The Photospheric Temperatures of Betelgeuse during the Great Dimming of 2019/2020: No New Dust Required

Abstract

The processes that shape the extended atmospheres of red supergiants, heat their chromospheres, create molecular reservoirs, drive mass loss, and create dust remain poorly understood. Betelgeuse’s V -band “Great Dimming” event of 2019 September/2020 February and its subsequent rapid brightening provides a rare opportunity to study these phenomena. Two different explanations have emerged to explain the dimming; new dust appeared in our line of sight attenuating the photospheric light, or a large portion of the photosphere had cooled. Here we present five years of Wing three-filter ( A , B , and C band) TiO and near-IR photometry obtained at the Wasatonic Observatory. These reveal that parts of the photosphere had a mean effective temperature ( T eff ) significantly lower than that found by Levesque & Massey. Synthetic photometry from MARCS-model photospheres and spectra reveal that the V band, TiO index, and C -band photometry, and previously reported 4000–6800 Å spectra can be quantitatively reproduced if there are multiple photospheric components, as hinted at by Very Large Telescope (VLT)-SPHERE images in Montargès et al. If the cooler component has Δ T eff  ≥ 250 K cooler than 3650 K, then no new dust is required to explain the available empirical constraints. A coincidence of the dominant short- (∼430 days) and long-period (∼5.8 yr) V -band variations occurred near the time of deep minimum (Guinan et al. 2019a). This is in tandem with the strong correlation of V mag and photospheric radial velocities, recently reported by Dupree et al. (2020b). These suggest that the cooling of a large fraction of the visible star has a dynamic origin related to the photospheric motions, perhaps arising from pulsation or large-scale convective motions.