Deuterium fractionation of a distant cold dark cloud along the line of
  sight of W51
  Herschel/HIFI observations toward the compact HII region W51 has revealed the
presence of a cold dense core along its line of sight in a high-velocity stream
located just in front of W51. This detection has been made possible through
absorption measurements of low-energy transitions of HDO, NH3, and C3 against
the bright background emitted by the star-forming region. We present a
follow-up study of this core using the high sensitivity and high spectral
resolution provided by the IRAM 30-meter telescope. We report new detections of
this 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 report
interferometric observation of this last species using the IRAM/NOEMA
telescope, revealing the fragmented nature of the source through the detection
of 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 (~2.5e4
cm-3) and temperature (~10 K) of this component, typical of what is found in
dark 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 deuterated
species indicates that W51-core is similar to an early-phase low-mass
star-forming region, formed from the interaction between the W51 giant
molecular cloud and the high-velocity stream in front of it. The W51 complex
being at about 5 kpc, these findings lead to what is the first detection of the
earliest phase of low-mass star-forming region at such a large distance.
