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Description
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 used 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 ~200AU of the disk-like structure. This molecule is a particularly useful temperature probe because transitions between energy levels with different K_a quantum numbers operate only through collisions. Based on the H_2_CS line ratios, the temperature is between ~100-175K in the inner ~150AU, and drops to ~75K at ~200AU. At the current resolution (0.5"~70AU), 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 1000AU scales down to ~100AU, 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 <~100AU scales. The temperature at 100AU is higher in IRAS 16293A than in the embedded Class 0/I disk L1527, consistent with the higher luminosity of the former.
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