2021AJ....162..246H

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.