Astronomers believe brown dwarfs may have weather patterns

17 Jun 2002

According to an astronomy team that provided a presentationat ScienceNOW! in Melbourne, Australia, evidence has been foundto show that failed stars known as brown dwarfs may haveweather patterns including wind, clouds, and storms.

The observation was made by Dr. Chris Tinney of the Anglo-AustralianObservatory and Andrew Tolley, who is earning a degreeat the University of Oxford. They utilized the Anglo-AustralianTelescope in New South Wales, Australia's largest optical telescope.

The astronomers observed the brown dwarf LP944-20 anddetailed the changes in the brown dwarf's surface chemistry as itrotated. Brown dwarfs are objects formed in the same way as starsand from the same material, but which do not have the criticalmass required to burn nuclear fuel, which is about seven percentof a solar mass or approximately 70 times the mass of the planetJupiter. It has been theorized that dust exists in some coolbrown dwarfs and it has recently been discovered that browndwarfs rotate rapidly with periods of hours. Those premisesindicate the likelihood of brown dwarfs showing weather patterns,as is the case in large planets in this solar system.

The dominant source of spectral features in cool stars andwarm brown dwarfs is titanium oxide (TiO). However, attemperatures below approximately 2000 degrees Celsius a range ofmaterials can condense out of a brown dwarf's atmosphere, severalof which contain titanium such as Ti₂O₃, Ti₄O₇, and CaTiO₃. Thatreaction "soaks up" the titanium which would otherwise exist attitanium oxide, which weakens the spectral features of thetitanium oxide. Ti₂O₃ and Ti₄O₇ exist on this planet only asman-made chemicals, but CaTiO₃ is a naturally occurring mineralcalled perovskite. In brown dwarf atmospheres CaTiO₃ forms asmicrometer-sized smoke particles rather than as rocks.

Stars are hot enough that their surfaces are a smooth mix ofvaporized material. Because brown dwarfs tend to smolder attemperatures below 2000 degrees, the outer layers are cool enoughfor chemicals to be emitted as smoke-like particles.

Tinney and Tolley utilized the Taurus Tuneable Filter, aninstrument developed by the Anglo-Australian Observatory, tofocus on a very narrow wavelength band and measure tinyfluctuations. The narrow band matched the titanium oxide tracermolecule, which was strong enough for the astronomers to trackthe formation of cloud particles.

Having mastered the technique, Tinney and Tolley willobserve additional brown dwarf stars in the upcoming months.