  Although stars accrete mass throughout the first few Myr of their lives, the
physical mechanism that drives disk accretion in the T Tauri phase is
uncertain, and diagnostics that probe the nature of disk accretion have been
elusive, particularly in the planet formation region of the disk. Here we
explore whether an accretion process such as the magnetorotational instability
could be detected through its "heat signature", the energy it deposits in the
disk atmosphere. To examine this possibility, we investigate the impact of
accretion-related mechanical heating and energetic stellar irradiation (FUV and
X-rays) on the thermal-chemical properties of disk atmospheres at planet
formation distances. We find that stellar FUV irradiation (Lyman alpha and
continuum), through its role in heating and photodissociation, affects much of
the upper warm (400-2000 K) molecular layer of the atmosphere, and the
properties of the layer are generally in good agreement with the observed
molecular emission features of disks at UV, near-infrared, and mid-infrared
wavelengths. At the same time, the effect of FUV irradiation is restricted to
the upper molecular layer of the disk, even when irradiation by Ly alpha is
included. The region immediately below the FUV-heated layer is potentially
dominated by accretion-related mechanical heating. As cooler (90-400 K) CO,
water, and other molecules are potential diagnostics of the mechanically-heated
layer, emission line studies of these diagnostics might be used to search for
evidence of the magnetorotational instability in action.
