Deuterium fractionation across the infrared-dark cloud G034.77−00.55 interacting with the supernova remnant W44

Context. Supernova remnants (SNRs) may regulate star formation in galaxies. For example, SNR-driven shocks may form new molecular gas or compress pre-existing clouds and trigger the formation of new stars. <BR /> Aims: To test this scenario, we measured the deuteration of N<SUB>2</SUB>H<SUP>+</SUP>, D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> - a well-studied tracer of pre-stellar cores - across the infrared-dark cloud (IRDC) G034.77-00.55, which is known to be experiencing a shock interaction with the SNR W44. <BR /> Methods: We use N<SUB>2</SUB>H<SUP>+</SUP> and N<SUB>2</SUB>D<SUP>+</SUP>J = 1−0 single pointing observations obtained with the 30m antenna at the Instituto de Radioas-tronomia Millimetrica to infer D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> towards five positions across the cloud, namely a massive core, different regions across the shock front, a dense clump, an<SUB>+</SUB>d ambient gas. <BR /> Results: We find D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> in the range 0.03−0.1, which is several orders of magnitude larger than the cosmic D/H ratio (~10<SUP>−5</SUP>). The D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> across the shock front is enhanced by more than a factor of 2 (D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> ~ 0.05 - 0.07) with respect to the ambient gas (≤0.03) and simila<SUB>+</SUB>r to that measured generally in pre-stellar cores. Indeed, in the massive core and dense clump regions of this IRDC we measure D<SUB>frac</SUB><SUP>N<SUB>2</SUB>H<SUP>+</SUP></SUP> ~ 0.01. <BR /> Conclusions: We find enhanced deuteration of N<SUB>2</SUB>H<SUP>+</SUP> across the region of the shock, that is, at a level that is enhanced with respect to regions of unperturbed gas. It is possible that this has been induced by shock compression, which would then be indirect evidence that the shock is triggering conditions for future star formation. However, since unperturbed dense regions also show elevated levels of deuteration, further, higher-resolution studies are needed to better understand the structure and kinematics of the deuterated material in the shock region; for example, to decipher whether it is still in a relatively diffuse form or is already organised in a population of low-mass pre-stellar cores.