VLTI/AMBER observations of the red giant Arcturus. Panel a: observed spectrum normalized to the continuum. The positions of the CO lines and the Mg I line are marked with the ticks. Panels b–d: visibilities observed at 7.3, 14.6, and 21.8 m, respectively. The leftmost error bars represent the typical total errors, while the smaller error bars represent the typical errors without the systematic errors resulting from the absolute calibration. Panel e: observed closure phase. The errors are shown in the same manner as in panels b–d. Panels f–h: differential phases observed at the 7.3, 14.6, and 21.8 m baselines, respectively. The errors are shown in the same manner as in panels b–d.