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We present Submillimeter Array observations of H2CO and N2H+ emission in thedisks around the T Tauri star TW Hya and the Herbig Ae star HD 163296 at 2"-6"resolution and discuss the distribution of these species with respect to COfreeze-out. The H2CO and N2H+ emission toward HD 163296 does not peak at thecontinuum emission center that marks the stellar position but is insteadsignificantly offset. Using a previously developed model for the physicalstructure of this disk, we show that the H2CO observations are reproduced ifH2CO is present predominantly in the cold outer disk regions. A model whereH2CO is present only beyond the CO snow line (estimated at a radius of 160 AU)matches the observations well. We also show that the average H2CO excitationtemperature, calculated from two transitions of H2CO observed in these twodisks and a larger sample of disks around T Tauri stars in the DISCS (the DiskImaging Survey of Chemistry with SMA) program, is consistent with the COfreeze-out temperature of 20 K. In addition, we show that N2H+ and H2CO linefluxes in disks are strongly correlated, indicative of co-formation of thesespecies across the sample. Taken together, these results imply that H2CO andN2H+ are generally present in disks only at low temperatures where CO depletesonto grains, consistent with fast destruction of N2H+ by gas-phase CO, and insitu formation of H2CO through hydrogenation of CO ice. In this scenario H2CO,CH3OH and N2H+ emission in disks should appear as rings with the inner edge atthe CO midplane snow line. This prediction can be tested directly usingobservations from ALMA with higher resolution and better sensitivity.

H2CO and N2H+ in Protoplanetary Disks: Evidence for a CO-ice Regulated Chemistry