Space Weather on Brown Dwarfs

Brown Dwarfs

Brown Dwarfs are objects somewhere between planets and stars. Brown Dwarfs have temperatures and densities comparable to gas planets (like Venus, Saturn and Jupiter) but are expected to form like stars, namely, from a cloud of interstellar matter which collapses under its own weight. New-born brown dwarfs are therefore discovered in typical star forming regions like the Trapezium cluster (right image) in the Orion nebula (left image).

Brown dwarfs are exceptionally interesting because they interlink the human expertise on stars and the one accumulated on planets. It challenges both branches of astrophysics and hence fosters a closer interaction of both. Its close relation to planets even suggests a synergy with meteorologists which are entirely devoted to Earth.

Observations have shown that Brown Dwarfs are variable which seems so be related to the formation of dust, comparable to the formation of rain in the Earth atmosphere. Brown Dwarfs have not yet been directly observed. The Hubble Space Telescope did observe Venus (left panel), a giant gas planet in our solar system which guides scientists imaginations for the somewhat hotter Brown Dwarfs. A closer look at Saturn (right panel) shows huge storms shaping clouds and turbulence.

Computer simulations of Brown Dwarf's weather

I have performed computer simulations for a Brown Dwarf atmosphere in which dust (small solid particles) can form. The dust formation is initiated by eddies which travel through the atmospheric gas as result of the convective energy transport. This is very much alike a pot of boiling water, except that boiling water usually does not form small solid particles.

A sensitive feedback loop sets in as soon as the first dust particles are formed because of the strong capacity of the dust to absorb the stellar light. The much they absorb, the much they have to re-radiate because of energy conservation. The result is an efficient cooling mechanism which supports the formation of even more dust. Such mechanisms, which support the cause of their existence, are called an instability . Instabilities are the very nature of evolution in every branch of science.

Results: The travelling eddies (synonymous for turbulence ) initiate dust formation in a non-predictable way. The initially small-scale structure (left picture) evolve into larger and larger pattern (right picture), comparable to clouds on Earth. No dust would form without turbulence.

What relation has a Brown Dwarf to a car engine?

The basic of every science is the cosmological principle which states, that the physical laws apply everywhere in the universe.

The relation between a Brown Dwarf and a car engine is simply, that in both the same physical laws act. The motion of the dust(soot)/gas mixtures is in both cases described by the same equations, the Navier-Stokes equations.