Edge-On Disk Study (EODS): I. Thermal structure of the Flying Saucer disk

Context. The dust and gas temperature in protoplanetary disks play critical roles in determining their chemical evolution and influencing planet formation processes. Aims. We attempted an accurate measurement of the dust and CO temperature profile in the edge-on disk of the Flying Saucer. Methods. We used the unique properties of the Flying Saucer-its edge-on geometry and its fortunate position in front of CO clouds with different brightness temperatures -to provide independent constraints on the dust temperature. We compared it with the dust temperature derived using the radiative transfer code DISKFIT and the CO gas temperature. Results. We find clear evidence of a substantial gas temperature vertical gradient, with a cold (10 K) disk mid-plane and a warmer CO layer where T(r)≈27 (r/100 au)-0.3 K. Direct evidence of CO depletion in the mid-plane, below about 1 scale height, is also found. At this height, the gas temperature is 15-20 K, consistent with the expected CO freeze-out temperature. The dust disk appears optically thin at 345 GHz, and exhibits moderate settling.