Oral Presentation
Singly- and doubly-deuterated formaldehyde in massive star-forming regions
Presenter: Sarolta Zahorecz (Osaka Prefecture University / NAOJ)
Low temperatures and high densities are necessary for the effective formation of deuterated molecules. During the star formation process, the presence of deuterated molecules is expected to be enhanced in cold, dense pre-stellar cores and to decrease after protostellar birth. The deuteration process differs depending on species; for example, deuteration of N$_2$H$^+$ happens in the gas phase, while that of CH$_3$OH happens on the surface of dust grains.
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 not fully understood. 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 the formation processes and time-scales traced by H$_2$CO deuteration. Our recent single dish APEX observations suggested that formaldehyde and its deuterated species form mostly on grain surfaces although some gas-phase contribution is expected at the warm high-mass protostellar object stage. Since the single dish beam is large, and since these high-mass star-forming regions are clustered and complex, it is unclear whether the emission arises from the protostellar sources or from starless/pre-stellar cores associated with them. Therefore, interferometric observations and a larger sample size are needed for detailed studies. We extended our study of the $J$=3$\rightarrow$2 rotational lines of HDCO and D$_2$CO at 193 GHz and 175 GHz toward eleven high-mass star-forming regions in total hosting objects at different evolutionary stages with the SEPIA Band 5 receiver on APEX and high-resolution follow-up observations of eight objects with ALMA Band 6 were performed to reveal the size of the H$_2$CO emission and to give an estimate of the deuteration fractions HDCO/H$_2$CO and D$_2$CO/HDCO at scales of $\sim$6'' (0.04-0.15 pc at the distance of our targets).
Our observations show that the deuteration fraction of H$_2$CO decreases with an order of magnitude from the earlier HMPO phases to the latest evolutionary stage (UC~HII), from $\sim$0.13 to $\sim$0.01.
Our combined APEX and ALMA data support our single dish results: the deuteration fraction of formaldehyde decreases with evolutionary stage, but higher sensitivity observations are needed to provide more stringent constraints to the D/H ratio obtained from HDCO and D$_2$CO during the HMSC phase.
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