Widespread deuteration across the IRDC G035.39-00.33

TL;DR

This study maps deuterated molecules in IRDC G035.39-00.33, revealing widespread deuteration indicative of early star formation stages and suggesting the cloud's age exceeds its free-fall time by a significant margin.

Contribution

First detailed mapping of N2D+ in IRDC G035.39-00.33, demonstrating widespread deuteration and providing insights into the cloud's evolutionary stage.

Findings

Deuterium fraction averages 0.04, with a maximum of 0.09.

Deuteration levels are about 1000 times higher than interstellar D/H ratio.

IRDC age estimated at ~3 million years, much longer than free-fall time.

Abstract

Infrared Dark Clouds (IRDCs) are cold, dense regions that are usually found within Giant Molecular Clouds (GMCs). Ongoing star formation within IRDCs is typically still deeply embedded within the surrounding molecular gas. Characterising the properties of relatively quiescent IRDCs may therefore help us to understand the earliest phases of the star formation process. Studies of local molecular clouds have revealed that deuterated species are enhanced in the earliest phases of star formation. In this paper we test this towards IRDC G035.39-00.33. We present an 80 arcsec by 140 arcsec map of the J=2-1 transition of N2D+, obtained with the IRAM-30m telescope. We find that N2D+ is widespread throughout G035.39-00.33. Complementary observations of N2H+(1-0) are used to estimate the deuterium fraction, N(N2D+)/N(N2H+). We report a mean deuterium fraction of 0.04+-0.01, with a maximum of 0.09+-0.02. The mean deuterium fraction is ~3 orders of magnitude greater than the interstellar [D]/[H] ratio. High angular resolution observations are required to exclude beam dilution effects of compact deuterated cores. Using chemical modelling, we find that the average observed values of the deuterium fraction are in agreement with an equilibrium deuterium fraction, given the general properties of the cloud. This implies that the IRDC is at least ~3Myr old, which is ~8 times longer than the mean free-fall time of the observed deuterated region.