THE DUSTY OR NOT-SO-DUSTY ATMOSPHERES
OF BROWN DWARFS
During the last few years, astronomers started to realize the importance
of condensation inside the atmospheres of very cool stars and brown dwarfs.
This process, comparable to cloud formation, changes significantly the appearance of such cool objects. It also has effects similar to the green house
effect on the inside of brown dwarfs. However, clouds and other condensates
have the capability to rain out and the condensates can settle to the bottom
of the atmospheres. There have been hints that the rather warm objects do not rain out as much as the cooler objects. This work proofs these hints and
establishes models and parameters that describe these conditions.
An international team of astronomers used the most powerful telescopes and the most powerful supercomputers to accomplish this task. They
analyzed a sample of so called L dwarfs. These L dwarfs are a class of very cool
objects that consists mostly of brown dwarfs. Only the coolest stars fall into
this class. Most of the L dwarfs are cooler than the coolest stars and cannot
reach the conditions which would qualify them as stars and hence are brown
dwarfs.
The scientists used several spectrographs of the 10-m (400-inch) Keck I telescope on Mauna Kea, Hawaii to perform a spectral analysis of the light
of the L dwarfs. They analyzed the light of the L dwarfs in the optical and
the infrared wavelength range. The spectrographs were operated in both low
and high resolution modes.
The analyses are backed by the most sophisticated computer models to date. To calculate these models, the astrophysicists deployed parallel
supercomputers starting with the local parallel computers of the University
of Georgia all the way to the parallel supercomputers of the San Diego Supercomputer Center (SDSC) and the National Energy Research Scientific
Computing Center (NERSC) of the Lawrence Berkeley National Laboratory of the Department of Energy. This includes the world's 3rd fastest computer
"Seaborg" at NERSC.
The models simulate the effects of two limiting extremes: dust distributed throughout the atmospheres and dust which was rained out of atmospheres
completely. By comparison with L dwarfs of different temperatures the astronomers concluded that the atmospheres of warmer ones have a high dust
content whereas the atmospheres of the cooler ones have rained out their dust content to the bottom of the atmosphere. Future models which are
currently being developed will simulate also partial rain out of the dust, since
the intermediate L dwarfs cannot be simulated with any current model.
The models predict surface temperatures of L dwarfs between 1100O C (2000O F) and 1800O C (3300O F) - as comparison, the sun has a surface
temperature of 5500O C (9900O F). The interesting result is that the lower
temperature is as low as the temperatures which are expected for extrasolar
giant gas planets and, therefore, closing the scale from stars to planets.
For more information contact :
Dr. Andreas Schweitzer
Dept. of Physics & Astronomy and Center for Simulational Physics
University of Georgia
Athens, GA 30602-2451
Phone 706-583-8227
Fax 706-542-2492
e-mail: andy@physast.uga.edu