Spectra of H¹³CN J = 2–1 and HC¹⁵N J = 2–1 for two sources as examples. Left: H¹³CN J = 2–1 hfs in G081.87+00.78 cannot be resolved due to line broadening. Right: H¹³CN J = 2–1 hfs in G081.75+00.59 can be resolved with a narrow line width. The vertical bold red lines indicate the hyperfine transitions for H¹³CN J = 2–1: F = 1–1, 1–2, 3–2, 2–1, 1–0, and 2–2 (from left to right).