Left upper: HN¹³C (1–0) (filled yellow) and Gaussian fitting profile (red line). The RMS is 0.29 mK at a velocity resolution of 24.20 km s⁻¹. Right upper: six hyperfine lines of C₂H (1–0), that is, J = 3∕2–1/2 F = 2–1, J = 1∕2–1/2 F = 1–0, J = 1∕2–1/2 F = 1–0, J = 1∕2–1/2 F = 1–0, and J = 1∕2–1/2 F = 1–0, marked from 1–6. The RMS is 0.43 mK at a velocity resolution of 12.14 km s⁻¹. As describedin Sect. 3, we only show the profile of the hyperfine lines without Gaussian fitting. Left lower: HNCO (4_0,4–3_0,3) (blue line and filled yellow) overlaid with HNCO (11_0,11–10_0,10) (red line, divided by 3). The RMS is 0.27 mK for HNCO (4_0,4–3_0,3) at a velocity resolution of 23.97 km s⁻¹. The RMS is 1.66 mK for HNCO (11_0,11–10_0,10) at a velocity resolution of 26.16 km s⁻¹. Right lower: HC₃N (18–17) (blue line and filled yellow) overlaid with HC₃N (10–9) (red line). The RMS is 1.14 mK for HC₃N (18–17) at a velocity resolution of 22.53 km s⁻¹. The RMS is 0.31 mK for HC₃N (10–9) at a velocity resolution of 23.17 km s⁻¹.