Surface map of a brown dwarf

Astronomers study exotic bodies outside the atmosphere

January 29, 2014

Brown dwarfs are failed stars which did not have enough mass to ignite nuclear fusion at their core. Researchers from the Max Planck Institute for Astronomy are among the astronomers who have now released the first surface map of such a celestial body. They have also published measurements which record the atmosphere at different depths. The results herald the beginning of a new era of research on brown dwarfs in which astronomers will be able to compare models for cloud formation on these bodies – and ultimately on huge gas planets – based on observations.

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, 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.

The analogy used to describe how this works pictures an observer hovering high above the equator and watching how the earth below is rotating. An object that is just coming into view will initially move towards the observer with great speed; as it passes directly beneath the person, its distance from him or her will hardly change at all; if the object finally rotates out of view across the horizon, its distance from the observer will change much faster.

The movement towards or away from the observer can be indirectly proven by the Doppler effect: light changes its wavelength systematically depending on whether, and how fast, the source of the light is moving towards or away from the observer; the extent of the change depends on how fast it is moving (and in which of the two directions). For brighter spots on the surface of a rotating star, a pattern of overlapping wavelength movements emerges.

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, 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.”

Biller and her colleagues performed the brightness measurements in April 2013 using the GROND astronomical camera on the 2.2-metre telescope at the ESO’s La Silla Observatory. GROND was built by the High Energy Group at the Max Planck Institute for Extraterrestrial Physics in Garching in collaboration with the Tautenburg State Observatory and the ESO.

The latest findings may herald the start of a new phase in the research on brown dwarfs, in which theorists formulate models for the cloud structure of this celestial body – and test these models by comparing them with detailed observations.

“Our observations are just the start,” says Beth Biller. Researchers should be able to use the next generation of telescopes, in particular the European Extremely Large Telescope whose mirror diameter measures 39 metres, to produce surface maps of even more distant brown dwarfs – and eventually a surface map for the young giant planets of other stars.


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