2011A&A...527A..39B

Formaldehyde is an organic molecule that is abundant in the interstellar medium. High deuterium fractionation is a common feature in low-mass star-forming regions. Observing several isotopologues of molecules is an excellent tool for understanding the formation paths of the molecules. We seek an understanding of how the various deuterated isotopologues of formaldehyde are formed in the dense regions of low-mass star formation. More specifically, we adress the question of how the very high deuteration levels (several orders of magnitude above the cosmic D/H ratio) can occur using H₂CO data of the nearby ρ Oph A molecular cloud. From mapping observations of H₂CO, HDCO, and D₂CO, we have determined how the degree of deuterium fractionation changes over the central 3'x3' region of ρ Oph A. The multi-transition data of the various H₂CO isotopologues, as well as from other molecules (e.g., CH₃OH and N₂D⁺) present in the observed bands, were analysed using both the standard type rotation diagram analysis and, in selected cases, a more elaborate method of solving the radiative transfer for optically thick emission. In addition to molecular column densities, the analysis also estimates the kinetic temperature and H₂ density. Toward the SM1 core in ρ Oph A, the H₂CO deuterium fractionation is very high. In fact, the observed D₂CO/HDCO ratio is 1.34±0.19, while the HDCO/H₂CO ratio is 0.107±0.015. This is the first time, to our knowledge, that the D₂CO/HDCO abundance ratio is observed to be greater than 1. The kinetic temperature is in the range 20-30K in the cores of ρ Oph A, and the H₂ density is (6-10)x10⁵cm^–3. We estimate that the total H₂ column density toward the deuterium peak is (1-4)x10²³cm^–2. As depleted gas-phase chemistry is not adequate, we suggest that grain chemistry, possibly due to abstraction and exchange reactions along the reaction chain H₂CO -HDCO -D₂CO, is at work to produce the very high deuterium levels observed.