Weather on brown dwarfs
WASHINGTON UNIVERSITY IN ST. LOUIS NEWS RELEASE
Posted: July 23, 2002

Until recently, "How's the weather up there?" was a question for pilots, NBA stars and friendly giants.

Today, however, you might also ask a dwarf. A brown dwarf, that is.

Brown dwarfs, which have been described as "failed stars," are celestial bodies more massive than planets like Jupiter but not large enough to sustain the thermonuclear reactions that make a star shine.

In the June 1, 2002 issue of Astrophysical Journal Letters, Katharina Lodders, a senior research scientist in the Planetary Chemistry Lab at Washington University in St. Louis -- along with researchers from UCLA, NASA and other institutions -- reported the first evidence for the existence of changing weather patterns on brown dwarfs. They are the first non-planetary objects to exhibit such phenomena.

Lodders' role was to model what compounds may exist under the temperatures and pressures in brown dwarf atmospheres.

"The thermodynamic modeling tells us that liquid iron is settling into clouds," said Lodders.

"There are lots of Earth-like analogies to suggest what the 'weather' is like. The appearance might be described as a sort of fog and clouds, but there are still details to be sorted out to get the most accurate picture."

Brown dwarfs bear similarities to both stars and planets. Like stars, their evolutionary life cycles can last billions of years and they contain the same elements in roughly similar proportions as our sun and other stars. Yet, lacking sufficient mass to become self-sustaining heat sources by nuclear burning, brown dwarfs cool as they age, though for the vast majority of their life cycle they do remain substantially warmer than the gas-giant planets in our solar system.

Like planets and stars, they possess gravitational fields and atmospheres that get cooler as one travels farther from the core to the outside. These two factors facilitate the settling of condensates into clouds once the atmospheres become cool enough to "freeze out" condensates.

Where Earth clouds are made of water vapor, the intense heat of brown dwarfs gives rise to metallic gases which can then form clouds of, say, liquid iron.

Lodders is a specialist in applying thermodynamics to study the chemical make-up, from elemental ingredients, of everything from stardust particles to planets and stars.

An analogy might be examining various amounts of egg, flour and sugar, then predicting what they form under different conditions; essentially thermodynamics tells you whether you're making cookies or cake, and rules out bread for lack of yeast.

First discovered in 1995, brown dwarfs pale in comparison to actual stars but do emit a kind of dim glow that enables astronomers to detect and study them. The nearest known one to Earth is about 19 light years away, about 1,200,000 times the Sun-Earth distance.

A specialized rating system describes how cool the coolest dwarfs are and how a typical brown dwarf cools as it ages, changing from "M" for the coolest stars to "L" to "T" for brown dwarfs.

Lodders and her collaborators were most interested in the older and cooler T dwarfs because their more planet-like atmospheres allowed for the possibility of complex, even almost Jupiter-like chemistry.

In one experiment described in their recent article, the group collected a series of data called absorption spectra. Plotted graphically, absorption spectra exhibit "dents" called bands when certain wavelengths of light are absorbed by particular chemical compounds and these bands can then be compared to giant databases of known compounds.

In essence, absorption spectra become a kind of "fingerprint," revealing what types of compounds are present in the atmosphere.

Once spectral data were collected, the group sorted them from the hotter L dwarfs down to cooler T dwarfs, thus creating a model sequence of the life cycle of a brown dwarf and how its composition changes through time. Two elements present within brown dwarfs are iron and hydrogen, and under certain conditions they form a gas called iron hydride, which, however, disappears once it gets too cool. The simplest example of the "disappearance" process is steam condensing to form liquid water when the steam temperature decreases, but iron hydride does something unusual.

Rather than simply condense into a liquid form of itself, iron hydride decomposes into liquid iron, leaving the hydrogen left in the gas

Researchers predicted that conditions in warmer L dwarfs would favor the existence of iron hydride, and bands in the absorption spectrum confirmed the compound's presence. They also predicted that the significant temperature decreases encountered as one progresses down from, say, an L5 dwarf to an L8 dwarf, would force iron hydride to condense into liquid iron, thus reducing its concentration in the atmosphere. Correspondingly, they expected the iron hydride absorption band to become weaker, which is what they found.

The big surprise came as they continued to analyze the spectra through the transition from L to T dwarfs. If iron hydride was beginning to "condense out" in the later L dwarfs, then even more Condensation would be expected in the cooler T dwarfs. With this in mind, the group might have predicted a steadily weaker signal that would eventually fade altogether but what they saw was a signal that would not die.

"From the chemistry, we would not expect that there is a way to get extra iron hydride back into the system as an object gets cooler" explained Lodders.

"Once it gets cooler, it condenses out. It's like wintertime in St. Louis--the air is very dry because the cold freezes all the moisture out. But interestingly enough, in the cooler brown dwarfs, the iron hydride bands become stronger again, or put another way, they never really disappear. So the question then was, 'how do we explain this?'"

Reappearing signals weren't the only thing that puzzled the scientists. Equally cryptic was the observation that brown dwarfs, as they age, generally appear fainter, but there is a brief period during which they actually seem to brighten.

The researchers hypothesized that perhaps there was something in between the extreme atmospheric conditions of clear vs. cloudy. Based on this new interpretation, the group devised an exploratory model of partial cloud-clearing in cool dwarfs. When they put their model to the test, they found it accurately described the characteristics of a very broad range of brown dwarfs.

The group then surmised that the cooling of the brown dwarfs leads to cloud-clearings caused by atmospheric weather patterns. Those "storms" eventually sweep clouds aside, allowing the bright infrared light trapped below to escape. It is this phenomenon that is believed to be responsible for the bizarre "brightening" effect and for the strong-again iron hydride bands.

"You do not expect iron hydride in the coolest brown dwarfs because it is condensed into the iron liquid clouds," Lodders said.

"If it's condensed, it cannot be in the gas. This means if it shows up in the spectrum, the only way you are seeing it is by looking through the clouds. And if you have cloud-clearings, that means you have weather."

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