Water Vapor Detected in Protoplanetary Disks
- From: baalke@xxxxxxxxxxxxx
- Date: Tue, 18 Mar 2008 17:57:26 -0700 (PDT)
Caltech New Release
For Immediate Release
March 18, 2008
Water Vapor Detected in Protoplanetary Disks
PASADENA, Calif.--Water is an essential ingredient for forming
planets, yet has remained hidden from scientists searching for it in
protoplanetary systems, the spinning disks of particles surrounding
newly formed stars where planets are born. Now the detection of water
vapor in the inner part of two extrasolar protoplanetary disks brings
scientists one step closer to understanding water's role during
terrestrial planet formation.
By maximizing the spectroscopic capabilities of NASA's Spitzer Space
Telescope and high-resolution measurements from the Keck II Telescope
in Hawaii, researchers from the California Institute of Technology
and other institutes found water molecules in disks of dust and gas
around two young stars. DR Tau and AS 205A, respectively around 457
and 391 light-years away from Earth, are each at the center of a
spinning disk of particles that may eventually coalesce to form
planets.
"This is one of the very few times that water vapor has been detected
in the inner part of a protoplanetary disk--the most likely place for
terrestrial planets to form," says Colette Salyk, a graduate student
in geological and planetary sciences at Caltech. She is the lead
author of a group of scientists reporting their findings in the March
20 issue of the Astrophysical Journal Letters.
Salyk and her colleagues first harnessed light-emission data captured
by Spitzer to inspect dozens of young stars with protoplanetary
disks. They honed in on DR Tau and AS 205A because these presented a
large number of water emission lines--spikes of brightness at certain
wavelengths that are a unique fingerprint for water vapor. "Only
Spitzer is capable of observing these particular lines in a large
number of disks because it operates above Earth's obscuring
water-vapor-rich atmosphere," says Salyk.
To determine in what part of the disk the vapor resides, the team
made high-resolution measurements at shorter wavelengths with
NIRSPEC, the Near-InfraRed cross-dispersed echelle grating
Spectrometer for the Keck II Telescope. Unlike Spitzer, which
observed water lines blended together into clumps, NIRSPEC can
resolve individual water lines in selected regions where the
atmospheric transmission is good. The shape of each line relays
information on the velocity of the molecules emitting the light.
"They were moving at fast speeds," says Salyk, "indicating that they
came from close to the stars, which is where Earthlike planets might
be forming."
"While we don't detect nearly as much water as exists in the oceans
on Earth, we see only a very small part of the disk--essentially only
its surface--so the implication is that the water is quite abundant,"
remarks coauthor Geoffrey Blake, professor of cosmochemistry and
planetary sciences and professor of chemistry at Caltech.
The presence of water in the inner disk may indicate its stage on the
road to planet formation. A planet like Jupiter in our solar system
grew as its gravitational field trapped icy solids spinning in the
outer part of the sun's planetary disk. However, before Jupiter
gained much mass, these same icy solids could have traveled towards
the star and evaporated to produce water vapor such as that seen
around DR Tau and AS 205A.
Although they have not detected icy solids in the extrasolar disks,
says Salyk, "our observations are possible evidence for the migration
of solids in the disk. This is an important prediction of
planet-forming models."
These initial observations portend more to come, says coauthor Klaus
Pontoppidan, a Caltech Hubble Postdoctoral Scholar in Planetary
Science. "We were surprised at how easy it is to find water in
planet-forming disks once we had learned where to look. It will take
years of work to understand the details of what we see."
Indeed, adds Blake, "This is a much larger story than just one or two
disks. With upcoming observations of tens of young stars and disks
with both Spitzer and NIRSPEC, along with our data in hand, we can
construct a story for how water concentrations evolve in disks, and
hopefully answer questions like how Earth acquired its oceans."
Other authors on the paper are Fred Lahuis of Leiden Observatory in
the Netherlands and SRON, the Netherlands Institute for Space
Research; Ewine van Dishoeck, also of Leiden Observatory; and Neal
Evans of the University of Texas at Austin.
###
Contact: Elisabeth Nadin
(626) 395-3631
enadin@xxxxxxxxxxx
.
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