H2CO and N2H+ in Protoplanetary Disks: Evidence for a CO-ice Regulated
  Chemistry
  We present Submillimeter Array observations of H2CO and N2H+ emission in the
disks 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 CO
freeze-out. The H2CO and N2H+ emission toward HD 163296 does not peak at the
continuum emission center that marks the stellar position but is instead
significantly offset. Using a previously developed model for the physical
structure of this disk, we show that the H2CO observations are reproduced if
H2CO is present predominantly in the cold outer disk regions. A model where
H2CO 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 excitation
temperature, calculated from two transitions of H2CO observed in these two
disks and a larger sample of disks around T Tauri stars in the DISCS (the Disk
Imaging Survey of Chemistry with SMA) program, is consistent with the CO
freeze-out temperature of 20 K. In addition, we show that N2H+ and H2CO line
fluxes in disks are strongly correlated, indicative of co-formation of these
species across the sample. Taken together, these results imply that H2CO and
N2H+ are generally present in disks only at low temperatures where CO depletes
onto grains, consistent with fast destruction of N2H+ by gas-phase CO, and in
situ 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 at
the CO midplane snow line. This prediction can be tested directly using
observations from ALMA with higher resolution and better sensitivity.
