Tracing the contraction of the pre-stellar core L1544 with HC${}^{17}$O${}^{+}$ $J$ = 1-0 emission

Abstract

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. 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 HC${}^{17}$O${}^{+}$ $J=$1-0 observations towards the dust peak in L1544. We observed the HC${}^{17}$O${}^{+}$ (1-0) spectrum with the Institut de Radioastronomie Millimétrique (IRAM) 30m telescope. By using new state-of-the-art collisional rate coefficients, a physical model for the core and the fractional abundance profile of HC${}^{17}$O${}^{+}$, 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 C${}^{17}$O molecule. The observed HC${}^{17}$O${}^{+}$(1-0) and C${}^{17}$O(1-0) lines have been 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 HC${}^{17}$O${}^{+}$(1-0) observations. The double peaks observed in the HC${}^{17}$O${}^{+}$(1-0) hyperfine components are due to the contraction motions at densities close to the critical density of the transition ($\sim$10${}^5$ cm${}^{-3}$) and to the fact that the HCO${}^{+}$ fractional abundance decreases toward the centre.