Brightness temperature T_B in Kelvin at ALMA wavelengths λ in μm, for Bands 3 to 9 of the G-star α Cen A (left, blue) and the K-star α Cen B (right, red). In addition to the observational rms errors (solid bars), the estimated absolute errors, including calibration uncertaities, are shown as dashed error bars. The stellar photospheres, represented by extrapolations to PHOENIX model atmospheres of Brott & Hauschildt (2005) for the respective stars (T_eff, log g, [Fe/H]), and shown as black dashed lines. The ALMA bands are indicated below. A solar model chromosphere (VAL IIIC, Vernazza et al. 1981) is shown as long dashes, with data for the Sun from Loukitcheva et al. (2004) as black open circles.