Structure of the outer layers of cool standard stars

TL;DR

This study investigates how chromospheres and stellar winds in cool giants affect their long-wavelength emissions, revealing flux excesses at millimeter and centimeter wavelengths that impact their use as calibration standards.

Contribution

It provides the first detailed comparison of observed spectral energy distributions with models, highlighting the influence of chromospheres and winds on long-wavelength flux in late-type giants.

Findings

Most stars show flux excess at (sub)millimeter and centimeter wavelengths.

Chromospheres dominate flux at wavelengths shorter than 1 mm.

Ionized winds are the main contributors at longer wavelengths.

Abstract

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. Methods: The observational spectral energy distributions are compared with the theoretical predictions of the MARCS atmosphere models for a sample of 9 K- and M-giants. The discrepancies found are explained using basic models for flux emission originating from a chromosphere or an ionized wind. Results: For 7 out of 9 sample stars, a clear flux excess is detected at (sub)millimeter and/or centimeter wavelengths. The precise start of the excess depends upon the star under consideration. The flux at wavelengths shorter than about 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionized wind is the main flux contributor at longer wavelengths. Conclusions: Although the optical to mid-infrared spectrum of the studied K- and M-giants is well represented by a radiative equilibrium atmospheric model, the presence of a chromosphere and/or ionized stellar wind at higher altitudes dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges. The presence of a flux excess also has implications on the role of these stars as fiducial spectrophotometric calibrators in the (sub)millimeter and centimeter wavelength range.