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Dust determines the temperature structure of protoplanetary disks. However,dust temperature determinations almost invariably rely on a complex modeling ofthe Spectral Energy Distribution. We attempt a direct determination of thetemperature of large grains emitting at mm wavelengths.} We observe the edge-ondust disk of the Flying Saucer, which appears in silhouette against the COJ=2-1 emission from a background molecular cloud in $\rho$ Oph. The combinationof velocity gradients due to the Keplerian rotation of the disk and intensityvariations in the CO background as a function of velocity allows us to directlymeasure the %absorbing dust temperature. The dust opacity can then be derivedfrom the emitted continuum radiation. The dust disk absorbs the radiation fromthe CO clouds at several velocities. We derive very low dust temperatures, 5 to7 K at radii around 100 au, which is much lower than most model predictions.The dust optical depth is $> 0.2$ at 230 GHz, and the scale height at 100 au isat least 8 au (best fit 13 au). However, the dust disk is very flat (flaringindex -0.35), which is indicative of dust settling in the outer parts.

The shadow of the Flying Saucer: A very low temperature for large dust grains