2016MNRAS.458.1990B

Infrared Dark Clouds (IRDCs) are cold, dense regions that are usually found within Giant Molecular Clouds. Ongoing star formation within IRDCs is typically still deeply embedded within the surrounding molecular gas. Characterizing 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 Institut de Radioastronomie Millimetrique 30 m telescope 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, D ≡ N(N₂D⁺)/N(N₂H⁺). We report a mean D=0.04±0.01, with a maximum of D=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 Dare in agreement with an equilibrium deuterium fraction, given the general properties of the cloud. This implies that the IRDC is at least ∼3 Myr old, which is ∼8 times longer than the mean free-fall time of the observed deuterated region.