The Chemistry of Multiply Deuterated Molecules in Protoplanetary Disks. I. The Outer Disk

Abstract

We present new models of the deuterium chemistry in protoplanetary disks,\nincluding, for the first time, multiply deuterated species. We use these models\nto explore whether observations in combination with models can give us clues as\nto which desorption processes occur in disks. We find, in common with other\nauthors, that photodesorption can allow strongly bound molecules such as HDO to\nexist in the gas phase in a layer above the midplane. Models including this\nprocess give the best agreement with the observations. In the midplane, cosmic\nray heating can desorb weakly bound molecules such as CO and N$_2$. We find the\nobservations suggest that N$_2$ is gaseous in this region, but that CO must be\nretained on the grains to account for the observed DCO$^+$/HCO$^+$. This could\nbe achieved by CO having a higher binding energy than N$_2$ (as may be the case\nwhen these molecules are accreted onto water ice) or by a smaller cosmic ray\ndesorption rate for CO than assumed here, as suggested by recent theoretical\nwork.\n For gaseous molecules the calculated deuteration can be greatly changed by\nchemical processing in the disk from the input molecular cloud values. On the\ngrains singly deuterated species tend to retain the D/H ratio set in the\nmolecular cloud, whereas multiply deuterated species are more affected by the\ndisk chemistry. Consequently the D/H ratios observed in comets may be partly\nset in the parent cloud and partly in the disk, depending on the molecule.\n