SOFIA upGREAT/FIFI-LS Emission-line Observations of Betelgeuse during the Great Dimming of 2019/2020

Abstract

We report NASA-DLR SOFIA upGREAT circumstellar [O I] 63.2 μm and [C II] 157.7 μm emission profiles and FIFI-LS [O I] 63.2 μm, [O I] 145.5 μm, and [C II] 157.7 μm fluxes obtained shortly after Betelgeuse's 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the [O I] 63.2 μm flux and V magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V ≃ 0.88 and revealed a 3σ non-detection at a quarter of the previous CGS flux measurement made when V ≃ 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the V-band brightness went to its lowest value on record, V ≃ 1.61, with the SOFIA observations being made when V _FIFI-LS ≃ 1.51 and V _upGREAT ≃ 1.36. The upGREAT spectra show a [O I] 63.2 μm flux larger than previous space observatory measurements obtained when V ≃ 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the [C II] 157.7 μm profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and V photometry, reveals that it is unlikely that the S1 circumstellar envelope and [O I] 63.2 μm fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The [O I] 63.2 μm profile is hard to reconcile with existing outflow velocity models.