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CNR: Alamanacco della Scienza

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N. 3 - 12 feb 2014
ISSN 2037-4801

International info   a cura di Cecilia Migali

Tecnologia

Surface map of a brown dwarf

Brown dwarfs are peculiar: they have a greater mass than planets, but are not massive enough to start a nuclear fusion process in their core. In March 2013, researchers announced the discovery of a system consisting of two brown dwarfs orbiting each other just 6.5 light years away. This presented astronomers with the opportunity to study this unusual object in greater detail than ever before. Now scientists have published two studies on these objects, named Luhman 16A and Luhman 16B after the astronomy professor who discovered them.

The first study, published in 'Nature' headed by Ian Crossfield from the Max Planck Institute for Astronomy in Heidelberg, presents something that previously never existed for brown dwarfs: a surface map of Luhman 16B, produced using a method known as Doppler imaging. This method uses the fact that light from a rotating star is shifted slightly in frequency as the star rotates. An approximate map of the stellar surface can then be reconstructed from these systematic shifts.

For their measurements, the scientists used data that they had collected in May 2013 using the Crires spectrograph; this instrument is installed on an eight-metre mirror of the Very Large Telescope at the Paranal Observatory, which is operated by the European Southern Observatory (Eso).

“Previous observations have inferred that brown dwarfs have mottled surfaces, but now we can start to directly map them”, says Crossfield. The researchers assume from this that they are seeing patchy cloud cover, similar to what they can see on Jupiter’s surface.

The maps obtained by Ian Crossfield and his colleagues correspond to satellite weather maps of our home planet. “In the future, we should be able to see cloud patterns form, evolve and dissipate on Luhman 16B”, says Crossfield. Exo-meteorologists may even be able to predict whether a visitor to Luhman 16B can expect clear or cloudy skies.

For us humans, however, the weather forecast for Luhman 16B may simply be “extremely unpleasant weather” at all times: at temperatures in excess of 1,000 degrees Celsius, clouds of gaseous iron and various minerals hover in an atmosphere of hydrogen.

The second study, illustrated in 'Astrophysical Journal Letters', headed by Beth Biller (currently at the University of Edinburgh, but at the Max Planck Institute for Astronomy when she conducted this research) literally goes to greater depths: as brighter and darker clouds are moved into view and removed again by a brown dwarf’s rotation, the overall brightness also changes. By simultaneously observing brightness variations at different wavelengths, Biller and her colleagues were able to reconstruct what happens in different layers of the atmosphere in both Luhman 16A and 16B.

The measurements were conducted simultaneously in seven different wavelength regimes. The amount of light emitted by the gas in each case depends directly on the temperature of the gas, and the seven wavelength regimes therefore correspond most probably to layers at different depths at different temperatures in the brown dwarf’s atmosphere. “We learned that the weather patterns on these brown dwarfs are quite complex,” says Biller. The cloud structure varies, depending on atmospheric depth. “We are definitely dealing with more than one layer of cloud”.

Per saperne di più: - www.mpg.de/7870755/surface-map-brown-dwarf