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 “ground state” 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
NASA’s Cosmochemistry Research and Analysis Program, NASA Headquarters.

For images and more information, refer to:

http://www.nasa.gov/centers/goddard/...t_factory.html