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Article: A spatially resolved, semiempirical model for the extended atmosphere of α orionis (M2 IAB)

TitleA spatially resolved, semiempirical model for the extended atmosphere of α orionis (M2 IAB)
Authors
KeywordsCircumstellar Matter
Stars: Atmospheres
Stars: Chromospheres
Stars: Mass Loss
Supergiants
Issue Date2001
PublisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205
Citation
Astrophysical Journal Letters, 2001, v. 551 n. 2 PART 1, p. 1073-1098 How to Cite?
AbstractWe have constructed a detailed mean density and temperature model for the extended outer atmosphere of the O-rich supergiant Betelgeuse (α Ori [M2 Iab]), which extends from 1.0 to 10.0 stellar radii. A one-dimensional model is based on fitting NRAO1 VLA centimeter visibility data, and two-dimensional 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 R* the mean electron temperature increases to a value of ≃3800 K, then declines down below Teff, and then rises to photospheric values. The peak mean model temperature is less than the typical chromospheric temperatures found in previous models. Observations of Hα 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 ≃ 1.45 R*) with the dominant chromospheric UV emission region and find an areal filling factor of ≤1/4. In the extended semiempirical model the 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 two-dimensional 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 ∼ 2 greater than that derived from the one-dimensional model. However, previous Hα 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 11 wind absorption features provide an estimate of the mass-loss rate of 3.1(±1.3) × 10-6 M⊙ yr-1. This further implies that the cool material dominates the mass of the extended atmosphere and that the radio-emitting region is within the base of the outflow 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 R* where Tdust ∼ 360 K. This region lies outside the semiempirical model but simple extrapolations suggest that at this radius Te ∼ 220 K, and the mean hydrogen density nH ∼ 3 × 106 cm-3. We address the difficult question of whether the mean thermal model based on the radio data can be consistent with the observed off-limb Hα scattering emission if inhomogeneities are present.
Persistent Identifierhttp://hdl.handle.net/10722/174818
ISSN
2015 Impact Factor: 5.487
2015 SCImago Journal Rankings: 3.369
ISI Accession Number ID
References

 

DC FieldValueLanguage
dc.contributor.authorHarper, GMen_US
dc.contributor.authorBrown, Aen_US
dc.contributor.authorLim, Jen_US
dc.date.accessioned2012-11-26T08:47:39Z-
dc.date.available2012-11-26T08:47:39Z-
dc.date.issued2001en_US
dc.identifier.citationAstrophysical Journal Letters, 2001, v. 551 n. 2 PART 1, p. 1073-1098en_US
dc.identifier.issn2041-8205en_US
dc.identifier.urihttp://hdl.handle.net/10722/174818-
dc.description.abstractWe have constructed a detailed mean density and temperature model for the extended outer atmosphere of the O-rich supergiant Betelgeuse (α Ori [M2 Iab]), which extends from 1.0 to 10.0 stellar radii. A one-dimensional model is based on fitting NRAO1 VLA centimeter visibility data, and two-dimensional 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 R* the mean electron temperature increases to a value of ≃3800 K, then declines down below Teff, and then rises to photospheric values. The peak mean model temperature is less than the typical chromospheric temperatures found in previous models. Observations of Hα 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 ≃ 1.45 R*) with the dominant chromospheric UV emission region and find an areal filling factor of ≤1/4. In the extended semiempirical model the 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 two-dimensional 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 ∼ 2 greater than that derived from the one-dimensional model. However, previous Hα 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 11 wind absorption features provide an estimate of the mass-loss rate of 3.1(±1.3) × 10-6 M⊙ yr-1. This further implies that the cool material dominates the mass of the extended atmosphere and that the radio-emitting region is within the base of the outflow 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 R* where Tdust ∼ 360 K. This region lies outside the semiempirical model but simple extrapolations suggest that at this radius Te ∼ 220 K, and the mean hydrogen density nH ∼ 3 × 106 cm-3. We address the difficult question of whether the mean thermal model based on the radio data can be consistent with the observed off-limb Hα scattering emission if inhomogeneities are present.en_US
dc.languageengen_US
dc.publisherInstitute of Physics Publishing Ltd. The Journal's web site is located at http://iopscience.iop.org/2041-8205en_US
dc.relation.ispartofAstrophysical Journal Lettersen_US
dc.subjectCircumstellar Matteren_US
dc.subjectStars: Atmospheresen_US
dc.subjectStars: Chromospheresen_US
dc.subjectStars: Mass Lossen_US
dc.subjectSupergiantsen_US
dc.titleA spatially resolved, semiempirical model for the extended atmosphere of α orionis (M2 IAB)en_US
dc.typeArticleen_US
dc.identifier.emailLim, J: jjlim@hku.hken_US
dc.identifier.authorityLim, J=rp00745en_US
dc.description.naturelink_to_subscribed_fulltexten_US
dc.identifier.doi10.1086/320215en_US
dc.identifier.scopuseid_2-s2.0-0035918131en_US
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-0035918131&selection=ref&src=s&origin=recordpageen_US
dc.identifier.volume551en_US
dc.identifier.issue2 PART 1en_US
dc.identifier.spage1073en_US
dc.identifier.epage1098en_US
dc.identifier.isiWOS:000168383600037-
dc.publisher.placeUnited Kingdomen_US
dc.identifier.scopusauthoridHarper, GM=35554359900en_US
dc.identifier.scopusauthoridBrown, A=36008017300en_US
dc.identifier.scopusauthoridLim, J=7403453870en_US

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