The distribution of deuterated formaldehyde within Orion-KL

Abstract

We report the first high angular resolution imaging (3.4\arcsec $\times$ 3.0\arcsec) of deuterated formaldehyde (HDCO) toward Orion--KL, carried out with the Submillimeter Array (SMA). We find that the spatial distribution of the formaldehyde emission systematically differs from that of methanol: while methanol is found towards the inner part of the region, HDCO is found in colder gas that wraps around the methanol emission on four sides. The HDCO/H$_2$CO ratios are determined to be 0.003--0.009 within the region, up to an order of magnitude higher than the D/H measured for methanol. These findings strengthen the previously suggested hypothesis that there are differences in the chemical pathways leading to HDCO (via deuterated gas phase chemistry) and deuterated methanol (through conversion of formaldehyde into methanol on the surface of icy grain mantles).