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

¹ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
e-mail: charlotte.vastel@irap.omp.eu
² CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’ Hères, France
⁵ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
⁶ Sorbonne Universités, UPMC Univ. Paris 06, UMR 8112, LERMA, 75005 Paris, France

Received: 11 July 2016
Accepted: 14 November 2016

Abstract

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 30m 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 (2_0,2–1_0,1, 3_0,3–2_0,2), 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 (~2.5 × 10⁴ 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.