Structure of the outer layers of cool standard stars

Abstract

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. Methods: The observational spectral energy distributions are compared with the theoretical predictions of the MARCS atmosphere models for a sample of 9 K- and M-giants. The discrepancies found are explained using basic models for flux emission originating from a chromosphere or an ionized wind. Results: For 7 out of 9 sample stars, a clear flux excess is detected at (sub)millimeter and/or centimeter wavelengths. The precise start of the excess depends upon the star under consideration. The flux at wavelengths shorter than about 1 mm is most likely dominated by an optically thick chromosphere, where an optically thick ionized wind is the main flux contributor at longer wavelengths. Conclusions: Although the optical to mid-infrared spectrum of the studied K- and M-giants is well represented by a radiative equilibrium atmospheric model, the presence of a chromosphere and/or ionized stellar wind at higher altitudes dominates the spectrum in the (sub)millimeter and centimeter wavelength ranges. The presence of a flux excess also has implications on the role of these stars as fiducial spectrophotometric calibrators in the (sub)millimeter and centimeter wavelength range.

13 pages, 6 figures, 7 pages of online material, submitted to A&A

References in corpus (7)

• The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory
• Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. II. 70 micron Imaging
• Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
• Theoretical model atmosphere spectra used for the calibration of infrared instruments
• Structure of the outer layers of cool standard stars
• Continuum emission around AGB stars at 1.2 mm
• Carbon monoxide in the solar atmosphere II. Radiative cooling by CO lines

Cited by in corpus (13)

• The Herschel-PACS photometer calibration: Point-source flux calibration for scan maps
• Herschel celestial calibration sources: Four large main-belt asteroids as prime flux calibrators for the far-IR/sub-mm range
• Structure of the outer layers of cool standard stars
• The inner dust shell of Betelgeuse detected by polarimetric aperture-masking interferometry
• The Herschel PACS photometer calibration - A time dependent flux calibration for the PACS chopped point-source photometry AOT mode
• Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER
• Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars VI. First chromosphere model of a late-type giant
• Far-infrared photometric observations of the outer planets and satellites with Herschel-PACS
• High spectral resolution spectroscopy of the SiO fundamental lines in red giants and red supergiants with VLT/VISIR
• Multi-wavelength Radio Continuum Emission Studies of Dust-free Red Giants
• Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 micron spectra
• Herschel-PACS photometry of faint stars
• Spectroscopic analysis of cool giants and supergiants