Heat, Dust, and Water in Protostellar Accretion Disks
November 05, 2009
Abstract: Requirements for magnetic coupling and accretion in the active layer of a protostellar disk are re-examined, and some implications for molecular emission from the layer are derived. Dust, especially small dust, is a powerful poison for the electrical conductivity of these disks, and hence for magnetic coupling. On the other hand, if all of the dust mass were in very large grains, then the opacity would be too low to explain infrared observations. Even for optimally sized grains, we find that the actively accreting layer is marginally optically thin to its own thermal radiation, so that narrow, highly saturated emission lines of water and other molecular species would be expected if accretion is driven by turbulence (rather than magnetized winds, for example). Such lines have recently been observed, indicating the presence of hot (1000 K?) water at 1 AU in a few rapidly accreting disks. These lines probably form above the main part of the active layer. Nevertheless, infrared molecular-line observations may be a powerful diagnostic of protostellar active layers in the planet-forming zone.