Cool Herschel/Hot Spitzer: The distribution of water in protoplanetary disks

One of the key mission objectives of Herschel is to observe water in various environments, but in protoplanetary disks in particular. Water has an immediate r^ance to the formation of planets and to our own origin. We propose to obtain deep PACS/SPIRE line spectra of water, OH and CO in protoplanetary disks already known to have strong rotational water vapor emission at midinfrared (1036 micron) wavelengths, as detected by Spitzer (the newly discovered midinfrared molecular forest). The midinfrared lines trace warm to hot gas formed within a few AU from the central star. The proposed observations will relate the oxygen chemistry and transport of water in the inner few AU with that of the cooler outer disk 1100 AU as traced by the PACS water lines. The line ratios of water in the Spitzer spectral range suggest that water vapor may be strongly depleted in the disk surface beyond 1 AU although gas and dust temperatures are high enough to maintain abundant water out to at least 10 AU. It was suggested by Meijerink et al. 2009 that this depletion of water could be due to a ..cold finger effect., in which the surface water vapor is carried downwards into colder regions of the disk where it freezes out and is bound in the disk midplane as part of the general dust growth and settling. If so, the surface abundance structure of water is a direct tracer of the midplane snowline. Based on model fits to the midinfrared lines, we predict very strong differences in the 50200 micron water lines, depending on the distribution of water in the outer disk surface. Our program is unique in that 1) we specifically target disks known to have strong water emission from the inner disk and 2) we go significantly deeper than others ensuring that we can constrain the abundance of water vapor down to 10^10 per hydrogen in the outer disk, thus putting very strong constraints of the efficacy of the proposed cold finger effect. truncated!, Please [truncated!, Please see actual data for full text]