What causes the large extensions of red supergiant atmospheres?

Abstract

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.

The rise-time of Type II supernovae

“…Alternatively, the fast rise-times and long plateaus could be simultaneously explained if RSG have more extended envelopes or denser outer layers (Wittkowski et al 2012;Arroyo-Torres et al 2015;Ohnaka et al 2013), or suffer mass-loss prior to explosion, as has also been recently suggested (Gezari et al 2015;Valenti et al 2015;Smith et al 2015). The very early rising light-curve in this case would not be due to shock-heated cooling but would be the delayed prolonged shock breakout emission of typical interacting SNe (Moriya et al 2011).…”

Large dust grains in the wind of VY Canis Majoris

“…The source of momentum for the wind remains unclear: the acceleration of dust grains by radiation pressure seems the most promising mechanism, but the formation of grains is inhibited by the extreme luminosity of RSGs. The outer atmospheres of RSGs are ∼20% larger than expected for a stellar photosphere and have high molecular abundances -this extension is likely to be caused by radiation pressure on molecular lines (Arroyo-Torres et al 2015), but the dust species that can form at these distances (<3 R * ) are not abundant enough to supply the momentum required to drive the wind (Bladh & Höfner 2012), while the iron-rich silicates that most efficiently absorb the star's radiation cannot form until ∼20 R * (extrapolating from the study of AGB stars in Woitke 2006). However, the acceleration of the wind to terminal velocity is observed to begin around 10 R * (Richards et al 1998); the only abundant species that can form so close are Ca-Al or Mg rich silicates (e.g.…”

From the atmosphere to the circumstellar environment in cool evolved stars

“…In a series of papers, Wittkowski et al (2012) and Arroyo-Torres et al (2013, 2015 used the VLTI/AMBER instrument to observe six red supergiants and confirmed the presence of extended molecular layers for their objects. They compared the data to hydrostatic PHOENIX model atmospheres showing that the PHOENIX models fit well the spectra, i.e.…”