Deep HIFI searches for cold water vapor in protoplanetary disks

Water vapor inside planetforming disks is expected to show largevariations. In the warm (virgul200 K) inner few AU of the disk all oxygenis locked up in water vapor. In the colder outer region water quicklyfreezes out onto dust grains. However, efficient photodesorption bystellar ultraviolet radiation will return an appreciable fraction backinto the gas phase in the upper disk layers. This coupled waterice/vapor system explains the Spitzer observations of warm water inseveral disks, and predicts the presence of cold water vapor in theouter disk.Through the .Water in Star Forming Regions. (WISH) Key Program, wehave recently clearly detected the groundstate line of cold watervapor to TW Hya and very tentatively detected it to DM Tau. Both linesare factors 1050 weaker than expected. We hypothesize that up to 99%of icy grains have settled to the disk.s midplane thus .freeze drying.the upper disk layers.The WISH data do not go deep enough to probe cold water vapor contentnow that we know that the outer disk is freezedried. The only cleardetection is TW Hya, which has a distance of only 51 pc. At moretypical distances of 100140 pc, line strengths are lower by factors58, beyond the sensitivity of WISH.We propose much deeper HIFI observations of the H2O 110101 line at557 GHz of four targets: DM Tau (to confirm or reject WISH.s .verytentative. detection), and HD100546, HD163296, and AA Tau. These disksare, after TW Hya, the closest and largest disks that offer the bestchances of detection. Because of the sensitivity of H2O 110101 tocold water vapor in the outer disk, our observations probe asofyetunexplored grain settling in the outer disk and enrichment of themidplane with icy grains. This can boost planet formation, since icygrains coagulate more easily.Our proposal also contains a small request for velocityresolved CO109 observations with HIFI to study warm (virgul200 K) gas in these fourdisks, and investigate i truncated! [truncated!, Please see actual data for full text]