Probing the atmospheres of ultra cool dwarfs with optical and infrared variability

Dr. Coryn Bailer-Jones (Max-Planck-Institut für Astronomie, Heidelberg)

Donnerstag, den 1.3.2001 um 10 Uhr c.t. im Raum PN-114

Abstract:

Large area infrared sky surveys have discovered a large population of faint, cool, compact objects in the solar neighbourhood which appear to be a low temperature continuation of the M dwarfs. The optical and infrared spectra of these L dwarfs -- as they are now known -- reveal highly broadened neutral alkali lines and volatile molecules, indicating surface temperatures below 2000K. At these low temperatures, dust is expected to form, and this is verified by recent atmospheric models. L dwarfs can be low mass stars, brown dwarfs or even giant planets, depending on their age. Methane is seen to form in cooler L dwarfs, indicating a natural transition of the L dwarf sequence into the even cooler T dwarfs.

I will present results of a program to monitor variability in a set of late M and L dwarfs. These data provide evidence for variability on timescales of a few hours in a number of objects. In several cases this variability does not appear to be due to a simple rotational modulation of the light curve, indicating short-term instability of surface features. Potential causes of this will be discussed, including the formation of dust clouds. Although this is a plausible scenario, very little is known -- or even predicted -- about the dust dynamics in these cool photospheres. Variability monitoring potentially opens up an observational means for testing dust models and studying weather systems in cool, exo-solar bodies.