Deuterium Fractionation in the Oph-H-MM1 Dense Core of the L1688 Low Mass Star-Forming Region

Abstract

Molecular clouds fragment to form dense cores, which are the first stage of star formation. Such objects are cold, with temperature of ~10 K and density of ${{10}^{4}}{-} {{10}^{7}}$ cm^–3, with predominance of thermal motions and high deuterium fraction. These objects give us information about the initial conditions of star formation and thus they are very important to understand this process. High abundance of deuterated species indicates that a dense core is close to the onset of star formation. In this work, we study deuterium fractionation, which occurs due to chemical reactions that take place under cold core conditions. To measure deuterium fraction, we use nitrogen-bearing species, because they stay longer in the gas phase at low temperatures. We choose the L1688 low-mass star-forming region as one of the closest ones, containing a large number of cold dense cores. We use spectral maps of two lines, N₂H⁺(1–0) and N₂D⁺(1–0), towards one of the dense cores in L1688, Oph-H-MM1, observed with the IRAM 30 m telescope. We measure column densities of N₂D⁺ and N₂H⁺ and deuterium fraction as the ratio of column densities. The map shows an increase in deuterium fraction towards the core center, which is consistent with theoretical predictions.