Deuteration in infrared dark clouds

TL;DR

This study investigates the deuteration levels and kinematic properties of dense gas in infrared dark clouds, revealing insights into early star formation stages through observations of N$_2$H$^+$ and N$_2$D$^+$ molecules.

Contribution

It provides one of the largest observational samples of IRDCs with detailed measurements of deuteration ratios and kinematics, offering new insights into early star formation processes.

Findings

Deuteration ratios range from 0.003 to 0.14.

Most sources show significant turbulence, with some having subthermal velocity dispersion.

No correlation between embedded 70 μm sources and deuteration detection.

Abstract

Much of the dense gas in molecular clouds has a filamentary structure but the detailed structure and evolution of this gas is poorly known. We have observed 54 cores in infrared dark clouds (IRDCs) using N$_2$H$^+$ (1-0) and (3-2) to determine the kinematics of the densest material, where stars will form. We also observed N$_2$D$^+$ (3-2) towards 29 of the brightest peaks to analyse the level of deuteration which is an excellent probe of the quiescent of the early stages of star formation. There were 13 detections of N$_2$D$^+$ (3-2). This is one of the largest samples of IRDCs yet observed in these species. The deuteration ratio in these sources ranges between 0.003 and 0.14. For most of the sources the material traced by N$_2$D$^+$ and N$_2$H$^+$ (3-2) still has significant turbulent motions, however three objects show subthermal N$_2$D$^+$ velocity dispersion. Surprisingly the presence or absence of an embedded 70 $\mu$m source shows no correlation with the detection of N$_2$D$^+$ (3-2), nor does it correlate with any change in velocity dispersion or excitation temperature. Comparison with recent models of deuteration suggest evolutionary time-scales of these regions of several free-fall times or less.