  Characterizing the physical conditions at disk scales in Class 0 sources is
crucial for constraining the protostellar accretion process and the initial
conditions for planet formation. We use ALMA 1.3 mm and 3 mm observations to
investigate the physical conditions of the dust around the Class 0 binary IRAS
16293-2422 A (sep <100 au) down to ~10 au scales. The circumbinary material's
spectral index, alpha, has a median of 3.1 and a dispersion of ~0.2, providing
no firm evidence of mm-sizes grains therein. Continuum substructures with
brightness temperature peaks of T_b~60-80 K at 1.3 mm are observed near the
disks at both wavelengths. These peaks do not overlap with strong variations of
alpha, indicating they trace high-temperature spots instead of regions with
significant optical depth variations. The lower limits to the inferred dust
temperature in the hot spots are 122, 87 and 49 K. Depending on the assumed
dust opacity index, these values can be several times higher. They overlap with
high gas temperatures and enhanced complex organic molecular (COM) emission.
This newly resolved dust temperature distribution is in better agreement with
the expectations from mechanical instead of the most commonly assumed radiative
heating. In particular, we find that the temperatures agree with shock heating
predictions. This evidence and recent studies highlighting accretion heating in
Class 0 disks suggest that mechanical heating (shocks, dissipation powered by
accretion, etc.) is important during the early stages and should be considered
when modeling and measuring properties of deeply embedded protostars and disks.
