Recently , studies of brown dwarfs have demonstrated that they possess strong magnetic fields and have the potential to produce radio and optical auroral emissions powered by magnetospheric currents .
This emission provides the only window on magnetic fields in the coolest brown dwarfs and identifying additional benchmark objects is key to constraining dynamo theory in this regime .
To this end , we conducted a new red optical ( 6300 - 9700 Å ) survey with the Keck telescopes looking for H \alpha emission from a sample of late L dwarfs and T dwarfs .
Our survey gathered optical spectra for 29 targets , 18 of which did not have previous optical spectra in the literature , greatly expanding the number of moderate resolution ( R \sim 2000 ) spectra available at these spectral types .
Combining our sample with previous surveys , we confirm an H \alpha detection rate of 9.2 \pm ^ { 3.5 } _ { 2.1 } % for L and T dwarfs in the optical spectral range of L4 - T8 .
This detection rate is consistent with the recently measured detection rate for auroral radio emission from ( ) , suggesting that geometrical selection effects due to the beaming of the radio emission are small or absent .
We also provide the first detection of H \alpha emission from 2MASS 0036+1821 , previously notable as the only electron cyclotron maser radio source without a confirmed detection of H \alpha emission .
Finally , we also establish optical standards for spectral types T3 and T4 , filling in the previous gap between T2 and T5 .