  A blue depression is found in the spectra of M dwarfs from 4000 to 4500A.
This depression shows an increase toward lower temperatures though is
particularly sensitive to gravity and metallicity. It is the single most
sensitive feature in the optical spectra of M dwarfs. The depression appears as
centered on the neutral calcium resonance line at 4227A and leads to nearby
features being weaker by about two orders of magnitude than predicted. We
consider a variety of possible causes for the depression including temperature,
gravity, metallicity, dust, damping constants, and atmospheric stratification.
We also consider relevant molecular opacities which might be the cause
identifying AlH, SiH, and NaH in the spectral region. However, none of these
solutions are satisfactory. In the absence of a more accurate determination of
the broadening of the calcium line perturbed by molecular hydrogen, we find a
promising empirical fit using a modified Lorentzian line profile for the
calcium resonance line. Such fits provide a simplistic line-broadening
description for this calcium resonance line and potentially other un-modelled
resonance lines in cool high-pressure atmospheres. Thus we claim the most
plausible cause of the blue depression in the optical spectra of M dwarfs is a
lack of appropriate treatment of line broadening for atomic calcium. The broad
wings of the calcium resonance line develop at temperatures below about 4000K
and are analogous to the neutral sodium and potassium features which dominate
the red optical spectra of L dwarfs.
