Electron Density and Turbulence Gradients within the Extended Atmosphere of the M Supergiant Betelgeuse (α Orionis)

The extended atmosphere of the M supergiant Betelgeuse is complex with cool plasma dominating the structure by mass and small amounts of embedded hotter chromospheric plasma. A major challenge is to understand the interrelationship and juxtaposition of these different components, which in turn may provide clues to the nature of the process of nonradiative heating and the mechanisms that drive mass loss. We examine the chromospheric C II] λ2325 multiplet emission line electron density diagnostic using spatially scanned HST STIS echelle spectra. Escape probability models for the electron density-sensitive line ratios reveal that the mean electron density decreases by 0.6 dex as the sight line goes from disk center to ±75 mas. Radiative transfer simulations using spherical model atmospheres show that this trend can be explained if the electron density declines with radius by nearly 2 dex across ΔR ~ 2R*. The emission profiles indicate that the chromospheric material corotates with the star and then becomes decoupled by ±75 mas from disk center. We find no evidence for radial outflow in the chromospheric plasma. We find that the strongest C II] λ2325 emission lines are opacity broadened and that the gradient of atmospheric turbulence is surprisingly small. Using empirical constraints, we derive a relation between the relative C II column densities in the cool and chromospheric atmospheric components and the excitation temperature. These UV chromospheric results and previous radio analyses suggest that the chromosphere is pervasive but has a small filling factor at ~3R*, suggestive of confinement and heating in magnetic structures.