Modeling the variable chromosphere of α Orionis

A spectral analysis of the prototypical red supergiant star a Ori that is based on near-UV, optical, and near-IR high-dispersion spectra obtained between 1992 September and 1999 July with the Space Telescope Imaging Spectrograph and the Goddard High Resolution Spectrograph on the Hubble Space Telescope, the Utrecht Echelle Spectrograph, and the SoFin Echelle Spectrograph is presented. With detailed non-LTE radiative transfer calculations in spherical geometry, we model the mean conditions in the stellar chromosphere from H alpha and the Mg II resonance doublet. The H alpha absorption line emerges from an extended chromosphere. Temporal changes of its velocity structure are determined from detailed fits to near-UV Si I lines, and chromospheric expansion velocities around 4 km s(-1) are found in 1992, whereas the chromosphere was collapsing onto the photosphere with a velocity of 5 km s(-1) in 1998- 1999. The H alpha core depth is correlated over time with weaker depression changes seen in prominent TiO band heads that dominate the optical spectrum. From elaborate spectral synthesis calculations, we isolate unblended metal absorption lines in the near-IR and determine T-eff = 3500 K and log (g) = -0.5 for solar metallicity and 12 +/- 0.5 km s(-1) for macrobroadening and v sin i. Semiempirical fits yield chromospheric temperatures not in excess of 5500 K, but with long-term changes by similar to 400 K. The model extends over 5000 R-. and requires supersonic microturbulence values ranging to 19 km s(-1), in strong contrast with the photospheric value of only 2 km s(-1). We observe Doppler shifts of 4-8 km s(-1) in the scattering cores of many double-peaked near-UV emission lines which correlate with changes in the intensity ratio of their emission components. The red emission components were much stronger in 1992, indicating a phase of enhanced chromospheric outflow, for which we determine a spherical mass-loss rate of 6 x 10(-7) M-. yr(-1). We present a discussion of chromospheric pulsation in this massive star. Detailed modeling of the observed Mg II h and k line asymmetry is also presented. We demonstrate that a chromospheric Mn I blend strongly contributes to this puzzling asymmetry.