Far Ultraviolet Continuum Emission: Applying this Diagnostic to the Chromospheres of Solar-Mass Stars

TL;DR

This study measures the FUV continuum flux in solar-mass stars using Hubble's COS, revealing a correlation between stellar rotation and chromospheric heating, and compares stellar data with solar models to infer thermal structures.

Contribution

It provides high-quality FUV continuum measurements for solar-mass stars and links stellar rotation to chromospheric heating rates using new observational data.

Findings

FUV continuum brightness temperature increases with decreasing stellar rotation period.

Rapidly rotating stars have FUV brightness temperatures similar to solar faculae.

The thermal structure of the brightest solar faculae helps estimate heating in stellar chromospheres.

Abstract

The far ultraviolet (FUV) continuum flux is recognized as a very sensitive diag- nostic of the temperature structure of the Sun's lower chromosphere. Until now analysis of the available stellar FUV data has shown that solar-type stars must also have chromospheres, but quantitative analyses of stellar FUV continua require far higher quality spectra and comparison with new non-LTE chromosphere models. We present accurate far ultraviolet (FUV, 1150-1500^{\circ}) continuum flux measurements for solar-mass stars, made feasible by the high throughput and very low detector background of the Cosmic Origins Spectrograph (COS) on the Hubbble Space Telescope. We show that the continuum flux can be measured above the detector background even for the faintest star in our sample. We find a clear trend of increasing continuum brightness temperature at all FUV wavelengths with decreasing rotational period, which provides an important measure of magnetic heating rates in stellar chromospheres. Comparison with semiempirical solar flux models shows that the most rapidly rotating solar-mass stars have FUV continuum brightness temperatures similar to the brightest faculae seen on the Sun. The thermal structure of the brightest solar faculae therefore provides a first-order estimate of the thermal structure and heating rate for the most rapidly rotating solar-mass stars in our sample.