Stardust Factory Solves 25-year-old Mystery of Impossible Dust (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 23 Feb 2006 18:25:21 -0500 (EST)
Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md. February 21, 2006
Phone: (301) 286-5017
Stardust Factory Solves 25-year-old Mystery of Impossible Dust
Researchers using a "stardust factory" at NASA's Goddard Space Flight
Center, Greenbelt, Md., have solved a mystery of how dying stars make
silicate dust at high temperatures. Understanding this process helps us
understand our origin, because this dust will become part of another
generation of stars and planets, just as previous generations of stars
contributed dust grains into our solar system that at least on one planet
led to life.
Dying stars heat up internally while expelling their outer layers of gas
into space. The gas expands and cools, allowing some matter in it to
condense into dust grains. Observations over the last quarter century show
dust grains made of silicon and oxygen (SiO or amorphous silicate grains)
condensing at 1,300 degrees Fahrenheit (more than 700 degrees Celsius) in
the billowing clouds of gas (nebulae) surrounding old stars. The
prevailing theory said that this temperature was too high to condense
solid silicate grains -- the silicon and oxygen should have remained in
the gas.
"Even though theory said it was impossible, stars made dust grains at high
temperatures anyway -- it was happening right before our eyes," said Dr.
Joseph Nuth of Goddard, lead author of a paper on this research recently
submitted to the Astrophysical Journal. "So we went to our laboratory at
Goddard where we vaporize material in a vacuum and observe how it
condenses to see what we were missing."
The experiment revealed that the "vapor pressure" at which the dust grains
condense was too high in the theory. Just as fog (water vapor) condenses
out of the air when the temperature drops or the humidity rises, SiO will
condense out of nebular gas at certain temperatures and pressures. Warm
air holds more water as gas than cold air, which is why 100 percent
humidity -- the amount of water gas required to completely saturate the
air -- feels so much more uncomfortable on a hot summer day. Similarly, at
high temperatures, it takes more SiO gas in the circumstellar outflow
before it will become completely saturated and condense into dust grains.
The pressure at which the SiO gas starts to condense is called its
saturated vapor pressure -- 100 percent humidity for SiO gas. The
experiment revealed that the actual value at 1,300 degrees F was about
100,000 times lower than what was predicted by the theory. The lower
actual value means that SiO gas can form dust grains in a 1,300
degree-nebula at concentrations about 100,000 times lower than previously
believed. "If weather forecasters had made a similar prediction about the
vapor pressure for water, they would say rain was impossible -- they would
think there was never enough water in the air to make it rain," said Nuth.
"We plugged the actual, lower saturated vapor pressure values from our
experiment into the theory, and it was almost good enough. The modified
theory predicted that the SiO gas was very close to condensing into dust
grains, but there was still some factor missing," said Dr. Frank Ferguson
of the Catholic University of America, Washington, Co-author of the paper.
According to the researchers, the missing factor was that the SiO
molecules can lose energy by radiating it out into space. Molecules can
vibrate at different levels, each with more energy than the one below,
until, at the highest vibrational levels, they have so much energy that
they just break apart. If nothing excites a molecule, giving it energy by
hitting it for example, the molecule will spontaneously lose energy by
dropping to a lower-energy vibrational level, and will continue to do this
until it reaches the ³ground state² or lowest level possible. Since the
pressure is low in the outflowing nebular gas, a SiO molecule there does
not often collide with another gas molecule. It is also unlikely to be
excited by light from the dying star, since the nebula is expanding into
the darkness of deep space and only part of its field of view includes the
star itself. Under these circumstances a large population of ground-state
SiO molecules develops that contain minimal vibrational energy.
To begin forming a silicate dust grain, two SiO molecules have to stick
together (condense). This releases energy. That energy has to go somewhere
-- likely into more energetic vibrational levels. Two molecules already in
high-energy states are more likely to gain too much energy from the
condensation reaction, so they would simply split apart again. On the
other hand, two low-energy SiO molecules are more likely to remain stuck
together with the reaction energy going temporarily into higher-level
vibrational states until the larger molecule can radiate this energy into
space. Therefore when many of the SiO molecules in the nebula are in
low-energy vibrational states, they can condense at a slightly higher
temperature than their vapor pressure alone indicates because these
molecules are cooler than the surrounding gas.
"When we use the new vapor pressure and account for the vibrational levels
of the SiO molecules in the expanding gas, silicate dust condenses
easily," said Nuth. "This result shows how experiment, observation, and
theory all complement each other in the search to understand what really
happens in nature." The research was funded by NASA¹s Cosmochemistry
Research and Analysis Program, NASA Headquarters.
For images and more information, refer to:
http://www.nasa.gov/centers/goddard/news/topstory/2006/stardust_factory.html
.
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