Betelgeuse Just Is Not That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse

Abstract

We present optical spectrophotometry of the red supergiant (RSG) Betelgeuse from 2020 February 15, during its recent unprecedented dimming episode. By comparing this spectrum to stellar atmosphere models for cool supergiants, as well as spectrophotometry of other Milky Way RSGs, we conclude that Betelgeuse has a current effective temperature of 3600 ± 25 K. While this is slightly cooler than previous measurements taken prior to Betelgeuse’s recent lightcurve evolution, this drop in effective temperature is insufficient to explain Betelgeuse’s recent optical dimming. We propose that episodic mass loss and an increase in the amount of large-grain circumstellar dust along our sightline to Betelgeuse is the most likely explanation for its recent photometric evolution.