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

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 A spectra can be quantitatively reproduced if there are multiple photospheric components, as hinted at by Very Large Telescope (VLT)-SPHERE images in Montarges et al. If the cooler component has Delta 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- (similar to 430 days) and long-period (similar to 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.