The young protostellar disk in IRAS16293-2422 B is hot and shows
  signatures of gravitational instability
  Deeply embedded protostars are actively fed from their surrounding envelopes
through their protostellar disk. The physical structure of such early disks
might be different from that of more evolved sources due to the active
accretion. We present 1.3 and 3\,mm ALMA continuum observations at resolutions
of 6.5\,au and 12\,au respectively, towards the Class 0 source IRAS 16293-2422
B. The resolved brightness temperatures appear remarkably high, with $T_{\rm b}
>$ 100\,K within $\sim$30\,au and $T_{\rm b}$ peak over 400\,K at 3\,mm. Both
wavelengths show a lopsided emission with a spectral index reaching values less
than 2 in the central $\sim$ 20\,au region. We compare these observations with
a series of radiative transfer calculations and synthetic observations of
magnetohydrodynamic and radiation hydrodynamic protostellar disk models formed
after the collapse of a dense core. Based on our results, we argue that the gas
kinematics within the disk may play a more significant role in heating the disk
than the protostellar radiation. In particular, our radiation hydrodynamic
simulation of disk formation, including heating sources associated with
gravitational instabilities, is able to generate the temperatures necessary to
explain the high fluxes observed in IRAS 16293B. Besides, the low spectral
index values are naturally reproduced by the high optical depth and high inner
temperatures of the protostellar disk models. The high temperatures in IRAS
16293B imply that volatile species are mostly in the gas phase, suggesting that
a self-gravitating disk could be at the origin of a hot corino.
