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Researchers Solve Silicate Dust Mystery

This is an image of the Cat's Eye nebula taken by the Hubble Space Telescope Advanced Camera for Surveys instrument. Nebulae like this form when dying stars heat up and eject their outer layers of gas into space. Dust grains form in the gas as it expands into space and cools. This will be the fate of our sun near the end of its life about five billion years from now. Credit: NASA and the European Space Agency
by Staff Writers
Greenbelt MD (SPX) Feb 22, 2006
NASA scientists said Wednesday they think they have solved the longstanding mystery of how dying stars can manufacture silicate dust at high temperatures. Researchers at Goddard Space Flight Center said it is important to understanding this process, because silicate dust is a critical building block of stars and planets.

An ancient dying star contributed the dust grains that became our solar system, and at least one planet where life emerged.

Dying stars expel their outer layers of gas into space. As the gas expands and cools, some of its constituent matter condenses into dust grains.

Observations over the last quarter century of gas clouds surrounding old stars have revealed dust made of silicon and oxygen (SiO or amorphous silicate grains) condensing at 1,300 degrees Fahrenheit (more than 700 degrees Celsius). Scientists had thought 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 lead researcher Joseph Nuth. "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 the flaw in the theory: the vapor pressure at which the dust grains were supposed to condense was too high. Just as the water vapor in fog 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. Likewise, at high temperatures, more SiO gas is needed in the circumstellar outflow before it becomes completely saturated and condenses into dust grains.

The pressure at which the SiO gas starts to condense is called its saturated vapor pressure - the equivalent of 100 percent humidity. The Goddard tests revealed the actual value of 1,300 degrees F - about 100,000 times lower than what theory had predicted. The lower actual value means SiO gas can form dust grains in a 1,300 degree-nebula at pressures about 100,000 times lower than previously expected.

"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," Nuth said.

"We plugged the actual, lower saturated vapor pressure values from our experiment into the theory, and it was almost good enough," said co-research Frank Ferguson of Catholic University in Washington, D.C. "The modified theory predicted that the SiO gas was very close to condensing into dust grains, but there was still some factor missing."

As Nuth and Ferguson write in a paper they have submitted to the Astrophysical Journal, the missing factor was the energy SiO molecules lose by radiating it into space. Molecules can vibrate at different levels until, at the highest vibrational energy, they simply can break apart. If nothing excites a molecule - giving it energy by hitting it for example - the molecule will tend to lose energy spontaneously by dropping to a lower vibrational level, and will continue to do this until it reaches the ground state or lowest level possible.

Because the pressure of outflowing nebular gas is low, a SiO molecule there does not often collide with another gas molecule. It also is unlikely to be excited by light from the dying star, because 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 emerges containing minimal vibrational energy.

To begin forming a silicate dust grain, two SiO molecules must condense, releasing energy. That energy must go somewhere - likely to 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 split apart again.

On the other hand, two low-energy SiO molecules are more likely to stick 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," Nuth said. "This result shows how experiment, observation and theory all complement each other in the search to understand what really happens in nature."

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Greenbelt MD (SPX) Feb 22, 2006
A new map of the Milky Way shows the galaxy is ablaze from hundreds of millions of individual X-ray sources, enough to cause scientists to consider they may have underestimated the galactic population of these objects by as much as a hundredfold.

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