Observed J = 1–0 and 2–1 lines of HCS⁺ and its isotopologues towards TMC-1. The abscissa corresponds to the local standard of rest velocity of the lines adopting the rest frequencies given in Table B.1. The bottom panels show the same transitions of HC³³S⁺, which exhibit several hyperfine components. The abscissa in this case is the rest frequency adopting a velocity for the source of 5.83 km s⁻¹ (Cernicharo et al. 2020). The ordinate corresponds to the antenna temperature corrected for atmospheric and telescope losses in mK. The derived line parameters for the observed lines are given in Table B.1. The synthetic spectra (red line) are derived for the column densities shown in Table 1 and the LVG model described in Sections 3 and 4.2.