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

Arroyo-Torres, B.; Martí-Vidal, Iván; Marcaide, J. M.; Wittkowski, M.; Guirado, J. C.; Hauschildt, P. H.; Quirrenbach, A.; Fabregat, J.

doi:10.1051/0004-6361/201323264

Cited by 15 publications

(19 citation statements)

References 39 publications

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“…We also observe a monotonic decrease beyond 2.3 μm, which may be caused by pseudocontinuum contributions from CO or by contributions from water vapor. We observed the same phenomenon previously for the RSGs VY CMa (Wittkowski et al 2012), AH Sco, UY Sct, and KW Sgr (Arroyo-Torres et al (2013), as well as for the smallamplitude pulsating red giants RS Cap (Marti-Vidal et al 2011), BK Vir (Ohnaka et al 2012), α Tau (Ohnaka 2013), and β Peg (Arroyo-Torres et al 2014).…”

Section: Resultssupporting

confidence: 88%

“…In particular we calculated the effective temperature, the luminosity, the Rosseland-mean radius, the bolometric flux, and the distance in the same way as described in detail by Arroyo-Torres et al (2014). We used the BVJHKs magnitudes from Kharchenko (2001) and Cutri et al (2003) and the IRAS flux from IRAS (1988).…”

Section: Fundamental Parametersmentioning

confidence: 99%

“…To convert the magnitudes into fluxes, we used the zero values from Johnson (1965) and Cohen et al (2003). To deredden the flux values we used the color excess method applied to (V − K) and based on intrinsic colors from Ducati et al (2001), as described in Arroyo-Torres et al (2014). V602 Car and HD 95687 belong to the cluster CAR OB2 and we use the distance as determined by Humphreys et al (1978).…”

Section: Fundamental Parametersmentioning

confidence: 99%

“…Although this approach may be limited by the limited spectral resolution of our observations of R ∼ 1500 and by the low number of observations per source, it is appropriate for a first comparison of the extensions of RSG stars and other evolved stars. Figure 10 shows the resulting ratios (V 2 cont /V 2 CO ) for our sources vs. log(L/L ) and ΔV, considering the RSGs by Wittkowski et al (2012), Arroyo-Torres et al (2013) Arroyo- Torres et al (2014). Results for Mira stars obtained by Wittkowski et al (2011) are also shown for comparison.…”

Section: Characterization Of the Extension Of The Molecular Atmospherementioning

confidence: 99%

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What causes the large extensions of red supergiant atmospheres?

Arroyo-Torres

Wittkowski²,

Chiavassa

et al. 2015

A&A

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108

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Aims. This research has two main goals. First, we present the atmospheric structure and the fundamental parameters of three red supergiants (RSGs), increasing the sample of RSGs observed by near-infrared spectro-interferometry. Additionally, we test possible mechanisms that may explain the large observed atmospheric extensions of RSGs. Methods. We carried out spectro-interferometric observations of the RSGs V602 Car, HD 95687, and HD 183589 in the near-infrared K-band (1.92−2.47 μm) with the VLTI/AMBER instrument at medium spectral resolution (R ∼ 1500). To categorize and comprehend the extended atmospheres, we compared our observational results to predictions by available hydrostatic PHOENIX, available 3D convection, and new 1D self-excited pulsation models of RSGs. Results. Our near-infrared flux spectra of V602 Car, HD 95687, and HD 183589 are well reproduced by the PHOENIX model atmospheres. The continuum visibility values are consistent with a limb-darkened disk as predicted by the PHOENIX models, allowing us to determine the angular diameter and the fundamental parameters of our sources. Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model visibilities do not predict the large observed extensions of molecular layers, most remarkably in the CO bands. Likewise, the 3D convection models and the 1D pulsation models with typical parameters of RSGs lead to compact atmospheric structures as well, which are similar to the structure of the hydrostatic PHOENIX models. They can also not explain the observed decreases in the visibilities and thus the large atmospheric molecular extensions. The full sample of our RSGs indicates increasing observed atmospheric extensions with increasing luminosity and decreasing surface gravity, and no correlation with effective temperature or variability amplitude. Conclusions. The location of our RSG sources in the Hertzsprung-Russell diagram is confirmed to be consistent with the red limits of recent evolutionary tracks. The observed extensions of the atmospheric layers of our sample of RSGs are comparable to those of Mira stars. This phenomenon is not predicted by any of the considered model atmospheres including available 3D convection and new 1D pulsation models of RSGs. This confirms that neither convection nor pulsation alone can levitate the molecular atmospheres of RSGs. Our observed correlation of atmospheric extension with luminosity supports a scenario of radiative acceleration on Doppler-shifted molecular lines.

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Section: Resultssupporting

confidence: 88%

Section: Fundamental Parametersmentioning

confidence: 99%

Section: Fundamental Parametersmentioning

confidence: 99%

Section: Characterization Of the Extension Of The Molecular Atmospherementioning

confidence: 99%

See 2 more Smart Citations

What causes the large extensions of red supergiant atmospheres?

Arroyo-Torres

Wittkowski²,

Chiavassa

et al. 2015

A&A

Self Cite

108

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“…The uncertainties are of the order of 50 K. Mérand et al (2010) showed that for a K0III star, the uncertainty in the diameter due to different models do not exceed 0.5% (and is thus negligible for the determination of T eff ). It may be worse for M stars but the uncertainty should not exceed ∼100 K. We note that, recently, Arroyo-Torres et al (2014) derived an effective temperature of T eff = 3909 ± 187 K for our coolest star β Peg based on VLTI/AMBER diameters, more than 450 K warmer than the temperature of Mozurkewich et al (2003). Dehaes et al (2011) derived T eff = 3600 ± 300 K from modelling the spectral energy distributions (SED), for the same star.…”

Section: Stellar Parametersmentioning

confidence: 71%

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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On the atmospheric structure and fundamental parameters of red supergiants

Wittkowski¹,

Arroyo-Torres

Marcaide

et al. 2014

Proc. IAU

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We present near-infrared spectro-interferometric studies of red supergiant (RSG) stars using the VLTI/AMBER instrument, which are compared to previously obtained similar observations of AGB stars. Our observations indicate spatially extended atmospheric molecular layers of water vapor and CO, similar as previously observed for Mira stars. Data of VY~CMa indicate that the molecular layers are asymmetric, possibly clumpy. Thanks to the spectro-interferometric capabilities of the VLTI/AMBER instrument, we can isolate continuum bandpasses, estimate fundamental parameters of our sources, locate them in the HR diagram, and compare their positions to recent evolutionary tracks. For the example of VY CMa, this puts it close to evolutionary tracks of initial mass 25-32 M⊙. Comparisons of our data to hydrostatic model atmospheres, 3d simulations of convection, and 1d dynamic model atmospheres based on self-excited pulsation models indicate that none of these models can presently explain the observed atmospheric extensions for RSGs. The mechanism that levitates the atmospheres of red supergiant is thus a currently unsolved problem.

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

Cited by 15 publications

References 39 publications

What causes the large extensions of red supergiant atmospheres?

What causes the large extensions of red supergiant atmospheres?

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

On the atmospheric structure and fundamental parameters of red supergiants

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