Saturday, February 1, 2014

Raining Molten Iron on Luhman 16B

Brown dwarfs are substellar objects that are more massive than planets, but not massive enough to sustain hydrogen fusion and shine as full-fledged stars. These objects start out hot, and cool gradually as they age. When cooled below a temperature of ~2300 K, it is believed that silicate minerals and molten iron begin to condense to form patchy cloud systems in the atmosphere. At cooler temperatures of below ~1300 K, these clouds disappear, probably sinking into the warmer and unobservable deeper layers of the atmosphere.

Figure 1: Artist’s impression of weather on a brown dwarf. Credit: NASA/JPL-Caltech/T. Pyle (IPAC).

Using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, I. Crossfield et al. (2014) have created the first ever global weather map of a brown dwarf named Luhman 16B. This object is a member of a pair of brown dwarfs known together as Luhman 16AB. As the name suggests, Luhman 16AB was discovered by Kevin Luhman, an astronomer at Pennslyvania State University in March 2013 using data from NASA’s Wide-field Infrared Survey Explorer (WISE). At a distance of just 6.5 light-years away, Luhman 16AB are not only the two closest known brown dwarfs, they are also the third nearest system - only the Alpha Centauri system and Barnard’s star are closer.

Brown dwarfs are notoriously difficult to study due to their faintness and relatively small size. However, the close proximity of Luhman 16AB puts them within easy reach of VLT’s gaze. Although both brown dwarfs were observed in the same fashion, only Luhman 16B exhibits strong temporal variability of its thermal radiation. The observed variability is consistant with Luhman 16B’s rotation period of 4.9 hours. As the brown dwarf rotates, brighter and darker areas of its surface come in and out of view to produce the observed variability. The brighter regions are believed to represent upper cloud layers that obscure the deeper and hotter parts of the atmosphere. In contrast, the brighter regions are believed to be gaps in the upper cloud layers that allow the deeper and hotter layers of the atmosphere to be seen.

Figure 2: Surface map of brown dwarf Luhman 16B. The lightest and darkest regions shown correspond to brightness variations of roughly 10%. Credit: ESO/I. Crossfield.

Luhman 16B has an estimated temperature of ~1400 K, extremely inclement by any standard. The global cloud map of Luhman 16B hints at the complexity of weather patterns on brown dwarfs. Clouds on Luhman 16B are probably comprised of iron and silicate minerals in a largely hydrogen-helium atmosphere. Here, iron can precipitate from the clouds in showers comprising droplets of molten iron. Weather on Luhman 16B can truly be exotic.

“Previous observations have inferred that brown dwarfs have mottled surfaces, but now we can start to directly map them,” said Ian Crossfield of the Max Planck Institute for Astronomy, lead author of the study. “What we see is presumably patchy cloud cover, somewhat like we see on Jupiter. In the future, we will be able to watch cloud patterns form, evolve and dissipate - eventually, maybe exo-meteorologists will be able to predict whether a visitor to Luhman 16B can expect clear or cloudy skies,” added Crossfield.

I. Crossfield et al., “A global cloud map of the nearest known brown dwarf”, Nature 505, 654-656 (30 January 2014)