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Knowledge of the midplane temperature of protoplanetary disks is one of thekey ingredients in theories of dust growth and planet formation. However,direct measurement of this quantity is complicated, and often depends on thefitting of complex models to the data. In this paper we demonstrate a method todirectly measure the midplane gas temperature from an optically thick molecularline, if the disk is moderately inclined. The only model assumption that entersis that the line is very optically thick, also in the midplane region where wewish to measure the temperature. Freeze-out of the molecule onto dust grainscould thwart this. However, in regions that are expected to be warm enough toavoid freeze-out, this method should work. We apply the method to the CO 2-1line channel maps of the disk around HD 163296. We find that the midplanetemperature between 100 and 400 au drops only mildly from 25 K down to 18 K.While we see no direct evidence of the midplane being optically thin due tostrong CO depletion by freeze-out, we cannot rule it out either. The fact thatthe inferred temperatures are close to the expected CO freeze-out temperaturecould be an indication of this. Incidently, for the disk around HD 163296 wealso find dynamic evidence for a rather abrupt outer edge of the disk,suggestive of outside-in photoevaporation or truncation by an unseen companion.

Midplane temperature and outer edge of the protoplanetary disk around HD 163296