Deuterated formaldehyde in rho Ophiuchi A

TL;DR

This study maps deuterium fractionation in rho Ophiuchi A, revealing exceptionally high D2CO/HDCO ratios and suggesting grain chemistry as the main formation mechanism.

Contribution

It provides the first observation of D2CO/HDCO ratio exceeding 1, highlighting the role of grain chemistry in deuterium enrichment in star-forming regions.

Findings

D2CO/HDCO ratio is greater than 1 in rho Oph A.

Kinetic temperature is 20-30 K, H2 density is (6-10)x10^5 cm^-3.

High deuterium levels likely due to grain chemistry processes.

Abstract

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 rho 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 rho 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 20-30 K in the cores of rho Oph A, and the H2 density is (6-10)x10^5 cm-3. We estimate that the total H2 column density toward the deuterium peak is (1-4)x10^23 cm-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.