(Proposal ID) S10B-046

(PI) Lucas, Philip

(Proposal Title)
Discovering the coolest brown dwarfs and measuring the substellar mass function

(Abstract)
Since 1995, two new spectral types, L and T, have been established to describe 
brown dwarfs and the sample of methane dwarfs (T dwarfs) has steadily grown. 
Recent discoveries from UKIDSS now include brown dwarfs up to 300 K cooler 
than the coolest (700 K) T dwarfs previously known. Most of these have been 
classified as types T8.5 to T10, although one recent very strange, metal poor 
discovery through this programme has type T7.5p and is thought to be as cool 
or cooler than the T9 dwarfs. The new T10 discovery has an estimated 
temperature of 360-500 K, although Subaru/IRCS 3-5 mu m data will be required 
to measure this more accurately. The temperature range below 700 K is 
interesting in itself because of the new atmospheric physics. It also provides 
the essential data needed to determine the substellar mass function in the 
local field by breaking the age-mass degeneracy in brown dwarfs. This requires 
us to build a statistically useful sample of the coolest brown dwarfs. We 
propose to observe the best candidate late T and Y dwarfs from UKIDSS DR8, 
prior to the official data release, to detect the predicted ammonia absorption 
features and additional features. This is a continuation of past successful 
proposals, which have resulted in the publication of 79 mid-late T dwarfs.