2013MNRAS.428.2064H

We examine the thermal free-free millimetre fluxes expected from non-dusty and non-pulsating K through mid-M giant stars based on our limited understanding of their inhomogeneous chromospheres. We present a semi-analytic model that provides estimates of the radio fluxes for the mm wavelengths [e.g. Combined Array for Research in Millimetre-wave Astronomy (CARMA), Atacama Large Millimetre/submillimetre Array (ALMA) and Karl G. Jansky Very Large Array (JVLA) Q band] based on knowledge of the effective temperatures, angular diameters and chromospheric Mgii h&k emission fluxes. At 250GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and submm range will have a contribution from the chromospheric material that gives rise to the ultraviolet emission spectrum, as well as the cool molecular material known to exist above the photosphere. We predict a lower bound to the inferred brightness temperature of red giants based on heating at the basal flux limit if the upper chromospheres have filling factor ≃ 1. Multifrequency mm observations should provide important new information on the structuring of the inhomogeneous chromospheres, including the boundary layer, and allow tests of competing theoretical models for atmospheric heating. We comment on the suitability of these stars as mm flux calibrators.