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 , NH _ { 3 } , and C _ { 3 } 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 ) , H ^ { 13 } CO ^ { + } ( 1–0 ) , DNC ( 3–2 ) , HN ^ { 13 } C ( 1–0 ) , p-H _ { 2 } CO ( 2 _ { 0 , 2 } –1 _ { 0 , 1 } , 3 _ { 0 , 3 } –2 _ { 0 , 2 } ) , and in emission for o-NH _ { 2 } D. 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 ( \sim 2.5 \times 10 ^ { 4 } cm ^ { -3 } ) and temperature ( \sim 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 .