Far-Infrared and Millimeter Continuum Studies of K Giants: α Bootis and α Tauri

Cohen, Martin; Carbon, D. F.; Welch, William J.; Lim, T.; Schulz, B.; McMurry, A. D.; Forster, J. R.; Goorvitch, D.

doi:10.1086/429887

Cited by 9 publications

(13 citation statements)

References 46 publications

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“…It can be noted that Cohen et al (2005) find that the continuum of α Boo is reasonably well modelled by a classical model photosphere in the region of 40−125 μm, that is, wavelengths that sample the upper photosphere, but they also show that it is not until wavelengths beyond 1−3 mm that the continuum is formed in photospheric regions corresponding to where Ryde et al (2002) need to start cooling their upper photosphere. This is also a location which is well above the depth of the onset of the coexisting chromospheric temperature rise of the Arcturus model of Ayres & Linsky (1975).…”

Section: Discussionmentioning

confidence: 66%

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 μm spectra

Ryde

Lambert

Farzone

et al. 2014

A&A

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Context. The structures of the outer atmospheres of red giants are very complex. Recent interpretations of a range of different observations have led to contradictory views of these regions. It is clear, however, that classical model photospheres are inadequate to describe the nature of the outer atmospheres. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain spectro-interferometric observations and low-and high-resolution spectra. On the other hand high-resolution spectra in the mid-IR do not easily fit into this picture because they rule out any large sphere of water vapour in LTE surrounding red giants. Aims. In order to approach a unified scenario for these outer regions of red giants, more empirical evidence from different diagnostics are needed. Our aim here is to investigate high-resolution, mid-IR spectra for a range of red giants, spanning spectral types from early K to mid M. We want to study how the pure rotational lines of water vapour change with effective temperature, and whether we can find common properties that can put new constraints on the modelling of these regions, so that we can gain new insights. Methods. We have recorded mid-IR spectra at 12.2−12.4 μm at high spectral resolution of ten well-studied bright red giants, with TEXES mounted on the IRTF on Mauna Kea. These stars span effective temperatures from 3450 K to 4850 K. Results. We find that all red giants in our study cooler than 4300 K, spanning a wide range of effective temperatures (down to 3450 K), show water absorption lines stronger than expected and none are detected in emission, in line with what has been previously observed for a few stars. The strengths of the lines vary smoothly with spectral type. We identify several spectral features in the wavelength region that are undoubtedly formed in the photosphere. From a study of water-line ratios of the stars, we find that the excitation temperatures, in the line-forming regions, are several hundred Kelvin lower than expected from a classical photospheric model. Conclusions. All stars in our sample show several photospheric features in their 12 μm spectra, which can be modelled with a classical model photosphere. However, in all stars showing water-vapour lines (stars cooler than ∼4300 K), the water lines are found to be much deeper than expected. The line ratios of these pure-rotational lines reveal low excitation temperatures. This could either be due to lower temperatures than expected in the outer regions of the photospheres caused by for example extra cooling, or due to non-LTE level populations, affecting the source function and line opacities, but this needs further investigation. We have demonstrated that these diagnostically interesting water lines are a general feature of red giants across spectral types, and we argue for a general explanation of their formation rather than explanations requiring specific properties, such as dust. Since the water lines are neither weak (filled in by emission) ...

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Section: Discussionmentioning

confidence: 66%

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 μm spectra

Ryde

Lambert

Farzone

et al. 2014

A&A

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“…We use the Altenhoff et al (1994) compilation because it represents the largest stellar sample available. We later ( §5.3) compare α Tau to more recent interferometric observations by Cohen et al (2005). Robinson, Carpenter, & Brown (1998) give estimates of ne in α Tau for three epochs (1990, 1994, and 1996) based on near-uv C II spectra obtained with the Hubble Space Telescope: ne 10 9 cm −3 .…”

Section: Calibrating With α Tau (K5 Iii)mentioning

confidence: 85%

Chromospheric thermal continuum millimetre emission from non-dusty K and M red giants

Harper

O'Riain

Ayres

2012

Monthly Notices of the Royal Astronomical Society

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We examine the thermal free-free millimetre fluxes expected from non-dusty and nonpulsating K through mid-M giant stars based on our limited understanding of their inhomogeneous chromospheres. We present a semi-analytic model that provides estimates of the radio fluxes for the mm wavelengths (e.g., CARMA, ALMA, JVLA Q-band) based on knowledge of the effective temperatures, angular diameters and chromospheric Mg II h & k emission fluxes. At 250 GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and sub-mm range will have a contribution from the chromospheric material that gives rise to the ultraviolet emission spectrum, as well as the cool molecular material known to exist above the photosphere. We predict a lower bound to the inferred brightness temperature of red giants based on heating at the basal-flux limit if the upper chromospheres have filling factor 1. Multi-frequency mm observations should provide important new information on the structuring of the inhomogeneous chromospheres, including the boundary layer, and allow tests of competing theoretical models for atmospheric heating. We comment on the suitability of these stars as mm flux calibrators.

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“…We have used the UBVRIJKLMNH Photoelectric Catalogue (Morel & Magnenat 1978), the IRAS catalogue of Point Sources (IRAS PSC), Version 2.0 (Beichman et al 1988), observations in the Geneva Photometric System 4 (Rufener 1989), radio continuum data from Wendker (1995) and Cohen et al (2005), photometric data in the Johnson's 11-color system , the COBE DIRBE Point Source Catalog (Smith et al 2004), the 2MASS All-Sky Catalog of Point Sources (Skrutskie et al 2006), IR photometry from the Absolute Calibration Programme for ISO from Cohen et al 1 and J n -band photometry from Selby et al (1988). A summary of the available data can be found in Tables A.1 -A.5 in the Appendix.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

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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Far-Infrared and Millimeter Continuum Studies of K Giants: α Bootis and α Tauri

Cited by 9 publications

References 46 publications

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 μm spectra

Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 μm spectra

Chromospheric thermal continuum millimetre emission from non-dusty K and M red giants

Structure of the outer layers of cool standard stars

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