Characterising the physical and chemical properties of a young Class 0 protostellar core embedded in the Orion B9 filament

Abstract

Deeply embedded low-mass protostars can be used as testbeds to study the early formation stages of solar-type stars, and the prevailing chemistry before the formation of a planetary system. The present study aims to characterise further the physical and chemical properties of the protostellar core Orion B9–SMM3. The Atacama Pathfinder EXperiment (APEX) telescope was used to perform a follow-up molecular line survey of SMM3. The observations were done using the single pointing (frequency range 218.2–222.2 GHz) and on-the-fly mapping methods (215.1–219.1 GHz). These new data were used in conjunction with our previous data taken by the APEX and Effelsberg 100 m telescopes. The following species were identified from the frequency range 218.2–222.2 GHz: ${}^{13}\mathrm{CO}$, $\mathrm{C}^{18}\mathrm{O}$, SO, para-$\mathrm{H}_{2}\mathrm{CO}$, and $\mathrm{E}_{1}$-type $\mathrm{CH}_{3}\mathrm{OH}$. The mapping observations revealed that SMM3 is associated with a dense gas core as traced by $\mathrm{DCO}^{+}$ and p-$\mathrm{H}_{2}\mathrm{CO}$. Altogether three different p-$\mathrm{H}_{2}\mathrm{CO}$ transitions were detected with clearly broadened linewidths ($\Delta v\sim8.2\mbox{--}11~\mbox{km}\,\mbox{s}^{-1}$ in FWHM). The derived p-$\mathrm{H}_{2}\mathrm{CO}$ rotational temperature, $64\pm15~\mbox{K}$, indicates the presence of warm gas. We also detected a narrow p-$\mathrm{H}_{2}\mathrm{CO}$ line ($\Delta v=0.42~\mbox{km}\,\mbox{s}^{-1}$) at the systemic velocity. The p-$\mathrm{H}_{2}\mathrm{CO}$ abundance for the broad component appears to be enhanced by two orders of magnitude with respect to the narrow line value (${\sim}3\times10^{-9}$ versus ${\sim}2\times10^{-11}$). The detected methanol line shows a linewidth similar to those of the broad p-$\mathrm{H}_{2}\mathrm{CO}$ lines, which indicates their coexistence. The CO isotopologue data suggest that the CO depletion factor decreases from ${\sim}27\pm2$ towards the core centre to a value of ${\sim}8\pm1$ towards the core edge. In the latter position, the $\mathrm{N}_{2}\mathrm{D}^{+}/\mathrm{N}_{2}\mathrm{H}^{+}$ ratio is revised down to $0.14\pm0.06$. The origin of the subfragments inside the SMM3 core we found previously can be understood in terms of the Jeans instability if non-thermal motions are taken into account. The estimated fragmentation timescale, and the derived chemical abundances suggest that SMM3 is a few times $10^{5}~\mbox{yr}$ old, in good agreement with its Class 0 classification inferred from the spectral energy distribution analysis. The broad p-$\mathrm{H}_{2}\mathrm{CO}$ and $\mathrm{CH}_{3}\mathrm{OH}$ lines, and the associated warm gas provide the first clear evidence of a molecular outflow driven by SMM3.