# The inhomogeneous sub-millimeter atmosphere of Betelgeuse

**Authors:** E. O'Gorman, P. Kervella, G. M. Harper, A. M. S. Richards, L. Decin, M. Montargès, I. McDonald  
**arXiv:** [1706.06021](https://arxiv.org/abs/1706.06021)  
**Date:** 2017-06-19  
**Summary page:** https://www.summarizepaper.com/en/paper/1706.06021v1/

## Key points

- Betelgeuse is an M2 spectral-type supergiant with an extended atmosphere
- Heating mechanisms of Betelgeuse's atmosphere are not well understood
- ALMA long baseline observations reveal a mean temperature of 2760 K at about 1.3 times its radius, which is lower than both the photospheric effective temperature and temperatures at $\sim$$2 $R$_{\star}$
- Evidence for an inversion of mean temperature in red supergiants' atmospheres
- Emission asymmetries suggest localized heating taking place in Betelgeuse's atmosphere, possibly related to magnetic activity generated by large-scale photospheric convection
- This study offers new insights into how early-M spectral-type supergiants' atmospheres are heated and how this process contributes to their mass loss rates.

## Summary

Betelgeuse is an M2 spectral-type supergiant with an extended atmosphere whose heating mechanisms are not well understood. This lack of understanding also extends to the role these mechanisms play in driving the large mass-loss rates of such stars. To gain insight into this issue, the authors present ALMA long baseline observations of Betelgeuse's free-free emission at 338 GHz (0.89 mm) continuum. The observations reveal that the atmosphere has a mean temperature of 2760 K at about 1.3 times its radius ($\sim$1.3 R$_{\star}$), which is lower than both the photospheric effective temperature ($T_{\textrm{eff}} = 3690$ K) and temperatures at $\sim$$2 $R$_{\star}$. This finding provides evidence for an inversion of mean temperature in red supergiants' atmospheres. The emission is not spherically symmetric, with two notable deviations from a uniform disk detected in both images and visibilities. The most prominent asymmetry is located in the north-east quadrant of the disk and is highly elongated with an axis-ratio of 2.4, occupying $\sim$$5\%$ of the disk projected area. Its temperature is approximately 1000 K above the measured mean temperature at 1.3 R$_{\star}$. The other main asymmetry is located on the disk limb almost due east of the disk center and occupies $\sim$$3\%$ of the disk projected area. These emission asymmetries suggest localized heating taking place in Betelgeuse's atmosphere, which could be related to magnetic activity generated by large-scale photospheric convection. Therefore, this study offers new insights into how early-M spectral-type supergiants' atmospheres are heated and how this process contributes to their mass loss rates.