[meteorite-list] Grain Growth in Orion Nebula Protoplanetary Disks

[Ron Baalke]

W.M. Keck Observatory
Kamuela, Hawaii

Media Contact:
Laura K. Kraft, (808) 885-7887

January 9, 2006

Grain Growth in Orion Nebula Protoplanetary Disks

WASHINGTON, D.C. -- New observations of the Orion Nebula at infrared 
wavelengths reveal that small dust grains located in disks around young 
stars are growing, taking the initial steps toward forming planets despite 
bathing in a flood of radiation from highly luminous stars. The properties 
of dust in disks around young stars plays a pivotal role in understanding 
star formation and determining the origins of planets in our Solar system 
and in extrasolar planetary systems as well. The results are presented 
today at the 207th meeting of the American Astronomical Society in 
Washington, D. C.

"One of the key questions we are trying to address is whether or not 
planets can form around young stars in the seemingly hostile environment 
of the Orion Nebula," said Dr. Marc Kassis, support astronomer at the W. 
M. Keck Observatory and lead author of the poster sharing the results.

The Orion Nebula, located about 1500 light years away, is an energetic 
stellar nursery giving birth to thousands of young, Sun-like stars with 
protoplanetary disks. But a few of these newborn stars are 10 to 30 times 
the mass of our Sun and 10,000 times as bright. These massive stars bathe 
the entire region in harsh ultraviolet radiation which evaporates the 
protoplanetary disks of their lower mass neighbors.

"You would think that the strong ultraviolet radiation that is evaporating 
these disks would also inhibit planet formation, but the larger particles 
we see in these Orion disks seem to suggest otherwise," said team member 
Dr. Nathan Smith, Hubble Fellow at the University of Colorado.

To determine the relative sizes of the grains in these protoplanetary 
disks, the research team used the Long Wavelength Spectrometer on the Keck 
I 10-meter telescope and the Mid-Infrared Spectrometer and Imager at the 
3-meter NASA Infrared Telescope Facility, both situated 14,000 feet atop 
Mauna Kea on the island of Hawai`i.

In the optical part of the spectrum, these protoplanetary disks are dark 
and are sometimes viewed in silhouette against the bright nebula. In 
contrast, the dusty disks are extraordinarily bright in the infrared. The 
observations revealed broad spectral signatures of silicate grains, and 
the overall shape of the spectra was unlike the silicate emission of 
relatively smaller grains typical of the interstellar medium.

"The silicate profiles from the protoplanetary disks are generally 
flat-topped instead of peaked, indicating the grains have increased in 
size since the birth of these disks," said Dr. Kassis. "You wonder whether 
the grains will grow enough to start forming planets."

"Could our own solar system have formed in such an environment?" posed Dr. 
Ralph Shuping, support scientist for the Stratospheric Observatory for IR 
Astronomy (SOFIA). "Careful study of primitive materials in meteorites 
suggests that it was, and our observations show that the initial stages of 
grain growth that lead to planet formation can occur in protoplanetary 
disks born in Orion-like environments."

Most stars are born in clusters with bright, massive stars relatively 
nearby. The stars in clusters and their protoplanetary disks born in 
regions like the Orion Nebula can be exposed to the intense ultraviolet 
radiation from massive stars, stellar winds, jets, gravitational pulls 
from their neighbors, and supernova explosions. Yet, recent theoretical 
work and the study of primitive meteorites indicate that our Solar System 
may have been born in a region like the Orion Nebula.

"Some years ago, we thought ultraviolet radiation would be hazardous to 
disks," said Dr. John Bally at the University of Colorado. However, recent 
work by Drs. Henry Throop of the Southwest Research Institute and Bally 
showed that ultraviolet irradiation could promote the rapid formation of 
planets. "So, in disks where grains have grown and settled to the disk 
mid-plane, ultraviolet radiation can remove gas, leaving large particles 
behind to accumulate through their mutual gravitation into small, 
planet-like objects," added Dr. Bally.

The team's observations also hint at the composition of the grains. From 
details in the shape of the infrared spectra, the team is identifying the 
presence of silicate minerals such as olivine and fosterite; olivine being 
the same mineral found along the green sand beaches in Hawai`i.

"It's amazing to think that we can study the minerology of these tiny 
grains 1500 light years away!" remarked Dr. Shuping.

The team responsible for the discovery of grain growth in Orion Nebula 
protoplanetary disks is Ralph Shuping (USRA-SOFIA), Marc Kassis (W. M. 
Keck Observatory), Mark Morris (UCLA), and Nathan Smith and John Bally 
(University of Colorado). The team acquired data at NASA's IRTF through a 
collaboration with the instrument team that includes Joseph Adams (Cornell 
University), Joseph Hora (Harvard-Smithsonian Center for Astrophysics), 
James Jackson (Boston University), and Eric Tollestrup (UH-IfA, NASA 
IRTF).

This work was supported by the Colorado Center for Astrobiology and the 
UCLA Center for Astrobiology, both supported by the NASA Astrobiology 
Institute. The Infrared Telescope Facility is operated by the University 
of Hawaii under Cooperative Agreement no. NCC 5-538 with the National 
Aeronautics and Space Administration, Office of Space Science, Planetary 
Astronomy Program. Some of the observations for this research were 
provided by the W. M. Keck Observatory using Director's discretionary 
time, also known as "Team Keck." The W. M. Keck Observatory is operated by 
the California Association for Research in Astronomy (CARA), a non-profit 
501 (c) (3) corporation whose board of directors includes representatives 
from the California Institute of Technology, the University of California, 
and the National Aeronautics and Space Administration.

IMAGE CAPTIONS:

[Image 1:
http://keckobservatory.org/news/science/060109_orion/AAS-PR_Fig1.jpg 
(1.1MB)]
Close-up of the Trapezium region in the Orion Nebula. On the left, sources 
A-D are bright in the mid-infrared. On the right, the same sources are 
dark in optical wavelengths and sometimes are viewed in silhouette against 
the bright nebula.

Image credit: N. Smith, University of Colorado/Gemini/HST

[Image 2:
http://keckobservatory.org/news/science/060109_orion/AAS-PR_Fig2-markup.jpg 
(328KB)]
Dust emission from protoplanetary disks in Orion. On the left is a 
mid-infrared image (11.7 microns) of the Trapezium region in the Orion 
Nebula. On the right are spectra from Keck Observatory that show grains in 
one of the protoplanetary disks have grown well beyond the sizes typical 
of the interstallar medium.

Image credit: N. Smith, University of Colorado/Gemini/Keck