  T-type dwarfs present a broad and shallow absorption feature centred around
6950 A in the blue wing of the K doublet at 0.77 micron which resembles in
depth and shape the satellite absorption predicted by detailed collisional
broadening profiles. In our previous work, the predicted line satellite
position was however somewhat too blue compared to the observed feature. In
this paper we investigate whether new calculations of the energy surfaces of
the potentials in the K-H_2 system, including spin-orbit coupling, result in a
closer coincidence of the satellite with the observed position. We also
investigate the extent to which CaH absorption bands contribute to the feature.
We present model atmospheres and synthetic spectra, including gravitational
settling for an improved description of depth-dependent abundances of
refractory elements, and based on new K-H_2 line profiles using improved
interaction potentials. By comparison with a high signal-to-noise optical
spectrum of the T1 dwarf epsilon Indi Ba, we find that these new models do
reproduce the observed feature, while CaH does not contribute for the
atmospheric parameters considered. We also find that CaH is settled out so deep
into the atmosphere that even turbulent vertical mixing would appear
insufficient to bring significant amounts of CaH to the photosphere in dwarfs
later than ~L5. We conclude that previous identification of the feature at this
location in T and late L dwarf spectra with CaH was erroneous, as expected on
physical grounds: calcium condenses onto grains in early L dwarfs and thus
should have settled out of the photosphere in cooler brown dwarfs. This finding
revokes one observational verification for the cloud-clearing theory: a gradual
clearing of the cloud cover in early T dwarfs.
