March 1, 2004
Adaptive optics gives clear view of star formation
Lick Observatory's new laser guide star adaptive
optics show clearly that larger stars form like our sun--in flattened
By Robert Sanders
A team of astronomers affiliated with UCSC's Center for Adaptive Optics
have obtained sharp, twinkle-free images of the faint dusty disks of
distant massive stars using a recently mounted laser guide star system
at Lick Observatory.
A sodium dye laser beam pierces the sky over Mt. Hamilton's Lick
Observatory on July 22, 2003. The laser is the final piece of
the laser guide star adaptive optics system that allows twinkle-free
viewing of the entire nighttime sky. The beam, which reaches 60
miles into the upper atmosphere, is visible in scattered light
for several kilometers. The yellowish cast of the dome is due
to the streetlights of nearby San Jose. Photo:
Marshall Perrin/UC Berkeley
The star LkHa 198 seen through the adaptive optics system alone,
left, and through the AO system plus polarimeter, right. By viewing
only the polarized component of the light, the polarimeter makes
the dust envelope around the star more easily visible. Image:
Marshall Perrin, James Graham/UC Berkeley
The images clearly show that stars two to three times larger than
the sun form in the same way as solar-type stars--inside a swirling
spherical cloud that collapses into a disk, like that from which the
sun and its planets emerged.
The yellow laser beam piercing the heavens over Lick Observatory became
operational on the 10-foot Shane Telescope last year, expanding use
of the telescope's "rubber mirror" system, called adaptive
optics, to the entire nighttime sky. The addition of the laser makes
Lick the only observatory to provide a laser guide star for routine
The team, led by astronomers at UC Berkeley with colleagues at UCSC,
Caltech, and Lawrence Livermore National Laboratory (LLNL), reported
their results in the February 27 issue of the journal Science.
"The paradigm for stars like our sun is gravitational collapse
of a cloud to a protostar and a pancake-like accretion disk, but there's
some mass at which this can't work--the luminosity of the star becomes
sufficient to disrupt the disk, and it falls apart as fast as it pulls
together," said James R. Graham, professor of astronomy at UC Berkeley.
"Our data show that the standard model paradigm still works for
stars two to three times as massive as the sun."
"Without adaptive optics, we'd see only a big fuzzy blob from
the ground and would be unable to detect any of the fine structure around
the sources," added UC Berkeley graduate student Marshall D. Perrin.
"Our observations provide strong support for an emerging view that
low and intermediate mass stars form in a similar manner."
An adaptive optics system, which removes the blurring effects of atmospheric
turbulence, was added to Lick's Shane Telescope in 1996. However, like
all other telescopes with adaptive optics today, including the twin
10-meter Keck Telescopes in Hawaii, the Lick telescope has had to rely
upon bright stars in the field of view to provide the reference needed
to remove the blur. Only about 1 to 10 percent of the objects in the
sky are sufficiently near a bright star for such a "natural"
guide star system to work.
The sodium dye laser, developed by ace laser scientists Deanna M. Pennington
and Herbert Friedman of LLNL, finally completes the adaptive optics
system so that astronomers can use it to view any part of the sky, whether
or not a bright star is nearby.
Strapped to the bore of the Lick telescope, the laser shines a narrow
beam about 60 miles through the turbulent zone into the upper atmosphere,
where the laser light stimulates sodium atoms to absorb and re-emit
light of the same color. The sodium comes from micrometeorites that
flame out and evaporate as they enter the Earth's atmosphere.
The yellow glowing spot created in the atmosphere is equivalent to
a 9th magnitude star--about 40 times fainter than the human eye can
see. Nevertheless, it provides a steady light source just as effective
as a bright distant star.
"We use that light to measure the turbulence in the atmosphere
over our telescope hundreds of times per second, and then use that info
to shape a special flexible mirror in such a way that when the light,
both from the laser and the target you are looking at, bounces off it,
the effects of the turbulence are removed," said Claire Max, a
professor of astronomy and astrophysics at UCSC, deputy director of
the Center for Adaptive Optics, and a researcher at LLNL who has been
working for more than 10 years to develop a laser guide star system.
In one of the first tests of this system, Graham and Perrin turned
the telescope on rare, young, massive stars called Herbig Ae/Be stars
that are fuzzy from the ground and typically too faint to be imaged
by natural guide star adaptive optics. Herbig Ae/Be stars, with masses
between 1.5 and 10 times that of the sun and probably less than 10 million
years old, are thought to be the beginnings of massive stars--stars
that will end up like the hot, Type A stars Sirius and Vega.
Herbig Ae/Be stars were cataloged years ago by UCSC astronomer George
Herbig, now at the University of Hawaii.
The most massive of the Herbig Ae/Be stars are of great interest because
they are the ones that undergo supernova explosions that seed the galaxy
with heavy atoms, making solid planets and even life possible. They
also trigger star formation in nearby clouds.
What the astronomers saw was very similar to the known picture of T
Tauri stars, which are the formative stages of stars up to 50 percent
bigger than our sun and up to 100 million years old. Images of the two
Herbig Ae/Be stars clearly show a dark line bisecting each star, caused
by a disk blocking the star's bright glare, and a glowing spherical
halo of dust and gas enveloping the star and disk. In each star, two
jets of gas and dust can be seen emerging from the poles of the accretion
The two stars, catalogued as LkH-alpha 198 and LkH-alpha 233 (Lick
hydrogen-alpha sources), are 2,000 and 3,400 light-years away, respectively,
in a distant region of the Milky Way galaxy.
"Material from the protostellar cloud cannot fall directly into
the infant star, so it first lands in an accretion disk and only moves
inward to fall onto the star after it has shed its angular momentum,"
Perrin explained. "That process of angular momentum transfer, along
with the evolution of magnetic fields, leads to the launching of the
bipolar outflows. These outflows eventually clear away the envelope,
leaving a newborn star surrounded by an accretion disk. Over a few million
years, the rest of the material in the disk is accreted, leaving only
the young star behind."
Perrin added that the Hubble Space Telescope has provided "very
clear-cut, unambiguous images of disks and outflows around T Tauri stars,"
confirming theories about the formation of stars like our sun. But,
due to the relative rarity of Herbig Ae/Be stars, such clear data for
those stars has been lacking until now, he said.
Astronomers have proposed that very massive stars form from the collision
of two or more stars, or in a turbulent cloud unlike the swirling accretion
disk. Interestingly, a third star imaged the same night by Graham and
Perrin turned out to be two sun-like stars with a ribbon of gas and
dust between them, looking suspiciously like one star capturing matter
from the other.
Graham hopes to photograph more massive Herbig Ae/Be stars to see if
the standard star-formation model extends to even larger stars. The
detailed images of the Herbig Ae/Be stars owe as much to the new laser
guide star system as to a near-infrared imaging polarimeter built by
Perrin and added to the Berkeley Near Infrared Camera (IRCAL) already
mounted on the telescope.
"Without a polarimeter, light from the stars largely obscures
the structures around them," Perrin said. "The polarimeter
separates unpolarized starlight from polarized scattered light from
the circumstellar dust, which increases the detectability of that dust.
Now that we've developed this technique at Lick, it will be possible
to extend it to the 10-meter Keck Telescopes as the laser guide star
system there becomes operational."
The polarimeter splits the light from the image into its two polarizations
using a new type of birefringent crystal made of lithium, yttrium, and
fluorine (LiYF4), an improvement over the calcite crystals used to date.
Many other groups are developing lasers to be used as guide stars,
but Max's group has been ahead of its competitors since first demonstrating
the concept in the early 1990s at Livermore. Since then, she and colleagues
have been perfecting the laser and the software that allows the mirror--in
the case of Lick's 120-inch telescope, a 3-inch secondary mirror inside
the main telescope--to be flexed just right to remove the twinkle from
The 11- to 12-watt laser is a sodium dye laser tuned to the frequency
that will excite the cold sodium atoms in the atmosphere. The dye laser
is pumped by a green neodymium YAG laser, a bigger brother to the readily
available green milliwatt laser pointers.
"The reason we can now do science with the laser guide star system
is that its reliability and usability is so much improved," Graham
said. "The laser opens up adaptive optics to a much larger community."
"I think it's going to be a workhorse instrument at Lick,"
added Max. "The laser itself and adaptive optics system hardware
are pretty stable and pretty robust. What's going to happen now is that
people are going to do astronomy with it, they're going to develop new
techniques to observe with it, try it on new types of objects. In the
typical way, a good astronomer will come and do things with your instrument
that you never imagined."
Max and her colleagues have tested an identical laser guide star system
at the Keck Telescopes in Hawaii, but it is not yet ready for routine
use, she said.
"The Keck is using the same technology we have at Lick,"
Max said. "I expect to see this general technology used on most
telescopes, but with different kinds of lasers. People are inventing
new types of lasers right and left, so I think that game remains to
Other authors of the Science paper, aside from Graham, Perrin,
Max, and Pennington, are affiliated with the National Science Foundation's
Center for Adaptive Optics, centered at UCSC: assistant research astronomer
Paul Kalas of UC Berkeley, James P. Lloyd of the California Institute
of Technology, Donald T. Gavel of UCSC's Laboratory for Adaptive Optics,
and Elinor L. Gates of the UC Observatories/Lick Observatory.
The observations and development of the laser guide star were funded
by the National Science Foundation and the U.S. Department of Energy.
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