Gas versus solid-phase deuterated chemistry: HDCO and D<inf>2</inf>CO in massive star-forming regions
Astronomy and Astrophysics
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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 N 2 H + (formed in the gas phase) and CH 3 OH (formed on grain surfaces) can be used as evolutionary indicators and to constrain their dominant formation processes and timescales. Aims. Formaldehyde (H 2 CO) 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 H 2 CO with respect to that of N 2 H + , NH 3 , and CH 3 OH 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 D 2 CO at 193 GHz and 175 GHz 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 H 2 CO J = 3 → 2 data, the deuteration fractions HDCO/H 2 CO and D 2 CO/HDCO are estimated. Results. Our observations show that singly deuterated H 2 CO is detected toward all sources and that the deuteration fraction of H 2 CO increases from the HMSC to the HMPO phase and then sharply decreases in the latest evolutionary stage (UCHII). The doubly deuterated form of H 2 CO is detected only in the earlier evolutionary stages, with D 2 CO/H 2 CO showing a pattern that is qualitatively consistent with the pattern of HDCO/H 2 CO, within current uncertainties. Conclusions. Our initial results show that H 2 CO may display a similar D frac pattern as that of CH 3 OH in massive young stellar objects. This finding suggests that solid-state reactions dominate its formation.