Structure of the outer layers of cool standard stars

Context. Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims. Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. The goal of this study is to assess wether a set of standard near-infrared calibration sources are fiducial calibrators in the far-infrared, beyond 50 mu m. Methods. The observational spectral energy distributions were compared with the theoretical model predictions for a sample of nine K- and M-giants. The discrepancies found are explained using basic models for flux emission originating in a chromosphere or an ionised wind. Results. For seven out of nine sample stars, a clear flux excess is detected at (sub)millimetre and/or centimetre wavelengths, while only observational upper limits are obtained for the other two. The precise start of the excess depends upon the star under consideration. For six sources the flux excess starts beyond 210 mu m and they can be considered as fiducial calibrators for Herschel/PACS (60-210 mu m). Out of this sample, four sources show no flux excess in the Herschel/SPIRE wavelength range (200-670 mu m) and are good calibration sources for this instrument as well. The flux at wavelengths shorter than similar to 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionised wind is the main flux contributor at longer wavelengths. Conclusions. Although the optical to mid-infrared spectrum of the studied K- and M-type infrared standard stars is represented well by a radiative equilibrium atmospheric model, a chromosphere and/or ionised stellar wind at higher altitudes dominates the spectrum in the (sub)millimetre and centimetre wavelength ranges. The presence of a flux excess has implications on the role of the stars as fiducial spectrophotometric calibrators in these wavelength ranges.