Deuterium fractionation as a multi-phase component tracer in the Galactic Centre

Abstract

The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disc. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm ($>$100 K) and turbulent phase with little material in the pre-stellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO${}^{+}$, and N${}_2$H${}^{+}$ towards the CMZ molecular cloud G+0.693-0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent narrow component, with a line width of $\sim$9 km s${}^{-1}$, in addition to the warm, less dense and turbulent broad component with a line width of $\sim$20 km s${}^{-1}$. The very low D/H ratio $\le$6$\times$10${}^{-5}$ for HCO${}^{+}$ and N${}_2$H${}^{+}$, close to the cosmic value ($\sim$2.5$\times$10${}^{-5}$), and the high D/H ratios $>$4$\times$10${}^{-4}$ for HCN and HNC derived for the broad component, confirm the presence of high-temperatures deuteration routes for nitriles. For the narrow component we have derived D/H ratios $>$10${}^{-4}$ and excitation temperatures of $7$ K for all molecules, suggesting kinetic temperatures $\le$30 K and H${}_2$ densities $\ge$5$\times$10${}^4$ cm${}^{-3}$, at least one order of magnitude larger than for the broad component. The method presented in this Letter allows to identify clouds on the verge of star formation, i.e. under pre-stellar conditions, towards the CMZ. This method can also be used for the identification of such clouds in external galaxies.

11 pages, 4 figures, 2 tables, 2 appendix - Accepted for publication in ApJ Letters

References in corpus (11)

• Survey of ortho-H2D+(1_{1,0}-1_{1,1}) in dense cloud cores
• The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars
• Tracing shocks and photodissociation in the Galactic center region
• On the frequency of N2H+ and N2D+
• Discovery in space of ethanolamine, the simplest phospholipid head group
• Prebiotic precursors of the primordial RNA world in space: Detection of NH${}_2$OH
• The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone
• Cloud-cloud collision as drivers of the chemical complexity in Galactic Centre molecular clouds
• Probing the chemical complexity of amines in the ISM: detection of vinylamine (C${}_2$H${}_3$NH${}_2$) and tentative detection of ethylamine (C${}_2$H${}_5$NH${}_2$)
• First detection of C${}_2$H${}_5$NCO in the ISM and search of other isocyanates towards the G+0.693-0.027 molecular cloud
• Evidence for DCO+ as a probe of ionization in the warm disk surface

Cited by in corpus (7)

• Amides inventory towards the G+0.693-0.027 molecular cloud
• On the abiotic origin of dimethyl sulfide: discovery of DMS in the Interstellar Medium
• Collisional excitation of PO${}^{+}$ by $para$-H${}_2$
• H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium
• The GUAPOS project. VI: the chemical inventory of shocked gas
• Chemical complexity in star formation induced by stellar feedback: cores shock-formed by the supernova remnant W44
• Synthesis and Spectroscopic Characterization of Interstellar Candidate Ethynyl Thiocyanate: HCCSCN