  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
Millim\'etrique (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.
