Spectroscopic evidence for a large spot on the dimming Betelgeuse

TL;DR

This study analyzes Betelgeuse's 2019-2020 dimming event, providing spectroscopic evidence that a large dark spot on its surface caused the brightness decrease by lowering its effective temperature.

Contribution

The paper presents high-resolution spectroscopic data showing that Betelgeuse's dimming was due to a large dark spot, offering new insights into stellar surface activity during such events.

Findings

Betelgeuse's effective temperature dropped by at least 170 K during dimming.

Spectroscopic evidence links the dimming to a large dark spot on the star's surface.

The dimming was observed to be caused by surface temperature changes, not dust obscuration.

Abstract

During October 2019 and March 2020, the luminous red supergiant Betelgeuse demonstrated an unusually deep minimum of its brightness. It became fainter by more than one magnitude and this is the most significant dimming observed in the recent decades. While the reason for the dimming is debated, pre-phase of supernova explosion, obscuring dust, or changes in the photosphere of the star were suggested scenarios. Here, we present spectroscopic studies of Betelgeuse using high-resolution and high signal-to- noise ratio near-infrared spectra obtained at Weihai Observatory on four epochs in 2020 covering the phases of during and after dimming. We show that the dimming episode is caused by the dropping of its effective temperature by at least 170 K on 2020 January 31, that can be attributed to the emergence of a large dark spot on the surface of the star.