Deuterium in protoplanetary disks

Abstract

The deuterium to hydrogen ratio is a sensitive tracer of the conditions of the star formation process and observations of deuterium in disks can be used to constrain the location and timescales of comet formation. In this study, we searched for the deuterated species DCN and HDO toward the T Tauri protostar LkCa 15 and the Herbig Ae stars HD 163296 and MWC 480 using the Owens Valley Radio Observatory Millimeter Array. HDO was detected toward LkCa 15 and upper limits for the HDO in MWC 480 and DCN in LkCa 15 and HD 163296 were obtained. The DCN to HCN fraction in LkCa 15 was calculated from the resulting upper limit for the DCN abundance and previously observed HCN and H^(13)CN abundances toward the disk. HDO/H_2O was estimated through comparison with predicted H_2O abundances from chemical models, scaled by the HCN (modeled) /HCN (observed) abundances. The resulting deuterium fractionations in LkCa 15 were found to be similar to those found toward the T Tauri star disk TW Hya and those observed for molecular clouds and hot cores. The structure of emission toward LkCa 15 was modeled and is discussed.