Abstract
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.
Highlights
Deuterium was formed during the primordial nucleosynthesis in the first minutes after the Big Bang with a fractional abundance of ≈1.5 × 10−5 (Linsky 2003)
In this work we performed a detailed analysis of multiple transitions of N2H+, N2D+, HC18O+, DCO+ with recent highsensitivity observations, which allow us to investigate the molecular properties with high-signal-to-noise ratio (S/N) data across the whole L1544 core
Using a non-local thermodynamic equilibrium (non-LTE) approach combined with the molecular abundances computed with chemical models, we derived the excitation conditions of the molecules at the dust peak
Summary
Deuterium was formed during the primordial nucleosynthesis in the first minutes after the Big Bang with a fractional abundance of ≈1.5 × 10−5 (Linsky 2003) Enhancements of this value of several orders of magnitude have been found both in different environments of the interstellar medium (ISM) and in several components of the solar system (Ceccarelli et al 2014, and references therein). This has led to the idea of using the deuteration ratio D/H (the ratio between the abundance of a D-bearing molecule and its hydrogenated isotopologue) as a diagnostic tool to investigate the star formation process, and to understand how our own solar system was formed (Ceccarelli et al 2014). D-fractionation (i.e. the inclusion of a D-atom in hydrogenated species) is driven by
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