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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Jan 9 13:54:00 2006
Message-ID: <200601091748.k09Hmjp13405_at_zagami.jpl.nasa.gov>

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
Received on Mon 09 Jan 2006 12:48:44 PM PST