Spectroscopic evidence for a large spot on the dimming Betelgeuse

From October 2019 to March 2020, Betelgeuse demonstrated an unusually deep minimum of its brightness captured the attention of astronomers all over the world. We present spectroscopic evidence that the dimming of Betelgeuse is likely caused by the appearance of a dark star-spot on its surface.

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  • Betelgeuse is the bright reddish star located in the shoulder of the Orion constellation and can be seen by naked eye in the night sky. Actually, it is a huge astronomical time bomb, which may explode anytime within 100,000 years and turn into Black Hole or neutron star. It is not a surprise why this star attracts much attention in professional astronomical community and mass media being an important astronomical laboratory for investigation of stellar evolution, stellar atmosphere, pre-supernova explosion, and dust condensation.
  • September 2019, I just had moved from Weihai to Beijing to join the research group of Professor Gang ZHAO in the National Astronomical Observatories, Chinese Academy of Sciences. My original goal was to investigate the non-local thermodynamic equilibrium (NLTE) line formation in the atmosphere of Betelgeuse and obtain its chemical composition.
  • During December 2019, the scientists from both Xinglong and Weihai Observatories in China reported about an unusual decreasing of the flux in optical spectral domain. Based on their photometric observations, it was clearly seen that something strange was happening with Betelgeuse and nobody knew what it was.  
  • January 2020, the brightness of Betelgeuse continued to decrease totally capturing attention and imagination astronomers and public alike. The rumors about imminent explosion of Betelgeuse started spreading in mass media.
  • Professor Gang ZHAO suggested me to study the possible physical cause behind this phenomenon with our advantage of spectroscopic analysis. We had many discussions about what a complex star Betelgeuse is. Many physical processes are involved there, such as, huge convective flows, pulsations, magnetic field, material ejection, and dust condensation. Some of them are not well-understood and cannot be considered in theoretical models.  
  • Professor Gang ZHAO applied for the high-resolution observations at Weihai Observatory of Shandong University, where Weihai Echelle Spectrograph (WES) – the first fiber-fed echelle spectrograph for astronomical observation in China – allows to cover the range up to 11,000 Å (Gao et al., 2016). First near-IR spectrum was obtained on 31st January 2020, when Betelgeuse was almost at the minimum of its brightness. Professor Shao-Ming HU and Dr. Kai LI from Shandong University provided observational time and Dr. Dong-Yang GAO carried out observations and made spectral reduction. We knew that Betelgeuse is the brightest star in the night sky in the near-infrared (near-IR) wavelength range. This is the most suitable wavelength range for investigating red supergiants like Betelgeuse.
  • I downloaded high-resolution spectra of Betelgeuse from the visible to the near-IR ranges obtained on February 14th 2012 with the Echelle Spectro Polarimetric Device for the Observation of Stars (ESPaDOnS) attached to the 3.6 m telescope of the CFHT Observatory in Hawaii (Donati et al. 2006). At the beginning of February 2020 there were two spectra in my hands, obtained at pre-dimming and dimming episodes.  Everything what I could say at that time: “They are different!” Why they are so different? To get the answer on this question, several investigations were planned and executed.
  • There were no reliable theoretical methods based on high-resolution spectra in the literature that made our investigations highly challenging. Previous studies employed the low resolution (R ∼ 1000) spectrophotometry to determine the effective temperatures of red supergiants. It took almost one month to develop a new method based on 7700 – 7900 Å spectral range, which contains more than 2300 molecular lines. The strategy using TiO molecules for temperature estimation is not new. A novelty is to apply this strategy to the high-resolution spectra.
  • The preliminary result showed that the effective temperature of Betelgeuse at the dimming epoch is lower by ~150/200 K compared to the pre-dimming one. On March 2020, the study of Levesque and Massey (2020) demonstrated that “Betelgeuse Just Is Not That Cool: Effective Temperature Alone Cannot Explain the Recent Dimming of Betelgeuse”. They proposed that the circumstellar dust along the sight line to Betelgeuse is the most likely explanation for its recent photometric evolution.
  • We had decided to continue the observations and investigations which showed interesting results which encouraged us to start working on the main text of manuscript. Professor Shao-Ming HU, Dr. Kai LI, and Dr. Dong-Yang GAO provided additional observations of Betelgeuse on March and April and, finally, we were able to cover the period of the recovering of brightness.
  • We found out that effective temperature and brightness’s recovering are correlated very well (see Fig. 1a,c in the manuscript). All co-authors were looking for a reasonable physical explanation: “What could lead to a temperature drop by 170 K?”
  • Professor Aigen LI from the University of Missouri (USA) was invited to discuss the role of the dust condensation in this phenomenon. The fact, that Betelgeuse is surrounding by gaseous nebula and dust particles is well-known and confirmed in many studies. We even tried to explain the temperature decreasing by involving dust. However, some non-physical ideas were rejected after discussions.         
  • It was also not clear for us at that time, whether it a whole star became cooler or only some part of it? If the whole star became cooler it would mean, that Betelgeuse almost overcame its Hayashi limit (the lower limit of effective temperature for stars in hydrostatic equilibrium) and moved to the forbidden region of the Hertzsprung-Russell (H-R) diagram, where would not be able to maintain its hydrostatic equilibrium.
  • The more reasonable solution came after looking at the images of Betegeuse’s surfacefrom the European Southern Observatory’s Very Large Telescope (ESO-VLT) before and after its unprecedented dimming in January and December 2019 provided by Montargès et al. (2020). It was clearly seen, that the star has faded and also its shape was seemingly changing indicating that something happened in the photosphere of the star or in its surroundings. The dark region seen in the optical VLT observations could be explained by large spot, whose temperature is lower than the typical temperature of Betelgeuse before the dimming.
  • The manuscript was ready for submission on July 2020. At that time, the study of  Dharmawardena et al. (2020) was published. Their submillimetre observations and modelling showed that this dimming is likely due to changes in the photosphere (luminosity) of the star.
  • We submitted our manuscript in Nature Communications on July 2020. Three months later, on October, we received three reviewers’ reports. There were many remarks and all of them were highly useful and constructive. We worked to improve the manuscript and resolved the review comments. We have included the approbation of the method by applying the method to other 15 well-known red supergiants. For four representative stars with effective temperatures from 3440 K to 3729 K, we show the best fits in Figure 8 (in manuscript). Most of the lines in this spectral region are TiO with small mixture of metals. Junju DU, researcher from Shandong University, improved the fitting technique by employing a Markov chain Monte Carlo (MCMC) forward-modeling method. It was very important to estimate the errors of the parameters (temperature and surface gravity).
  • On January 2021, we submitted the revised version and, finally, on May, it was accepted for publication and only editorial work was remaining.
  • We present the spectroscopic evidence of a large cool spot on the surface of Betelgeuse. The presence of spots on the surface of red supergiants like Betelgeuse is a well-known phenomenon. These spots are probably a consequence of convective flows or cool convective cells, which are widely believed to be present in such stars. The developed method can be useful for future investigations of the atmospheres of M-type stars, both dwarfs and supergiants. 

Sofya Alexeeva

LAMOST fellow, National Astronomical Observatories, Chinese Academy of Sciences

I got my PhD on May 2017 in Moscow (Russia). I studied the formation of spectral lines under conditions of non-local thermodynamic equilibrium (NLTE) in the atmospheres of B - K type stars in the Institute of Astronomy of the Russian Academy of Sciences. From Sept. 2017 to Sept. 2019, I was working as postdoctoral scientist at Shandong University in Weihai (China). Since Sept. 2019 till now -- LAMOST fellow in the National Astronomical Observatories, Chinese Academy of Sciences, Beijing (China).