Betelgeuse remains steadfast in the infrared

ATel #13518; R. D. Gehrz, J. Marchetti, S. McMillan, T. Procter, A. Zarling, J. Bartlett (U. Minnesota), and N. Smith (U. Arizona)
on 24 Feb 2020; 01:20 UT
Credential Certification: Nathan Smith (nathans@as.arizona.edu)

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We recently obtained infrared photometry of Betelgeuse on 2020 Feb 21 UT at the 76 cm telescope of the O'Brien observatory in Minnesota, using the University of Minnesota Ga:Ge multi-filter bolometer and a 9.3 arcsec aperture. This was amid the presumed minimum of its recent dramatic fading at visual wavelengths (ATels 13341, 13365, 13410, and 13439). We derived the spectral energy distribution (SED) of Betelgeuse by comparison to Aldebaran, with these magnitudes: filter mag err [1.2] = -2.83 +/- 0.07 [2.2] = 3.96 +/- 0.08 [3.6] = -4.33 +/- 0.08 [4.9] = -4.18 +/- 0.11 N = -4,96 +/- 0.12 [7.91] = -4.6 +/- 0.3 [8.81] = -4.84 +/- 0.14 Examining the SED of Betelgeuse from roughly 1 to 9 micron, we see essentially no change as compared to previous observations by Gezari et al. (1993) and Gehrz & Woolf (1971). The resulting magnitudes are the same as they were 50 years ago to within the photometric uncertainty. (While a few filters show it to be fainter by around 0.2 mag, other filters show much less than 0.1 mag change. Some filters show a brighter current magnitude.) Since the mid-IR traces the total bolometric luminosity and is insensitive to moderate fluctuations in extinction, the lack of any significant changes in its mid-IR SED indicates that the bolometric luminosity of Betelgeuse is largely unchanged. This suggests that the recent dramatic fading observed at visual wavelengths is due mostly to local surface phenomena, such as changes in dust extinction or molecular opacity along the line of sight through the inner wind and complex atmosphere, and/or surface temperature fluctuations. The visual fading is not connected to a major change in total energy output from the star. Thus, while Betelgeuse may explode tomorrow or any time in the next few 1e5 yr, the unprecedented current visual faintness is unlikely to be a harbinger of its impending core collapse. We plan to continue these IR observations, and a more detailed comparison of these with past observations in the literature and our own unpublished measurements will appear in a forthcoming paper.