CO 3–2 (solid black) and ¹³CO 3–2 (dotted black) spectra at the peak position of Cha-MMS1 in brightness temperature scale. Green squares show the ratio of CO to ¹³CO intensity, and green arrows indicate the lower limits of this ratio after smoothing over four channels. The scale is given on the first right axis. The optical depth scale is shown on the second right axis. The red and blue dashed lines mark the inner integration limits, and the black dashed line indicates the systemic velocity of Cha-MMS1.