Zooming-In on star formation in the Orion Nebula using the Keck adaptive optics system
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.
The authors wish to recognize and acknowledge the very
significant cultural role and reverence that the summit
of Mauna Kea has always had within the indigenous
Hawaiian community. We are most fortunate to have the
opportunity to conduct observations from this mountain.
IMAGE CAPTIONS:
[Fig. 1:
http://www.astro.ucla.edu/~shuping/OrionProplyds/ProplydSchematic_300.jpg (433KB)]
Schematic of a typical proplyd in Orion: Ultraviolet (UV)
radiation from a nearby massive star eats away at the
protoplanetary disk surrounding a young star creating a
bubble of warm gas. The outer portions of the gas bubble
are then heated and removed by energetic UV radiation.
Material falling from the disk onto the central protostar
fuels twin gas jets. Artwork: Space Telescope Science
Institute.
[Fig. 2:
http://www.astro.ucla.edu/~shuping/OrionProplyds/Trap_HST-Keck_300.jpg (2.8MB)]
Hubble Space Telescope image of the bright proplyds
surrounding the Trapezium stars and some of the new
observations of individual proplyds using the 10-m
Keck II telescope with adaptive optics. The Keck/AO images
show the outlines of gas bubbles due to disk evaporation
that are roughly 50 – 100 AU in size. One AU (Astronomical
Unit) is the distance between the Earth and Sun. For
comparison, the diameter of our solar system is roughly
80 AU.
The Keck II images were presented at the American
Astronomical Society meeting in Seattle, Washington on
January 8, 2003.
IMAGE CREDIT: Keck images — R. Y. Shuping & J. Patience,
W. M. Keck Observatory; HST image — J. Bally.
[Fig. 3:
http://www.astro.ucla.edu/~shuping/OrionProplyds/Proplyds_NIRSPAO_300.jpg (3MB)]
The images on the right are spectra from the positions
outlined on the images at the left. In the spectra, the
light from the small slit on the left is spread out so
that its components can be studied. Gas evaporating away
at roughly 20 km/s can be seen easily in emission from
hydrogen and helium gas. In addition, two proplyds have
jets of gas with velocities over 50 km/s within 200 AU
of the host protostar. One AU (Astronomical Unit) is the
distance between the Earth and Sun. For comparison, the
diameter of our solar system is roughly 80 AU.
These images and spectra were presented at the American
Astronomical Society meeting in Seattle, Washington on
January 8, 2003.
IMAGE CREDIT: R. Y. Shuping & J. Patience, W. M. Keck
Observatory.
