Spectra of ¹²CO and ¹³CO toward starless cores. The ¹²CO lines are shown in the histogram, and the ¹³CO lines are shown as solid lines. The x‐axis is radial velocity (km s⁻¹) with respect to the local standard of rest, whereas the y‐axis is antenna temperature (K). As a result of frequency‐switching observations, a telluric ¹²CO line appears either in emission (filled circle) or in absorption (open circle). CS ﹩J=2{\mbox{--}} 1﹩ lines taken from Lee et al. (1999) including unpublished ones are also shown as solid lines for comparison. The CS lines are scaled up by a factor of 6 and shifted by 1 K. The dashed vertical lines indicate the systemic velocity derived from N₂H⁺ line.