Making sense of the spectral line profiles of Betelgeuse and other red supergiants

Spectropolarimetry of atomic lines in the spectra of Betelgeuse and other red supergiants (RSG) typically presents broad line profiles in linear polarization, but narrow profiles in intensity. By contrast, recent observations of the RSG RW Cep show broad intensity profiles, comparable to those in linear polarization. This observation suggests that the difference in the Stokes Q/U and I profile widths noted in many RSGs arises from the temporary atmospheric conditions of a given star. We propose an explanation for both cases based on the presence of strong velocity gradients steeper than the thermal broadening of the spectral line. Using simple analytical radiative transfer, we computed intensity line profiles in such scenarios. We find that they qualitatively match the observed broadenings: steep gradients are required for the narrow profiles of Betelgeuse, while shallow gradients are required for the broad profiles of RW Cep. Profile bisectors are also reasonably well explained by this scenario, despite the simple radiative transfer treatment used. These results give a comprehensive explanation of the intensity and polarization profiles. They also support the approximation of a single-scattering event used to explain the observed linear polarization in images inferred for the photosphere of Betelgeuse and other RSGs such as RW Cep, µ Cep and CE Tau. The atmospheres of RSGs appear capable, perhaps cyclically, of either producing steep velocity gradients that prevent photospheric plasma from reaching the upper atmosphere and thus hinder major mass-loss events, or allowing vertical movements to proceed unchanged, enabling plasma to rise, escape gravity, and form large dust clouds in the circumstellar environment. The origin of the velocity gradient and its modulation within the atmosphere remains an open question.