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Vigorous atmospheric motion in the red supergiant star Antares

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Abstract

Red supergiant stars represent a late stage of the evolution of stars more massive than about nine solar masses, in which they develop complex, multi-component atmospheres. Bright spots have been detected in the atmosphere of red supergiants using interferometric imaging1,2,3,4,5. Above the photosphere of a red supergiant, the molecular outer atmosphere extends up to about two stellar radii6,7,8,9,10,11,12,13,14. Furthermore, the hot chromosphere (5,000 to 8,000 kelvin) and cool gas (less than 3,500 kelvin) of a red supergiant coexist at about three stellar radii15,16,17,18. The dynamics of such complex atmospheres has been probed by ultraviolet and optical spectroscopy19,20,21,22. The most direct approach, however, is to measure the velocity of gas at each position over the image of stars as in observations of the Sun. Here we report the mapping of the velocity field over the surface and atmosphere of the nearby red supergiant Antares. The two-dimensional velocity field map obtained from our near-infrared spectro-interferometric imaging reveals vigorous upwelling and downdrafting motions of several huge gas clumps at velocities ranging from about −20 to +20 kilometres per second in the atmosphere, which extends out to about 1.7 stellar radii. Convection alone cannot explain the observed turbulent motions and atmospheric extension, suggesting that an unidentified process is operating in the extended atmosphere.

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Figure 1: Reconstructed velocity-resolved images of Antares.
Figure 2: Spatially resolved spectra over the stellar disk and atmosphere of Antares.
Figure 3: Velocity field map over the stellar disk and atmosphere of Antares.

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Acknowledgements

We thank the ESO VLTI team for supporting our VLTI/AMBER observations. This work is based on AMBER observations made with the VLTI of the ESO (program ID: 093.D-0468A/B). K.O. acknowledges the grant from the Universidad Católica del Norte.

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Authors

Contributions

K.O. wrote the telescope proposal and the first draft of the paper, carried out the observations, data reduction, and image reconstruction, and worked on data interpretation. G.W. and K.-H.H. were co-authors on the telescope proposal and worked on data reduction and interpretation.

Corresponding author

Correspondence to K. Ohnaka.

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Extended data figures and tables

Extended Data Figure 1 The u–v coverage of our VLTI/AMBER observations with four different Auxiliary Telescope configurations.

Extended Data Figure 2 Limb-darkened disk fit to the AMBER data of Antares.

a, Power-law-type limb-darkened (LD) disk diameter as a function of wavelength (red dots). The scaled, observed spectrum is shown in black. b, Limb-darkening parameter as a function of wavelength. In both panels, the error bars represent 1σ.

Extended Data Figure 3 Limb-darkened disk fit to the observed visibilities as a function of spatial frequency.

The fit at a wavelength channel is shown in the continuum in a and b, in the CO band head in c and d, and at the centre of one of the CO lines in e and f. The observed visibilities are plotted with the dots with the 1σ errors computed over Nf × Nexp frames as listed in Extended Data Table 1. The limb-darkened disk fit is shown with the curves.

Extended Data Figure 4 Comparison between the measured interferometric observables and those from the images reconstructed near the CO band head.

The filled circles in the top row (ad) show the wavelength channels in the observed spectrum, which correspond to the wavelengths of the images shown in Fig. 1a–d. The second, third, fourth and fifth rows show comparisons of the visibility at spatial frequencies lower than 55 arcsec−1, visibility at spatial frequencies higher than 55 arcsec−1, Fourier phase, and closure phase, respectively. In these panels, the observed data are plotted by the red dots with the error bars (1σ as described in the legend of Extended Data Fig. 3). The blue triangles represent the values from the image reconstruction. The reduced values including the visibilities, Fourier phases and closure phases, are given in the bottom row.

Extended Data Figure 5 Comparison between the measured interferometric observables and those from the images reconstructed across one of the CO lines.

The panels are shown in the same manner as Extended Data Fig. 4. The filled circles in the top row (ad) show the wavelength channels in the observed spectrum, which correspond to the wavelengths of the images shown in Fig. 1e–h.

Extended Data Table 1 Summary of VLTI/AMBER observations of Antares

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Ohnaka, K., Weigelt, G. & Hofmann, KH. Vigorous atmospheric motion in the red supergiant star Antares. Nature 548, 310–312 (2017). https://doi.org/10.1038/nature23445

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