Angular diameter measurements of cool giant stars in strong TiO bands and in the continuum

Quirrenbach, A.; Mozurkewich, David; Armstrong, J. T.; Buscher, David F.; Hummel, C. A.

doi:10.1086/172432

Cited by 53 publications

(49 citation statements)

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“…For example, angular diameters of normal K and M giant stars, including several objects common to our sample, were measured in the strong TiO bands at 712 nm and in the nearby pseudo-continuum at 754 nm (Quirrenbach et al 1993). The observed 712 nm/754 nm diameter ratios appeared to be larger than predicted by model photospheres, suggesting that the photospheric models cannot describe adequately the outer layers of these red giant stars, and the result may lend support for the presence of the extra molecular envelopes (Quirrenbach 2001).…”

Section: Abundance Analysis and Outer Molecular Envelopementioning

confidence: 99%

Cool luminous stars: the hybrid nature of their infrared spectra

Tsuji¹

2008

A&A

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Aims. We determine carbon, oxygen, and their isotopic abundances based on CO and OH spectra in 23 red giant stars, and identify possible origin of difficulty in abundance analysis of cool luminous stars. Methods. We apply the line-by-line analysis based on the classical micro-turbulent model and 1D model photospheres. Results. We found empirically that there is a critical value of log W/ν ≈ −4.75 (W is the equivalent width and ν the wavenumber) above which the observed lines do not follow the classical line formation theory based on the micro-turbulent model and that the classical abundance analysis can be applied only to the weak lines of log W/ν < ∼ −4.75. The carbon, oxygen, and their isotopic abundances in 23 K-M giant stars obtained from such weak lines are approximately consistent with evolutionary models, although the 12 C/ 13 C puzzle (observed 12 C/ 13 C ratios are too small compared with theoretical predictions) remains unsolved. It is already known that the strong lines of log W/ν > −4.4 are contaminated by the contribution from outer molecular layers. The less strong but saturated lines of −4.75 < log W/ν < ∼ −4.4 (intermediate-strength lines) cannot be understood at all with the abundance and turbulent velocity based on the weak lines. By studying the behavior of these lines and considering other observations such as the detection of H 2 O lines, not only in the late M giants but also in the early M and K giants, we show that the intermediate-strength lines are essentially identical to the strong lines in that they also include contamination from the extra molecular layers. Thus, the behavior of the intermediate-strength lines, including those with LEP as high as 2 eV, appears to be nothing but a manifestation of the warm molecular envelope or MOLsphere. Conclusions. The infrared spectra of K-M giant stars are a hybrid of at least two components originating in the photosphere and MOLsphere. Only the weak lines, mostly high excitation, are relatively free from the effect of MOLsphere and can be used to extract photospheric abundances. The strong lines and the intermediate-strength lines, which dominate the observed infrared spectra, are affected badly by contamination from the MOLsphere. For this reason, they provide little information about the photosphere, but instead can be important proves of the warm molecular envelope for which little is known yet. In the interpretation and analysis of the infrared spectra of K-M giant stars, it is essential to consider their hybrid nature.

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Section: Abundance Analysis and Outer Molecular Envelopementioning

confidence: 99%

Cool luminous stars: the hybrid nature of their infrared spectra

Tsuji¹

2008

A&A

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“…Figures 2-4 are available in electronic form at http://www.aanda.org the circumstellar environment (Quirrenbach et al 1993;Perrin et al 2004). Recent studies with VLTI/AMBER and VLTI/MIDI have provided information about the pulsation and the massloss of AGB stars (Ohnaka et al 2006Wittkowski et al 2007;Chiavassa et al 2010;Karovicova et al 2011Karovicova et al , 2013 and about the structure of the molecular distribution in AGB stars (Wittkowski et al 2008Martí-Vidal et al 2011).…”

Section: Introductionmentioning

confidence: 99%

VLTI/AMBER observations of cold giant stars: atmospheric structures and fundamental parameters

Arroyo-Torres

Martí-Vidal

Marcaide

et al. 2014

A&A

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Aims. The main goal of this research is to determine the angular size and the atmospheric structures of cool giant stars ( Oct, β Peg, NU Pav, ψ Peg, and γ Hya) and to compare them with hydrostatic stellar model atmospheres, to estimate the fundamental parameters, and to obtain a better understanding of the circumstellar environment. Methods. We conducted spectro-interferometric observations of Oct, β Peg, NU Pav, and ψ Peg in the near-infrared K band (2.13−2.47 μm), and γ Hya (1.9−2.47 μm) with the VLTI/AMBER instrument at medium spectral resolution (∼1500). To obtain the fundamental parameters, we compared our data with hydrostatic atmosphere models (PHOENIX). Results. We estimated the Rosseland angular diameters of Oct, β Peg, NU Pav, ψ Peg, and γ Hya to be 11.66 ± 1.50 mas, 16.87 ± 1.00 mas, 13.03 ± 1.75 mas, 6.31 ± 0.35 mas, and 3.78 ± 0.65 mas, respectively. Together with distances and bolometric fluxes (obtained from the literature), we estimated radii, effective temperatures, and luminosities of our targets. In the β Peg visibility, we observed a molecular layer of CO with a size similar to that modeled with PHOENIX. However, there is an additional slope in absorption starting around 2.3 μm. This slope is possibly due to a shell of H 2 O that is not modeled with PHOENIX (the size of the layer increases to about 5% with respect to the near-continuum level). The visibility of ψ Peg shows a low increase in the CO bands, compatible with the modeling of the PHOENIX model. The visibility data of Oct, NU Pav, and γ Hya show no increase in molecular bands. Conclusions. The spectra and visibilities predicted by the PHOENIX atmospheres agree with the spectra and the visibilities observed in our stars (except for β Peg). This indicates that the opacity of the molecular bands is adequately included in the model, and the atmospheres of our targets have an extension similar to the modeled atmospheres. The atmosphere of β Peg is more extended than that predicted by the model. The role of pulsations, if relevant in other cases and unmodeled by PHOENIX, therefore seems negligible for the atmospheric structures of our sample. The targets are located close to the red limits of the evolutionary tracks of the STAREVOL model, corresponding to masses between 1 M and 3 M . The STAREVOL model fits the position of our stars in the HertzsprungRussell (HR) diagram better than the Ekström model does. STAREVOL includes thermohaline mixing, unlike the Ekström model, and complements the latter for intermediate-mass stars.

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“…The average formal error of the diameter measurements -derived from the X 2 of the model fits -is 2.5%. Data for a subset of 12 stars have already been published (Quirrenbach et al 1993). …”

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confidence: 99%

Cool Giant Stars are Bigger at 712 nm than at 754 nm

Quirrenbach

Mozurkewich

Armstrong

et al. 2001

Symp. - Int. Astron. Union

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We used the Mark III Optical Interferometer on Mt. Wilson in 1991 and 1992 to measure the diameters of 42 stars with 10 nm wide filters centered at 712 nm and at 754 nm. These filters probe the stellar atmosphere in a strong TiO band (712 nm) and in a "continuum" band relatively free of TiO absorption (754 nm). The data were taken on a North-South baseline that could be configured to lengths between 3.0 m and 31.5 m. Observations of the target stars were interleaved with frequent measurements of calibrator stars. The square of the calibrated visibility amplitude V 2 was determined as a function of baseline length, and a uniform disk model fitted to these data (for details see Mozurkewich et al. 1991). The average formal error of the diameter measurements -derived from the X 2 of the model fits -is 2.5%. Data for a subset of 12 stars have already been published (Quirrenbach et al. 1993). Results and DiscussionK stars, MO stars, and carbon stars have identical diameters at 712 nm and at 754 nm. For stars with spectral types later than MO, the diameters are systematically larger at 712 nm than at 754 nm (see Figure 1). The diameter ratio increases with decreasing effective temperature (or increasing R -I color index), and it is larger for luminosity class I than for luminosity class II and III stars. For C and S stars, which do not have TiO absorption bands, the diameter ratio is consistent with one, as expected.The variation of the uniform disk diameter between 712 nm and 754 nm cannot be due to wavelength-dependent variations of the limb darkening, which is a much smaller effect. The observations can be qualitatively explained by the different TiO opacities in the two filters: the T == 1 surface occurs at a 304 use, available at https://www.cambridge.org/core/terms. https://doi

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Angular diameter measurements of cool giant stars in strong TiO bands and in the continuum

Cited by 53 publications

References 0 publications

Cool luminous stars: the hybrid nature of their infrared spectra

Cool luminous stars: the hybrid nature of their infrared spectra

VLTI/AMBER observations of cold giant stars: atmospheric structures and fundamental parameters

Cool Giant Stars are Bigger at 712 nm than at 754 nm

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