The Continued Unprecedented Fading of Betelgeuse

ATel #13410; Edward F. Guinan and Richard J. Wasatonic (Villanova University)
on 20 Jan 2020; 17:50 UT
Credential Certification: Edward Guinan (edward.guinan@villanova.edu)

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We report further on the recent unusual dimming of the red supergiant Betelgeuse (alpha Ori) reported previously in ATel #13341 and ATel #13365. We continue to carry out V-band and Wing TiO and near-IR photometry of the star. Since our last report, Betelgeuse has continued to gradually decrease in brightness. Our most recent photometry secured on 17.25 UT and 18.20 UT January 2020 yields: V = +1.494 mag and 1.506 mag, respectively. This is more than ~0.2 mag fainter than previously reported in ATel #13365 on 22.25 UT Dec. 2019. However during the last week or so the decline in brightness of the star may be slowing. As reported by Brian Skiff of Lowell Observatory (priv. commun.) visual estimates of Betelgeuse are available as far back as about 180-years ago. Systematic visual measures of the star have been made by AAVSO observers since the 1920s. More precise photoelectric photometry began nearly 100 yrs ago but systematic (mostly unpublished) photometry of Betelgeuse commenced about 40-years ago at Villanova Observatory by Scott Wacker and Guinan. Betelgeuse is now nearly as faint as (the slightly variable) B2 star Bellatrix (V ~+1.62 mag). Bellatrix (gamma Ori) is about 5° west of Betelgeuse in the constellation Orion. The analysis of the calibrated Wing photometry (Wing 1992: JAAVSO 21, 42) returns measures of the temperature (via calibrated Wing TiO- and near-IR (B-C) color-indices) as well as estimates of bolometric magnitude (m-bol). The Wing intermediate band A-filter is used to measure the temperature-sensitive TiO 719-nm (gamma; 0, 0) molecular band. The B (750-nm) and C (1020.4-nm) filters are centered on relatively line-free stellar continuum regions. The C-band filter measures have been calibrated with K-M stars with bolometric magnitudes returning proxy measures of the apparent bolometric magnitude (m-bol) (see Wasatonic et al. 2015: PASP, 127, 1010). During the 25-years of V-band / Wing Near-IR photometry, Betelgeuse is currently the coolest and least luminous yet observed. Since September 2019, the star's temperature has decreased by ~100 K while its luminosity (inferred from the C-band/m-bol observations) has diminished by nearly 25%. At face value using R'/R = [(T'/T)^4 / L'/L]^0.5 (where R', T' and L' are the current values of stellar Radius, Temperature & Luminosity), this implies an increase of the star's radius of ~9%. However, as pointed out by others, the current fainting episode could also arise from expelled, cooling gas/dust partially obscuring the star. The recent changes defined by our V-band/Wing photometry seem best explained from changes in the envelop-outer convection atmosphere of this pulsating, unstable supergiant. If these recent light changes are due to an extra-large amplitude light pulse on the ~420-day period, then the next mid-light minimum is expected during late January/early February, 2020. If Betelgeuse continues to dim after that time then other possibilities will have to be considered. The unusual behavior of Betelgeuse should be closely watched.