An observing program was executed close to the Betelgeuse's V-band minimum in February 2020 under several Director's Discretionary Time (DDT) programs.

These observations obtained with the EXES instrument were focused on high-spectral resolution spectra around 25 microns, encompassing forbidden [Fe II], [S I] and two water absorption features. Similar data were already obtained in 2015 and 2017, allowing evaluations of variations in line flux and width over time, with sufficient resolution (R~50000) to identify gas velocity changes.

Preliminary data appear to show that the water features at 25.24 microns are significantly deeper in 2020 compared to 2017. However, modeling is needed to confirm the celestial origin of the variation of the water features.

Graham Harper, University of Colorado Boulder, and his team demonstrated that [Fe II] and [S I] emission lines, originating from circumstellar regions well above the photosphere, display no significant change from 2015 to 2020 (paper published in ApJL). Their results suggest that while dust heating can be very sensitive to photometric variations, circumstellar gas in the regions probed by [Fe II] and [S I] lines may not be significantly heated by dust.

All EXES Betelgeuse calibrated data, including from 2020 observations, are publicly available on the IRSA archive under program IDs 75_0051, 05_0073, and 02_004.

The spectral orders near the [S I] 25.25 µm line

The spectral orders near the [S I] 25.25 µm line. The CY05 (red) and CY07 (blue) are uncorrected for order shape, and the CY05 Metis spectrum is shown in grey highlighting how the order shape can affect the stellar spectra. The most significant difference between CY05 and CY07 are the deepening of the strong water absorption lines in CY07 near 394.9 cm−1 . The bottom green line shows the sky spectrum. The spherical MARCS model spectra, normalized to 100 Jy, are described in Fig. 2. Again there is not a strong correspondence between the photospheric models and the observed structure, in particular the feature on the high frequency side of the [S I] 25.25 µm emission.

Harper et al. 2020
EXES
Absorption lines
Emission lines
Spectroscopy
atmospheres and chromospheres
dust
giants and AGB stars
massive stars
stellar evolution and models
Stellar Activity
variability
variable and pulsating stars