Radio continuum emission from winds, chromospheres, and coronae of cool giants and supergiants

Drake, S. A.; Linsky, J. L.

doi:10.1086/114043

Cited by 60 publications

(60 citation statements)

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“…This is the clearest demonstration of such a correlation for red giants, though past Mg II observations from IUE have also suggested a trend of this nature (Drake 1986;Drake & Linsky 1986;Judge & Stencel 1991). There is some ambiguity with regards to whether the optical spectra of Ca II H & K wind absorption support a wind velocity correlation with spectral type.…”

Section: The Stellar Wind Absorption Profilesmentioning

confidence: 70%

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

Wood

Müller

Harper

2016

ApJ

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We report on an ultraviolet spectroscopic survey of red giants observed by the Hubble Space Telescope, focusing on spectra of the Mg II h & k lines near 2800Å in order to study stellar chromospheric emission, winds, and astrospheric absorption. We focus on spectral types between K2 III and M5 III, a spectral type range with stars that are noncoronal, but possessing strong, chromospheric winds. We find a very tight relation between Mg II surface flux and photospheric temperature, supporting the notion that all K2-M5 III stars are emitting at a basal flux level. Wind velocities (V w ) are generally found to decrease with spectral type, with V w decreasing from ∼ 40 km s −1 at K2 III to ∼ 20 km s −1 at M5 III. We find two new detections of astrospheric absorption, for σ Pup (K5 III) and γ Eri (M1 III). This absorption signature had previously only been detected for α Tau (K5 III). For the three astrospheric detections the temperature of the wind after the termination shock correlates with V w , but is lower than predicted by the Rankine-Hugoniot shock jump conditions, consistent with the idea that red giant termination shocks are radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic simulation of the γ Eri astrosphere is provided to explore this further.

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Section: The Stellar Wind Absorption Profilesmentioning

confidence: 70%

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

Wood

Müller

Harper

2016

ApJ

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“…An extreme case of this scenario would be the existence of an outer cutoff radius r 0 to the ionised portion of the wind. Although doubtful for the targets in our study, this cutoff might be caused for example by the formation of dust at this location in the wind, "quenching" the ionised material (Drake & Linsky 1986). The spectral index would change for ν < ν 0 , where ν 0 is determined by the cutoff radius.…”

Section: Thermal Emission From An Ionised Windmentioning

confidence: 77%

Structure of the outer layers of cool standard stars

Dehaes

Bauwens

Decin

et al. 2011

A&A

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Context. Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims. Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. The goal of this study is to assess wether a set of standard near-infrared calibration sources are fiducial calibrators in the far-infrared, beyond 50 μm. Methods. The observational spectral energy distributions were compared with the theoretical model predictions for a sample of nine K-and M-giants. The discrepancies found are explained using basic models for flux emission originating in a chromosphere or an ionised wind. Results. For seven out of nine sample stars, a clear flux excess is detected at (sub)millimetre and/or centimetre wavelengths, while only observational upper limits are obtained for the other two. The precise start of the excess depends upon the star under consideration. For six sources the flux excess starts beyond 210 μm and they can be considered as fiducial calibrators for Herschel/PACS (60-210 μm). Out of this sample, four sources show no flux excess in the Herschel/SPIRE wavelength range (200-670 μm) and are good calibration sources for this instrument as well. The flux at wavelengths shorter than ∼1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionised wind is the main flux contributor at longer wavelengths. Conclusions. Although the optical to mid-infrared spectrum of the studied K-and M-type infrared standard stars is represented well by a radiative equilibrium atmospheric model, a chromosphere and/or ionised stellar wind at higher altitudes dominates the spectrum in the (sub)millimetre and centimetre wavelength ranges. The presence of a flux excess has implications on the role of the stars as fiducial spectrophotometric calibrators in these wavelength ranges.

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“…(free-free) opacity dominated, for example in a warm wind of a hybrid bright giant, e.g., (Harper et al 1995), then for the same power-law model we would have The S l P l1.28. di †erence in spectral index is quite small, especially given the typical S/N of observations and the di †erent wavelengths used to construct an empirical spectral index (see, e.g., Drake & Linsky 1986 ;Drake, Linsky, & Elitzur 1987). In order to use empirical spectral indices to constrain stellar atmospheric models, it is imperative that simultaneous high S/N observations are made at as many wavelengths as is feasible.…”

Section: Controlling Physicsmentioning

confidence: 99%

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

Harper

Brown

Lim

2001

ApJ

116

135

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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.

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Radio continuum emission from winds, chromospheres, and coronae of cool giants and supergiants

Cited by 60 publications

References 0 publications

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars*

Structure of the outer layers of cool standard stars

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

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