Winds and Chromospheres of Cool (Super-) Giants

Crowley, Cian; Espey, B. R.; Harper, G. M.; Roche, Joseph

doi:10.1063/1.3099101

Cited by 2 publications

(1 citation statement)

References 12 publications

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“…However, as Airapetian et al (2010) point out, this simulation assumes fully-ionized plasma, although the gas around K-M giants is only weakly ionized. The MHD simulation of Airapetian et al (2010) reproduces the observed terminal wind velocities and mass-loss rates of K giants, but the agreement between the modeled and observed radial velocity fields is not yet satisfactory (a comparison of the model of Airapetian et al 2010 and the observed data summarized in Crowley et al 2009 is given in Harper 2011). Since the simulation of Airapetian et al (2010) assumes a nearly iso-thermal chromosphere, it does not include the cool, molecular component.…”

Section: Discussionmentioning

confidence: 99%

Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER

Ohnaka¹,

Hofmann²,

Schertl³

et al. 2012

A&A

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Context. The mass-loss mechanism in normal K-M giant stars with small variability amplitudes is not yet understood, although the majority among red giant stars are precisely of this type. Aims. We present high-spatial and high-spectral resolution observations of the M7 giant BK Vir with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the physical properties of the outer atmosphere by spatially resolving the star in the individual CO first overtone lines. Methods. BK Vir was observed between 2.26 and 2.31 μm using the 16-32-48 m telescope configuration with an angular resolution of 9.8 mas and a spectral resolution of 12 000. Results. The uniform-disk diameters observed in the CO first overtone lines are 12−31% larger than those measured in the continuum. We also detected asymmetry in the CO line-forming region, which manifests itself as non-zero/non-π differential and closure phases. The data taken 1.5 months apart show possible time variation on a spatial scale of 30 mas (corresponding to 3× stellar diameter) at the CO band head. Comparison of the observed data with the MARCS photospheric model shows that whereas the observed CO line spectrum can be satisfactorily reproduced by the model, the angular sizes observed in the CO lines are much larger than predicted by the model. Our model with two additional CO layers above the MARCS photosphere reproduces the observed spectrum and interferometric data in the CO lines simultaneously. This model suggests that the inner CO layer at ∼1.2 R is very dense and warm with a CO column density of ∼10 22 cm −2 and temperatures of 1900−2100 K, while the outer CO layer at 2.5−3.0 R is characterized by column densities of 10 19 −10 20 cm −2 and temperatures of 1500−2100 K. Conclusions. Our AMBER observations of BK Vir have spatially resolved the extended molecular outer atmosphere of a normal M giant in the individual CO lines for the first time. The temperatures derived for the CO layers are higher than, or equal to, the uppermost layer of the MARCS photospheric model, implying the operation of some heating mechanism in the outer atmosphere. This study also illustrates that testing photospheric models only with the spectra of strong molecular or atomic features can be misleading.

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Section: Discussionmentioning

confidence: 99%

Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER

Ohnaka¹,

Hofmann²,

Schertl³

et al. 2012

A&A

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Multi-Wavelength Radio Continuum Emission Studies of Dust-Free Red Giants

O’Gorman

Harper

Brown

et al. 2013

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Multi-wavelength centimeter continuum observations of non-dusty, non-pulsating K spectral-type red giants directly sample their chromospheres and wind acceleration zones. Such stars are feeble emitters at these wavelengths however, and previous observations have provided only a small number of modest signal-to-noise measurements slowly accumulated over three decades. We present multi-wavelength Karl G. Jansky Very Large Array thermal continuum observations of the wind acceleration zones of two dust-free red giants, Arcturus (α Boo: K2 III) and Aldebaran (α Tau: K5 III). Importantly, most of our observations of each star were carried out over just a few days, so that we obtained a snapshot of the different stellar atmospheric layers sampled at different wavelengths, independent of any long-term variability. We report the first detections at several wavelengths for each star including a detection at 10 cm (3.0 GHz: S band) for both stars and a 20 cm (1.5 GHz: L band) detection for α Boo. This is the first time single (non-binary) luminosity class III red giants have been detected at these continuum wavelengths. Our long-wavelength data sample the outer layers of α Boo's atmosphere where its wind velocity is approaching (or possibly has reached) its terminal value and the ionization balance is becoming frozen-in. For α Tau, however, our long-wavelength data are still sampling its inner atmosphere, where the wind is still accelerating probably due to its lower mass-loss rate. We compare our data with published semi-empirical models based on ultraviolet data, and the marked deviations highlight the need for new atmospheric models to be developed. Spectral indices are used to discuss the possible properties of the stellar atmospheres, and we find evidence for a rapidly cooling wind in the case of α Boo. Finally, we develop a simple analytical wind model for α Boo based on our new long-wavelength flux measurements.

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Winds and Chromospheres of Cool (Super-) Giants

Cited by 2 publications

References 12 publications

Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER

Spatially resolving the outer atmosphere of the M giant BK Virginis in the CO first overtone lines with VLTI/AMBER

Multi-Wavelength Radio Continuum Emission Studies of Dust-Free Red Giants

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