Telescopes reveal brown dwarf’s ‘iron rain’

U. ARIZONA (US) — The Spitzer and Hubble space telescopes show sandy and iron storm clouds that enshroud these strange objects, which are not quite planets and not quite stars.

A team of astronomers used NASA’s Spitzer and Hubble space telescopes simultaneously to peer into the stormy atmosphere of a brown dwarf, creating the most detailed “weather map” yet for this class of strange objects. The forecast shows wind-driven, planet-sized clouds.

This artist’s illustration shows the atmosphere of a brown dwarf called 2MASSJ22282889-431026. The results were unexpected, revealing offset layers of material as indicated in the diagram. For example, the large, bright patch in the outer layer has shifted to the right in the inner layer. The observations indicate this brown dwarf is marked by wind-driven, planet-size clouds. (Credit: NASA/JPL-Caltech)


Brown dwarfs form out of condensing gas, like stars, but fail to accrue enough mass to ignite the nuclear fusion process necessary to turn them into a star. Instead, they pass their lives as dimly glowing, constantly cooling, gas balls similar to gas planets with their complex, varied atmospheres.

The new research is a stepping-stone toward better understanding not only brown dwarfs but also the atmospheres of planets beyond our solar system.

“With Hubble and Spitzer, we were able to look at the layers of a brown dwarf, similar to the way doctors use medical imaging techniques to study the different tissues in your body,” says Daniel Apai, the principal investigator and assistant professor in astronomy and planetary sciences at the University of Arizona.

The researchers, whose study is published in the Astrophysical Journal Letters, turned Hubble and Spitzer simultaneously toward a brown dwarf called 2MASSJ22282889-431026. They found that its light varied in time, brightening and dimming as the body rotated around every 1.4 hours. But more surprising, the team also found that the timing of this brightening changed depending on whether they looked at it with Spitzer or Hubble using different wavelengths of infrared light (Hubble sees shorter-wavelength infrared light than Spitzer).

These variations are the result of different layers, or patches, of material swirling around the brown dwarf in windy storms as large as Earth itself. Spitzer and Hubble see different atmosphere layers because certain infrared wavelengths are blocked by vapors of water and methane high up, while other infrared wavelengths emerge from much deeper.

“What we see here is evidence for massive, organized cloud systems, perhaps akin to giant versions of the Great Red Spot on Jupiter or large-scale storm systems on Earth,” says Adam Showman, a theorist with the Lunar and Planetary Laboratory, who was involved in the research.

“We were expecting the phases of the light variations to be in sync between the two telescopes, so we were really surprised that they were offset,” says Esther Buenzli, a researcher in the department of astronomy. “This is the first time that we can probe variability at several different altitudes at the same time in the atmosphere of a brown dwarf.”

“The deeper layers appear to lag behind the higher layers,” Apai explains. “This tells us that the same or similar cloud distribution is present in the different layers, but the deeper you look, the later you will see the same clouds turning into view.

“We were very surprised to see such a big lag. Our best guess is this has to do with the brown dwarf’s atmospheric circulation. The bigger picture here is that we see a very large-scale atmospheric structure in this brown dwarf.”

“These out-of-sync light variations provide a fingerprint of how the brown dwarf’s weather systems stack up vertically,” adds Showman. “The data suggest that regions on the brown dwarf where the weather is moist and cloudy deep in the atmosphere coincide with balmier, drier conditions at higher altitudes—and vice versa.”

Ranging in size between Jupiter and the smallest stars, and commonly weighing in at 30-40 Jupiter masses, brown dwarfs are cool relative to other stars but quite hot by our Earthly standards. This particular object is about 600 to 700 degrees Celsius (1,100 to 1,300 degrees Fahrenheit). Being quite warm, they emit strongly in the infrared, wavelengths picked up by Spitzer and Hubble.

At the cooler, outer layers of the star, gas condenses into smoke-sized particles, including sand and iron, which fall down into the interior as a sandy and iron rain. Just like on Earth, the iron and sand “raindrops” heat up as they enter the deeper warmer layer and eventually evaporate, triggering a rain cycle.

Apai says the atmospheric dynamics on brown dwarfs are very different from those here on Earth.

“On our planet, we have only one species of cloud—water,” he explains, “But on this brown dwarf, there is such a wide temperature range that we have many different species of clouds.”

In three ongoing Spitzer programs, Apai and his co-investigators have successfully explored the properties of cloud covers in about 50 brown dwarfs.

“As different wavelengths probe different pressures and different rotational phases probe different latitudes we will be able to explore the two or even three-dimensional structure of the atmospheres,” Apai says.

Buenzli says that theorists are excited to model the new data: a new era of weather reporting has begun. By mapping cloud systems in brown dwarfs, these studies are opening a new field, extrasolar atmospheric dynamics, which bridges astronomy and planetary science.

“Brown dwarfs are fascinating and diverse,” says Apai. “Now we’ve got a new technique to chase their gigantic and violent storms.”

“Eventually, we want to probe the atmospheres of exoplanets in a similar fashion,” she says.

“Currently, we can’t get this type of data on exoplanets because their bright host stars blind our vision,” Apai says. “Brown dwarfs are the perfect laboratories for studying the exotic science of worlds beyond our own.”

Source: University of Arizona

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