2017A&A...602L...3Z

Context. The formation of deuterated molecules is favoured at low temperatures and high densities. Therefore, the deuteration fraction (D_frac) is expected to be enhanced in cold, dense prestellar cores and to decrease after protostellar birth. Previous studies have shown that the deuterated forms of species such as N₂H⁺ (formed in the gas phase) and CH₃OH (formed on grain surfaces) can be used as evolutionary indicators and to constrain their dominant formation processes and timescales.
Aims. Formaldehyde (H₂CO) and its deuterated forms can be produced both in the gas phase and on grain surfaces. However, the relative importance of these two chemical pathways is unclear. Comparison of the deuteration fraction of H₂CO with respect to that of N₂H⁺, NH₃, and CH₃OH can help us to understand its formation processes and timescales.
Methods. With the new SEPIA Band 5 receiver on APEX, we have observed the J=3-2 rotational lines of HDCO and D₂CO at 193GHz and 175GHz toward three massive star-forming regions hosting objects at different evolutionary stages: two high-mass starless cores (HMSC), two high-mass protostellar objects (HMPOs), and one ultracompact HII region (UC HII). By using previously obtained H₂CO J=3-2 data, the deuteration fractions HDCO/H₂CO and D₂CO/HDCO are estimated.
Results. Our observations show that singly deuterated H₂CO is detected toward all sources and that the deuteration fraction of H₂CO increases from the HMSC to the HMPO phase and then sharply decreases in the latest evolutionary stage (UCHII). The doubly deuterated form of H₂CO is detected only in the earlier evolutionary stages, with D₂CO/H₂CO showing a pattern that is qualitatively consistent with the pattern of HDCO/H₂CO, within current uncertainties.
Conclusions. Our initial results show that H₂CO may display a similar D_frac pattern as that of CH₃OH in massive young stellar objects. This finding suggests that solid-state reactions dominate its formation.