Temperature profiles of young disk-like structures: The case of IRAS
  16293A
  Temperature is a crucial parameter in circumstellar disk evolution and planet
formation, because it governs the resistance of the gas to gravitational
instability and sets the chemical composition of the planet-forming material.
We set out to determine the gas temperature of the young disk-like structure
around the Class 0 protostar IRAS 16293$-$2422A. We use Atacama Large
Millimeter/submillimeter Array (ALMA) observations of multiple H$_2$CS $J=7-6$
and $J=10-9$ lines from the Protostellar Interferometric Line Survey (PILS) to
create a temperature map for the inner $\sim$200 AU of the disk-like structure.
This molecule is a particularly useful temperature probe because transitions
between energy levels with different $K_{\rm{a}}$ quantum numbers operate only
through collisions. Based on the H$_2$CS line ratios, the temperature is
between $\sim$100$-$175 K in the inner $\sim$150 AU, and drops to $\sim$75 K at
$\sim$200 AU. At the current resolution (0.5$^{\prime\prime} \sim$70 AU), no
jump is seen in the temperature at the disk-envelope interface. The temperature
structure derived from H$_2$CS is consistent with envelope temperature profiles
that constrain the temperature from 1000 AU scales down to $\sim$100 AU, but
does not follow the temperature rise seen in these profiles at smaller radii.
Higher angular resolution observations of optically thin temperature tracers
are needed to establish whether cooling by gas-phase water, the presence of a
putative disk or the dust optical depth influences the gas temperature at
$\lesssim$100 AU scales. The temperature at 100 AU is higher in IRAS 16293A
than in the embedded Class 0/I disk L1527, consistent with the higher
luminosity of the former.
