Photometry of Betelgeuse with the STEREO Mission While in the Glare of the Sun from Earth

ATel #13901; Andrea Dupree (Center for Astrophysics | Harvard & Smithsonian), Edward Guinan (Villanova University), William T. Thompson (ADNET) and the STEREO/SECCHI/HI consortium.
on 28 Jul 2020; 22:43 UT
Credential Certification: Edward Guinan (edward.guinan@villanova.edu)

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During 2019/20 the red supergiant Betelgeuse (Alpha Ori; HD 39801; M1-2 Iab) underwent an unprecedented dimming reaching V ~ 1.61 mag 7-13 February 2020. From that time to the end of the 2020 observing season (early-May), the star recovered in brightness. The last reliable photoelectric measure of V = +0.398+/-0.012 mag was secured on April 27.6 UT, 2020 (priv. commun.: D. Corona ). Additional visual measures from the AAVSO indicate that the star may have reached mv ~ +0.3 mag during mid-May. According to the star's dominant ~420-430 d periodicity, Betelgeuse is expected to reach maximum brightness during August/September 2020; its next minimum is expected in early April 2021. Unfortunately at this critical time, Betelgeuse is out of view from mid-May to early August due its close angular proximity to the Sun (as seen from Earth). This summer, thanks to the STEREO mission, Betelgeuse cannot hide in the Sun's glare.

While unobservable from Earth, Betelgeuse and surrounding stars have been imaged from the Solar Terrestrial Relations Observatory spacecraft (STEREO-A) located about 69 degrees behind the Earth in the Earth's orbit.The outer Heliospheric Imager HI-2 was used during June and July 2020 after rolling the spacecraft 180 degrees to repoint on the other side of the Sun. A two hour block of time was required for each roll, exposures, and return to nominal orientation. Unbinned images were transmitted from the spacecraft. The CCD passband is broad, and the calibration of Tappin et al. (Solar Phys., 2015, 290, 2143) enabled the determination of V magnitudes from aperture photometry with AstroImageJ (Collins, K. et al., 2017, AJ, 153, 77). The wide field of view (70 degrees) of the HI-2 enabled selection of several M giant stars as comparison stars for transforming the instrumental magnitude of the star to the standard Johnson V-band system. These stars have similar Spectral Energy Distributions to Betelgeuse, minimizing color-V-mag corrections and permitting accurate V-mags to be computed. Exposure times varied between 0.5 and 2 sec in sequences of about 15 exposures.

V magnitudes are reported for 3 STEREO-A rolls: 2020 June 24.12 UT, V= 0.66 +/-0.015; 2020 July 13.87 UT, V=0.76 +/-0.015; 2020 July 20.54 UT, V=0.80+/-0.015. Here the V-magnitudes are means and errors are the standard deviation of the individual measurements that include transformation uncertainties. Surprisingly, instead of continuing to increase or level off in brightness, Betelgeuse has decreased by ~0.5 mag from mid-May to mid-July. Spanning the STEREO observations, the star dimmed at a rate of 5 mmag/day. The decrease in brightness over 26 days between our first and third observation is comparable to the photometric behavior in late October 2018, and slower than the faster decline, between October 22 and November 10 2019, that marked the beginning of the historic Great Fainting event in February 2020. The next 'minimum' according to the 420-430 d periodicity is predicted to occur in early April 2021. If this current decline continues, another dimming event may occur much earlier. However, Betelgeuse has (not-well understood) complex light variations and it is difficult to make predictions. It will be important to continue to follow Betelgeuse closely through 2020/21.

The HI instruments, part of the SECCHI suite on STEREO, were developed by a consortium that comprised the Rutherford Appleton Laboratory (UK),the University of Birmingham (UK), Centre Spatial de Liege (CSL, Belgium)and the Naval Research Laboratory (NRL, USA). We applaud the dedicated effort of the STEREO Mission Operations Center and the SECCHI Operations Team to acquire these observations.