Anomalously high deuterium fractionation in a galactic translucent cloud: A challenge to chemical models

Astronomy and Astrophysics, vol. 712, pp. L17

Abstract

Deuterated (D-) species have long been proposed to diagnose the physical conditions and chemical evolution of cold dense molecular clouds. While deuterium fractionation has been extensively measured in dense cores, observations in diffuse and translucent clouds remain rare. We report here the detection of DCN and DNC toward a translucent cloud ( A V = 1.2 ± 0.2 mag, n H 2 = 3.9 ± 0.2 × 10 2 cm −3 ) through sensitive absorption observations with the IRAM NOrthern Extended Millimeter Array (NOEMA). This detection reaches the lowest column-density and volume-density regime in which deuteration has been observed so far. Interestingly, the observed DCN/HCN and DNC/HNC abundance ratios (3.3 ± 0.6 × 10 −3 and 3.6 ± 1.2 × 10 −3 , respectively), which are more than two orders of magnitude higher than the element abundance [D]/[H] (1.5 × 10 −5 ), suggest an unexpected enhancement of deuterium fractionation in the translucent cloud. These results represent a significant departure from established chemical models considering deuterium fractionation, which predict negligible formation of D-molecules in such environments. Although it remains unclear how D-molecules built up their abundances in translucent gas, a dispersed dense core scenario could potentially explain the observed high deuterium fraction. This interpretation is consistent with the idea proposed by Price et al. (2003, MNRAS, 343, 1257) more than two decades ago, namely that a translucent cloud may be a transient dynamically evolving structure formed through the dissipation of a dense molecular cloud.

Authors 11

1. Gan Luo

   Institut de Radioastronomie Millimétrique · Nanjing University

   Affiliation as printed

   Institut de Radioastronomie Millimétrique

   School of Astronomy and Space Science, Nanjing University

2. Zhi-Yu Zhang

   Nanjing University

   Affiliation as printed

   School of Astronomy and Space Science, Nanjing University

3. Thomas G. Bisbas

   Zhejiang Lab

   Affiliation as printed

   Research Center for Computational Earth and Space Science, Zhejiang Lab

4. Di Li

   Chinese Academy of Sciences · National Astronomical Observatories · Tsinghua University

   Affiliation as printed

   New Cornerstone Science Laboratory, Department of Astronomy, Tsinghua University

   State Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences

5. S. Viti Aachen Leiden Observatory

   Leiden University · University of Bonn · Leiden Observatory · University College London

   Affiliation as printed

   Department of Physics and Astronomy, UCL, Gower Place

   Leiden Observatory, Leiden University

   Transdisciplinary Research Area (TRA Matter) and Argelander Institut für Astronomie, Universität Bonn

6. R. Neri

   Institut de Radioastronomie Millimétrique

   Affiliation as printed

   Institut de Radioastronomie Millimétrique

7. Junzhi Wang

   Guangxi University

   Affiliation as printed

   School of Physical Science and Technology, Guangxi University

8. Siyi Feng

   Xiamen University

   Affiliation as printed

   Department of Astronomy, Xiamen University

9. Ningyu Tang

   Anhui Normal University

   Affiliation as printed

   Department of Physics, Anhui Normal University, Wuhu

10. Daniel R. Rybarczyk

    University of Wisconsin–Madison

    Affiliation as printed

    University of Wisconsin-Madison, Department of Astronomy

11. Lingrui Lin

    Nanjing University

    Affiliation as printed

    School of Astronomy and Space Science, Nanjing University