disk and thus, as the disk evaporates and shrinks, the erosion rate decreases.

By using this knowledge, and fitting the evaporation model to the HST observations, the collaboration shows that the original circumstellar disks surrounding stars in the Trapezium were similar in appearance to disks around young stars in other systems, and more importantly to the hypothesized Solar proto-planetary disk from which our own nine planets formed.

Doug Johnstone (CITA)