Betelgeuse Updates

ATel #13439; Edward F. Guinan and Richard J. Wasatonic (Villanova University)
on 1 Feb 2020; 23:20 UT
Credential Certification: Edward Guinan (edward.guinan@villanova.edu)

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We provide a short update on the current unprecedented dimming of the bright red supergiant Betelgeuse. V-band and Wing TiO and near-IR photometry continues. This photometry now is accompanied by frequent photoelectric V-band and visual observations being carried out by AAVSO observers (see: www.aavso.org). We give an update on the star's recent behavior and also correct an error in our prior telegram (ATel #13410). Although the decrease in brightness appears to have slowed, our most recent observation on 30.15 UT, January 2020 is V = +1.614 +/-0.012 mag. The average of our last three measures over the prior 10-days is = +1.58 mag. Our V-mag measures are in very good agreement with corresponding values found by AAVSO observers. Betelgeuse is now ~1.0 mag fainter relative to observations made at the start of 2019/20 observing season during September. As discussed in the previous telegrams, Wing TiO and near-IR photometry returns measures of the star’s temperature (T) from TiO 719-nm band-head feature relative to the nearby reference continuum (the Wing B-filter at 752-nm). The Wing C-band filter (1024-nm) is centered on a (mostly) line-free spectral region near the peak of the M-star's spectral energy distribution (see Wing 1992: JAAVSO 21, 42). The Wing C-band observations serve as proxies for apparent bolometric magnitudes (m-bol) from which the star's luminosity(L) can be estimated. The analyses of the updated Wing ABC measures return estimates of Teff (T) and Luminosity (L). The recent mean values (T', L', R') were computed from observations made during the last 10-days of January 2020. These are compared with measures from September 2019 (L, T, R) made during the local light maximum. The star's radius (R) is estimated from L = 4 pi R^2 -sigma T^4. Correcting the error in our previous ATel (thanks to George Cooper for pointing this out), the change in radius R'/R is calculated from R'/R = [(L'/L)/(T'/T)^4 ]^0.25. Adopting T'/T = (3565 K /3650 K), and L'/L = 0.77, indicates an apparent radius decrease of R'/R ~0.92 since September 2019. But as a caveat, this estimate is bolometric-based and assumes the changes originate from a spherically symmetric star. The light changes also could arise from changes in circumstellar dust and/or variations in localized bright or dark regions on the star. Adopting a more refined determination of the dominant (probable pulsation) period of P = 430 days, the minimum brightness is expected on 21 (+/-7d) February, 2020. The most recent photometric observations indicate that Betelgeuse is currently the least luminous and coolest yet measured from our 25-years of photometry. Observations made over the last few weeks indicate that current dimming could be slowing. Observations (of all kinds) over the next few months will be important. We are preparing a paper on these results for publication.