Chromospheric Thermal Continuum Millimetre Emission from non-dusty K and M Red Giants

TL;DR

This paper models the millimetre thermal emission from non-dusty K and M red giants' chromospheres, predicting fluxes and brightness temperatures to inform observations and understand chromospheric structure and heating.

Contribution

It introduces a semi-analytic model to estimate mm fluxes from red giants' chromospheres based on their physical and emission properties.

Findings

Chromospheric optical depths at 250 GHz are less than unity.

Predicted lower bounds for brightness temperatures based on basal-flux heating.

Multi-frequency observations can reveal chromospheric inhomogeneity and test heating models.

Abstract

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., CARMA, ALMA, JVLA Q-band) based on knowledge of the effective temperatures, angular diameters and chromospheric Mg II h & k emission fluxes. At 250 GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and sub-mm range will have a contribution from the chromospheric mate- rial 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. Multi-frequency 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.