Tracing the contraction of the pre-stellar core L1544 with HC17O+ J=1-0 emission

Context. Spectral line profiles of several molecules observed towards the pre-stellar core L1544 appear double-peaked. For abundant molecular species this line morphology has been linked to self-absorption. However, the physical process behind the double-peaked morphology for less abundant species is still under debate. Aims. In order to understand the cause behind the double-peaked spectra of optically thin transitions and their link to the physical structure of pre-stellar cores, we present high-sensitivity and high spectral resolution (HCO-)-O-17 J =1-0 observations towards the dust peak in L1544. Methods. We observed the (HCO+)-O-17(1-0) spectrum with the Institut de Radioastronomie Millimetrique (IRAM) 30 m telescope. By using state-of-the-art collisional rate coefficients, a physical model for the core and the fractional abundance profile of (HCO+)-O-17, the hyperfine structure of this molecular ion is modelled for the first time with the radiative transfer code Loc applied to the predicted chemical structure of a contracting pre-stellar core. We applied the same analysis to the chemically related (CO)-O-17 molecule. Results. The observed (HCO+)-O-17 (1-0) and (CO)-O-17(1-0) lines were successfully reproduced with a non-local thermal equilibrium (LTE) radiative transfer model applied to chemical model predictions for a contracting pre-stellar core. An upscaled velocity profile (by 30%) is needed to reproduce the (HCO+)-O-17 (1-0) observations. Conclusions. The double peaks observed in the (HCO+)-O-17(1-0) hyperfine components are due to the contraction motions at densities close to the critical density of the transition (similar to 10(5) cm(-3)) and to the decreasing HCO+ fractional abundance towards the centre.

Automated summary

In this study, the double-peaked spectral profiles of (O-17 HCO+)-O-17 J=1-0 molecules in the pre-stellar core L1544 were observed and found to be due to the contraction motions at densities close to the critical density of the transition and the decreasing HCO+ fractional abundance towards the centre.