Cool Stars 14 - Submitted Abstract # 37
This version created on 05 October 2006

A New Mechanism for the L to T Transition


Mark Marley, NASA Ames Research Center
Andrew Ackerman, NASA Goddard Institute for Space Studies

A number of mechanisms have been suggested to explain the L to T type
transition, including sudden cloud thinning, cloud patchiness, and a
rapid increase in the cloud sedimentation efficiency.  The transition
has been difficult to understand because cloud opacity rapidly
diminshes over a very small range of effective temperatures near 1400
K.  Previous dynamical suggestions (e.g., sudden cloud deck thinning
or appearance of holes) have been less than fulfilling because they
lacked mechanisms to fully account for the observed characteristics
sudden transition.  We have been investigating a new mechanism that
naturally explains both the effective temperature range at which the
transition appears and the rapidity of the removal of cloud opacity.
To test this mechanism we have developed a new version of our cloud
sedimation model that, for the first time, couples the abundance of
condensate at each atmospheric layer to all other layers and accounts
for dynamical mixing and sedimentation throughout the atmosphere.  We
will discuss the new mechanism, report on model results, and propose
observational tests.

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