A Spatially Resolved, Semiempirical Model for the Extended Atmosphere of α Orionis (M2 Iab)

Abstract

We have constructed a detailed mean density and temperature model for the extended outer atmosphere of the O-rich supergiant Betelgeuse (a Ori [M2 Iab]), which extends from 1.0 to 10.0 stellar radii. A one-dimensional model is based on Ðtting NRAO1 VLA centimeter visibility data, and twodimensional models are constructed using the intensity contours of the 0.7 cm observations of Lim et al. As one moves in toward the star from about 10 the mean electron temperature increases to a value R * of^3800 K, then declines down below and then rises to photospheric values. The peak mean T eff , model temperature is less than the typical chromospheric temperatures found in previous models. Observations of Ha and chromospheric ultraviolet (UV) emission show that higher temperature components must also exist, but they do not dominate the weighted mean temperature structure. We tentatively identify the radius where the temperature distribution peaks (R^1.45with the dominant chromospheric R * ) UV emission region and Ðnd an areal Ðlling factor ofIn the extended semiempirical model the ¹1 4 . dominant source of electrons is from photoionized metals and is dominated by carbon. The low ionization of hydrogen leads to a dominance by H~(free-free) opacity at centimeter wavelengths. We derive simple estimates of the radio spectral indices for other similar M supergiants. We have constructed twodimensional models to examine whether the intensity asymmetry observed at 0.7 cm is most likely to result from density or temperature variations. Adopting an elliptical two-dimensional model, a density asymmetry along the axes of symmetry would need to be 20 : 1. If we assume the radial wind velocity is independent of angle the integrated mass-loss rate is only a factor of D2 greater than that derived from the one-dimensional model. However, previous Ha speckle observations that sample the same spatial regions suggest the asymmetry observed at 0.7 cm is not due to such a large-scale density asymmetry. A modest change in temperature can more easily provide the asymmetry, increasing both the opacity and the thermal source term. If the radial density structure is assumed to be the same as in 1992 September, when HST /GHRS spectra were obtained, then the Fe II wind absorption features provide an estimate of the mass-loss rate of 3.1(^1.3) ] 10~6yr~1. This further implies that the cool material dominates M _ the mass of the extended atmosphere and that the radio-emitting region is within the base of the outÑow observed in the circumstellar layers. A simple silicate dust model is constructed and the semiempirical model suggests an onset of dust formation at R^33 where K. This region lies outside R * T dust D 360 the semiempirical model but simple extrapolations suggest that at this radius K, and the mean T e D 220 hydrogen density cm~3. We address the difficult question of whether the mean thermal n H D 3 ] 106 model based on the radio data can be consistent with the observed o †-limb Ha scattering emission if inhomogeneities are present.

Imaging the spotty surface of Betelgeuse in theHband

“…Danchi et al 1994). The radio emission mostly arises from free-free processes in the gaseous envelope (Harper et al 2001). The UV emission is a result of a combination of scattered and fluorescent line photons, electron collisional excitation, and bound-free recombination in the chromosphere and/or ionized wind (Harper et al 2001;Lobel & Dupree 2001).…”

“…The radio emission mostly arises from free-free processes in the gaseous envelope (Harper et al 2001). The UV emission is a result of a combination of scattered and fluorescent line photons, electron collisional excitation, and bound-free recombination in the chromosphere and/or ionized wind (Harper et al 2001;Lobel & Dupree 2001). Although the H band covers an opacity minimum of the combined free-free and bound-free emission from both chromosphere and (partly) ionized wind in the gaseous envelope, we can not exclude these emission processes as the origin of the observed environment without detailed calculations based on the models which were constructed to explain the UV and radio observations (Harper et al 2001, e.g.).…”

The dust condensation sequence in red supergiant stars

“…Free-free emission from a chromosphere or ionised wind can be present in RSG (e.g. Harper et al 2001), but since its source function has a temperature above that of the photosphere, it cannot explain the additional extinction towards the near-IR 7 . The colour temperature of the excess is therefore indicative of dust rather than free-free emission.…”

The inhomogeneous submillimeter atmosphere of Betelgeuse

“…However, for the earlier M-type RSGs such as Betelgeuse the dominant source of opacity at submm wavelengths is from H − opacity; thermal free-free interactions between free electrons from photoionized metals and neutral hydrogen (Harper et al 2001). At these wavelengths, the thermal source function is simply the Planck function in the RayleighJeans limit, which is linear with the gas (electron) temperature.…”