Spectroscopic evidence for a large spot on the dimming Betelgeuse - PubMed (original) (raw)

Spectroscopic evidence for a large spot on the dimming Betelgeuse

Sofya Alexeeva et al. Nat Commun. 2021.

Erratum in

Abstract

During October 2019 and March 2020, the luminous red supergiant Betelgeuse demonstrated an unusually deep minimum of its brightness. It became fainter by more than one magnitude and this is the most significant dimming observed in the recent decades. While the reason for the dimming is debated, pre-phase of supernova explosion, obscuring dust, or changes in the photosphere of the star were suggested scenarios. Here, we present spectroscopic studies of Betelgeuse using high-resolution and high signal-to-noise ratio near-infrared spectra obtained at Weihai Observatory on four epochs in 2020 covering the phases of during and after dimming. We show that the dimming episode is caused by the dropping of its effective temperature by at least 170 K on 2020 January 31, that can be attributed to the emergence of a large dark spot on the surface of the star.

© 2021. The Author(s).

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1. The mean visual magnitude (V), surface gravity (log g), effective temperature (_T_eff), and metallicity ([Fe/H]) of Betelgeuse obtained on 2020 January 31, March 19, April 4, and April 6.

a Each V magnitude was calculated by averaging over all available magnitudes in the AAVSO database in a particular date of observations. b The uncertainty of surface gravity (log g) is taken as the difference between the 84th and 50th percentile as the upper limit, and the difference between the 50th and 16th percentile as the lower limit. c The uncertainty of the effective temperature (_T_eff) is taken as the difference between the 84th and 50th percentile as the upper limit, and the difference between the 50th and 16th percentile as the lower limit. d Metallicity ([Fe/H]) was calculated by averaging over all iron abundances from individual lines in a particular date of observations. The two horizontal dashed lines show the ranges of parameters (V, _T_eff, log g, [Fe/H]) for pre-dimming, which we have obtained with ESPaDOnS’s spectrum observed on 2012 February, 14. Error bars represent standard deviation. Source data are provided as a Source Data file.

Fig. 2

Fig. 2. The schemes of the stellar surface with a dark spot and the corresponding best fits of the narrow spectral region 7705–7811 Å for four dates of observations.

The temperature increasing is interpreted as the changing of the surface area of a dark spot. Relative flux is calculated as a ratio of the flux at a particular wavelength (_F_λ) to the flux in continuum (_F_c). The observed spectra are shown as brown dots. The theoretical spectra (black solid curves) were calculated with the obtained parameters from Table 1 and convolved with the instrumental profile. Everywhere V_mic = 3 km s−1, V sin_i = 5 km s−1, _V_mac = 16 km s−1. Titanium, carbon, nitrogen, and oxygen abundances are taken to be 4.95, 8.43, 8.60, and 8.80 dex, respectively. The molecular TiO (titanium oxide) lines are marked by vertical lines on each panel. a On 31-01-2020, the temperature of the spot is 3306 K if it occupies 50% of the surface area. b On 19-03-2020, the temperature of a spot is 3306 K if it occupies 33% of the surface area. c On 04-04-2020, the temperature of a spot is 3306 K if it occupies 10% of the surface area. d On 06-04-2020, the temperature of the whole star is 3646 K. Source data are provided as a Source Data file.

Fig. 3

Fig. 3. Conditional chemical abundances of carbon, nitrogen, and oxygen in the atmosphere of Betelgeuse at four different epochs: 31-01-2020, 19-03-2020, 04-04-2020, and 06-04-2020.

Chemical abundances of carbon, nitrogen, and oxygen are calculated under three scenarios. First scenario is effective temperature _T_eff = var, the chemical abundances are obtained at parameters according to Table 1 (blue filled circles). Second scenario is the chemical abundances are obtained at the assumption of constant effective temperature _T_eff = 3646 K and log g = 0.2 (green open squares). Third scenario is the chemical abundances are obtained at the assumption of constant effective temperature _T_eff = 3476 K and log g = 0.2 (pink open diamonds). a Carbon abundances calculated under three scenarios. b Nitrogen abundances calculated under three scenarios. c Oxygen abundances calculated under three scenarios. The dashed lines show solar abundances obtained in this study. Error bars represent standard deviation. Source data are provided as a Source Data file.

Fig. 4

Fig. 4. Chemical composition in the atmosphere of Betelgeuse at four different epochs: 31-01-2020, 19-03-2020, 04-04-2020, and 06-04-2020.

a Na abundances calculated under three scenarios. b Mg abundances calculated under three scenarios. c Ca abundances calculated under three scenarios. d Ti abundances were calculated under three scenarios. e Cr abundances were calculated under three scenarios. f Fe abundances were calculated under three scenarios. g Sr abundances calculated under three scenarios. Everywhere, the meanings of symbols are the same as in Fig. 3. The dashed lines show solar abundances obtained in this study. Error bars represent standard deviation. Source data are provided as a Source Data file.

Fig. 5

Fig. 5. The different strength of CN (cyanide) molecular line at 10,179 Å at different epochs can be explained by different temperatures at these epochs.

Relative flux is calculated as a ratio of the flux at a particular wavelength (_F_λ) to the flux in continuum (_F_c). The spectral observations are shown by dots. Solid curves are theoretical spectra calculated at the particular temperature specified on each panel, convolved with the instrumental profile. Carbon, nitrogen, and oxygen abundances are 8.43, 8.60, and 8.80 dex, respectively. This line was presented as an example. Similar behavior was found for other CN lines at λ λ 10,223, 10,367, 10,376, and 10,377 Å. a The CN molecular line became stronger on 14-02-2012 compared to 31-01-2020. On the b, the comparison of CN lines observed on 14-02-2012 and 19-03-2020. c This CN molecular line is stronger on 19-03-2020 compared to 04-04-2020, while CN molecular lines look similar at both epochs 14-02-2012 and 06-04-2020 for which the temperature difference is only 13 K (d). Source data are provided as a Source Data file.

Fig. 6

Fig. 6. Four corner plots with the final effective temperatures (_T_eff) of Betelgeuse obtained at four epochs 31-01-2020, 19-03-2020, 04-04-2020, and 06-04-2020.

a The best-fit _T_eff and its uncertainties were obtained on 31-01-2020. The panel corresponding to the fit of the region at 7700–7900 Å is shown in Supplementary Fig. 4a. b The best-fit _T_eff and its uncertainties were obtained on 19-03-2020. The panel corresponding to the fit of the region at 7700–7900 Å is shown in Supplementary Fig. 4b. c The best-fit _T_eff and its uncertainties were obtained on 04-04-2020. The panel corresponding to the fit of the region at 7700–7900 Å is shown in Supplementary Fig. 4c. d The best-fit _T_eff and its uncertainties were obtained on 06-04-2020. The panel corresponding to the fit of the region at 7700–7900 Å is shown in Supplementary Fig. 4d. On each panel, the diagonal shows the marginalized posteriors. The subsequent covariances between all the parameters are in the corresponding 2D histograms. The vertical lines represent the 16, 50, and 84 percentiles. The best-fit parameters for _T_eff and their uncertainties are presented on the top of each panel. Uncertainty for _T_eff is defined as the difference between the 84th and 50th percentile as the upper limit, and the difference between the 50th and 16th percentile as the lower limit. The data of observations are marked in dd-mm-yyyy format.

Fig. 7

Fig. 7. Best fits to the molecular bands of TiO for five sets of spectra.

The observed spectra of Betelgeuse obtained on 31-01-2020 (a), 19-03-2020 (b), 04-04-2020 (c), 06-04-2020 (d), and 14-02-2012 (e) are shown by brown dots. The solid line is a theoretical profile calculated with the particular effective temperature (_T_eff) and surface gravity (log g), which were obtained in this study and presented on each panel. Relative flux is calculated as a ratio of the flux at a particular wavelength (_F_λ) to the flux in continuum (_F_c). The 2365 TiO lines are marked by vertical lines on the a. Source data are provided as a Source Data file.

Fig. 8

Fig. 8. The best fits of the parts of spectra of Betelgeuse in 7750–7800 Å range for four represented supergiant stars.

Relative flux is calculated as a ratio of the flux at a particular wavelength (_F_λ) to the flux in continuum (_F_c). The solid black line is a theoretical profile calculated with the particular _T_eff obtained in this study. The observed spectra are shown by brown dots. The name of the star and its _T_eff are presented on each panel. a The best-fit for the coolest star in our sample, α Her, with effective temperature (_T_eff) 3440 K. b The best-fit for VX Sgr with _T_eff = 3533 K. c The best-fit for HD 217906 with _T_eff = 3621 K. d The best-fit for HD 146051 with _T_eff = 3729 K. Source data are provided as a Source Data file.

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