Description
Red Supergiants (RSGs) are high-mass He-burning evolved stars predicted to be the progenitors of Type II-Plateau core-collapse supernovae. Betelgeuse (α Orionis), the most studied Galactic RSG, displayed an unexpected and rapid descent in brightness from October 2019 through March 2020. Its minimum in February 2020 is the deepest recorded for the star. Explanations for this unanticipated event include a dust occultation caused by a prior mass loss event, a drop in the surface temperature of the star, or a combination of the two causes. We present optical spectroscopy of Betelgeuse using low-resolution spectral data obtained at five epochs during and after the dimming event using the 1-m telescope at Mount Laguna Observatory. The purpose of this thesis is to derive the effective temperature (Tef f ) of Betelgeuse at all epochs by comparing the equivalent width (Wλ) of the temperature-sensitive 7054 ̊A line with those measured from PHOENIX synthetic model spectra. Our analysis finds a strong positive correlation between Betelgeuse’s photometric and spectroscopic behavior: Decreases in apparent brightness correlate directly with decreases in inferred temperature, with a globally averaged mean temperature drop of ∆Tef f = 149 ± 12 K inferred during the deepest part of the light-curve minimum compared with the Tef f inferred after Betelgeuse had returned to normal brightness. The PHOENIX models predict a decline in V-band magnitude of ∆Vest = 0.70 ± 0.04 from this temperature drop. Compared with the observed dimming of ∆Vobs = 1.13 ± 0.03, this suggests that at least a significant fraction of Betelgeuse’s dimming was caused by a Tef f change. Invoking dust from a prior mass-loss event could explain the difference between ∆Vest and ∆Vobs. However, imaging of Betelgeuse’s surface during the Great Dimming revealed an inhomogeneous brightness profile; its southern hemisphere became notably dimmer than the northern hemisphere. Our results, though unable to completely dismiss dust as a contributor, support the conclusion that the observed dimming could have resulted from Betelgeuse’s southern hemisphere cooling significantly, ∆T ≥ 250 K, while the northern hemisphere maintained a comparatively normal temperature.
