The impact of winds on the spectral appearance of Red Supergiants

TL;DR

This study models how stellar winds influence the optical to mid-infrared spectra of Red Supergiants, revealing effects on spectral classification, molecular zones, and variability, with implications for understanding mass-loss impacts.

Contribution

It introduces wind-inclusive model atmospheres for RSGs and demonstrates their effects on spectra, explaining observed spectral type shifts and variability.

Findings

Higher mass-loss rates strengthen TiO absorption, causing later spectral types.

Winds can mimic extended molecular zones around RSGs.

Wind fluctuations can account for Betelgeuse's recent dimming without dust.

Abstract

The rate at which mass is lost during the Red Supergiant evolutionary stage may strongly influence how the star appears. Though there have been many studies discussing how RSGs appear in the mid and far infrared (IR) as a function of their mass-loss rate, to date there have been no such investigations at optical and near-IR wavelengths. In a preliminary study we construct model atmospheres for RSGs which include a wind, and use these models to compute synthetic spectra from the optical to the mid-infrared. The inclusion of a wind has two important effects. Firstly, higher mass-loss rates result in stronger absorption in the TiO bands, causing the star to appear as a later spectral type despite its effective temperature remaining constant. This explains the observed relation between spectral type, evolutionary stage and mid-IR excess, as well as the mismatch between temperatures derived from the optical and infrared. Secondly, the wind mimics many observed characteristics of a `MOLsphere', potentially providing an explanation for the extended molecular zone inferred to exist around nearby RSGs. Thirdly, we show that wind fluctuations can explain the spectral variability of Betelgeuse during its recent dimming, without the need for dust.