lunes, 15 de junio de 2009
Brown Dwarf Weather Report: Cloudy
Astronomers have made an interplanetary weather report, spotting brightness alterations in a brown dwarf's atmosphere. The brown dwarf in question is SIMP 0136, the brightest "T dwarf" in the northern hemisphere.
Although SIMP 0136 is classed as a 'failed star', have astronomers uncovered a phenomenon more commonly found in planetary atmospheres?
Etienne Artigau and his team at the Gemini South Observatory carried out an observing campaign for five days and realized that not only was SIMP 0136 varying in brightness, its atmosphere was changing. On each day, dark and light patches moved across its surface, indicating the motion of dust clouds.
Brown dwarfs are often considered to be 'failed stars' as they are too lightweight to allow nuclear containment in the core, facilitating fusion. The fusing of nuclei in the cores of stars generate energy and outward pressure, balancing out the inward pull of gravity. Brown dwarfs never enjoyed fusion (or if they did, it didn't last very long), so the cores of these sub-stellar bodies become 'electron degenerate' before any fusion is possible. This electron degeneracy counters the inward gravitational pressure, thereby maintaining the brown dwarfs shape.
So, they're not stars, but they can't really be called planets either. They have convective interiors (like our sun's convection zone) and lack any chemical differentiation. You can see why they can't be classed as a "star" (even though they are still ranked by stellar classifications), but they can't be classed as planets; they are in a no-mans land, too big to be a planet, too small to be a star.
That said, they certainly have more star-like properties than planet-like properties, hence the almost derogatory term 'failed star', a term I took great displeasure with in a recent light-hearted Astroengine article:
Why is the term “failed star” synonymous with brown dwarfs? On the one hand, brown dwarfs lack the mass to sustain nuclear fusion in their cores. On the other hand, who said brown dwarfs were trying to be stars? Who ever said that becoming a star was the pinnacle of stellar living? Perhaps brown dwarfs are perfectly happy the way they are. In a world of equality and political correctness, brown dwarfs could be viewed as “over-achieving Jupiters”, or gas supergiants. --Astroengine.com
Going back to Artigau's study of SIMP 0136, is this brown dwarf exhibiting a planet-like characteristic? This object is cool enough for solid grains to form in its atmosphere, so these brightness changes could indicate changes in atmospheric conditions. Is this a brown dwarf weather system?
Calling it "weather" is probably a bit strong, but it's certainly something to consider before calling brown dwarfs a failure in the stellar department.