Observations of the J = 5−4 transition of CH₃CO⁺ towards L483, L1544, L1527, and B1 (top panels); bottom panel: J = 7−6 line towards L1527. The abscissa corresponds to the rest frequency (in MHz) and the ordinate is the antenna temperature (in mK). The spectral resolution is 48.8 kHz for all sources except L1527, for which it is 198 kHz. The vertical dashed blue lines indicate the position of the K = 0 and K = 1 lines (detected in all sources except B1), and the cyan line corresponds to the U feature at 91 344 MHz (detected in all sources except L1527). The rest velocities of L1527 and L1544 were taken as v_LSR = 5.9 km s⁻¹ and 7.2 km s⁻¹, respectively, based on Sakai et al. (2009) and Vastel et al. (2015), respectively.