Volume 629, September 2019
|Number of page(s)||16|
|Section||Interstellar and circumstellar matter|
|Published online||26 August 2019|
High-sensitivity maps of molecular ions in L1544
I. Deuteration of N2H+ and HCO+ and primary evidence of N2D+ depletion★
Centre for Astrochemical Studies, Max-Planck-Institut für extraterrestrische Physik,
Garching bei München,
Accepted: 18 July 2019
Context. The deuterium fraction in low-mass prestellar cores is a good diagnostic indicator of the initial phases of star formation, and is also a fundamental quantity to infer the ionisation degree in these objects.
Aims. With the analysis of multiple transitions of N2H+, N2D+, HC18O+, and DCO+ we are able to determine the molecular column density maps and the deuterium fraction in N2H+ and HCO+ toward the prototypical prestellar core L1544. This is the preliminary step to derive the ionisation degree in the source.
Methods. We used a non-local thermodynamic equilibrium (non-LTE) radiative transfer code combined with the molecular abundances derived from a chemical model to infer the excitation conditions of all the observed transitions. This allowed us to derive reliable maps of the column density of each molecule. The ratio between the column density of a deuterated species and its non-deuterated counterpart gives the sought-after deuteration level.
Results. The non-LTE analysis confirms that, for the molecules analysed, higher-J transitions are characterised by excitation temperatures that are ≈1–2 K lower than those of the lower-J transitions. The chemical model that provides the best fit to the observational data predicts the depletion of N2H+ and to a lesser extent of N2D+ in the innermost region. The peak values for the deuterium fraction that we find are D/HN2H+ = 0.26−0.14+0.15 and D/HHCO+=0.035−0.012+0.015, in good agreement with previous estimates in the source.
Key words: ISM: clouds / ISM: molecules / ISM: abundances / radio lines: ISM / stars: formation
© E. Redaelli et al. 2019
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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