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Herschel/HIFI observations toward the compact HII region W51 has revealed thepresence of a cold dense core along its line of sight in a high-velocity streamlocated just in front of W51. This detection has been made possible throughabsorption measurements of low-energy transitions of HDO, NH3, and C3 againstthe bright background emitted by the star-forming region. We present afollow-up study of this core using the high sensitivity and high spectralresolution provided by the IRAM 30-meter telescope. We report new detections ofthis core in absorption for DCO+ (2-1, 3-2), H13CO+ (1-0), DNC (3-2), HN13C(1-0), p-H2CO (202-101, 303-202), and in emission for o-NH2D. We also reportinterferometric observation of this last species using the IRAM/NOEMAtelescope, revealing the fragmented nature of the source through the detectionof two cores, separated by 0.19 - 0.24 pc, with average sizes of less than 0.16- 0.19 pc. From a non-LTE analysis, we are able to estimate the density (~2.5e4cm-3) and temperature (~10 K) of this component, typical of what is found indark clouds. This component (called W51-core) has the same DCO+/HCO+ ratio(0.02) as TMC-1 and a high DNC/HNC ratio (0.14). Detection of these deuteratedspecies indicates that W51-core is similar to an early-phase low-massstar-forming region, formed from the interaction between the W51 giantmolecular cloud and the high-velocity stream in front of it. The W51 complexbeing at about 5 kpc, these findings lead to what is the first detection of theearliest phase of low-mass star-forming region at such a large distance.

Deuterium fractionation of a distant cold dark cloud along the line of sight of W51