VLTI-GRAVITY measurements of cool evolved stars - I. Variable photosphere and extended atmosphere of the Mira star R Peg

Astronomy & Astrophysics, May 2018

Context. Dynamic model atmospheres of Mira stars predict variabilities in the photospheric radius and in atmospheric molecular layers which are not yet strongly constrained by observations.Aims. Here we measure the variability of the oxygen-rich Mira star R Peg in near-continuum and molecular bands.Methods. We used near-infrared K-band spectro-interferometry with a spectral resolution of about 4000 obtained at four epochs between post-maximum and minimum visual phases employing the newly available GRAVITY beam combiner at the Very Large Telescope Interferometer (VLTI).Results. Our observations show a continuum radius that is anti-correlated with the visual lightcurve. Uniform disc (UD) angular diameters at a near-continuum wavelength of 2.25 μm are steadily increasing with values of 8.7 ± 0.1 mas, 9.4 ± 0.1 mas, 9.8 ± 0.1 mas, and 9.9 ± 0.1 mas at visual phases of 0.15, 0.36, 0,45, 0.53, respectively. UD diameters at a bandpass around 2.05 μm, dominated by water vapour, follow the near-continuum variability at larger UD diameters between 10.7 mas and 11.7 mas. UD diameters at the CO 2–0 bandhead, instead, are correlated with the visual lightcurve and anti-correlated with the near-continuum UD diameters, with values between 12.3 mas and 11.7 mas.Conclusions. The observed anti-correlation between continuum radius and visual lightcurve is consistent with an earlier study of the oxygen-rich Mira S Lac, and with recent 1D CODEX dynamic model atmosphere predictions. The amplitude of the variation is comparable to the earlier observations of S Lac, and smaller than predicted by CODEX models. The wavelength-dependent visibility variations at our epochs can be reproduced by a set of CODEX models at model phases between 0.3 and 0.6. The anti-correlation of water vapour and CO contributions at our epochs suggests that these molecules undergo different processes in the extended atmosphere along the stellar cycle. The newly available GRAVITY instrument is suited to conducting longer time series observations, which are needed to provide strong constraints on the model-predicted intra- and inter-cycle variability.

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VLTI-GRAVITY measurements of cool evolved stars - I. Variable photosphere and extended atmosphere of the Mira star R Peg

A&A VLTI-GRAVITY measurements of cool evolved stars? LETTER TO THE EDITOR M. Wittkowski 3 G. Rau 0 1 A. Chiavassa 7 S. Höfner 6 M. Scholz 4 5 P. R. Wood 9 W. J. de Wit 8 F. Eisenhauer 2 X. Haubois 8 T. Paumard 10 0 Department of Physics, The Catholic University of America , Washington, DC 20064 , USA 1 NASA Goddard Space Flight Center , Code 667, Greenbelt, MD 20771 , USA 2 Max Planck Institute for extraterrestrial Physics , Giessenbachstr., 85748 Garching , Germany 3 European Southern Observatory , Karl-Schwarzschild-Str. 2, 85748 Garching bei München , Germany 4 Sydney Institute for Astronomy, School of Physics, University of Sydney , Sydney NSW 2006 , Australia 5 Zentrum für Astronomie der Universität Heidelberg (ZAH), Institut für Theoretische Astrophysik , Albert-Ueberle-Str. 2, 69120 Heidelberg , Germany 6 Division of Astronomy and Space Physics, Department of Physics and Astronomy, Uppsala University , Box 516, 75120 Uppsala , Sweden 7 Université Côte d'Azur , Observatoire de la Côte d'Azur, CNRS, Lagrange, CS 34229, 06304 Nice Cedex 4 , France 8 European Southern Observatory , Casilla 19001, Santiago 19 , Chile 9 Research School of Astronomy and Astrophysics, Australian National University , Canberra ACT2611 , Australia 10 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot , Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon , France Context. Dynamic model atmospheres of Mira stars predict variabilities in the photospheric radius and in atmospheric molecular layers which are not yet strongly constrained by observations. Aims. Here we measure the variability of the oxygen-rich Mira star R Peg in near-continuum and molecular bands. Methods. We used near-infrared K-band spectro-interferometry with a spectral resolution of about 4000 obtained at four epochs between post-maximum and minimum visual phases employing the newly available GRAVITY beam combiner at the Very Large Telescope Interferometer (VLTI). Results. Our observations show a continuum radius that is anti-correlated with the visual lightcurve. Uniform disc (UD) angular diameters at a near-continuum wavelength of 2.25 m are steadily increasing with values of 8:7 0:1 mas, 9:4 0:1 mas, 9:8 0:1 mas, and 9:9 0:1 mas at visual phases of 0.15, 0.36, 0,45, 0.53, respectively. UD diameters at a bandpass around 2.05 m, dominated by water vapour, follow the near-continuum variability at larger UD diameters between 10.7 mas and 11.7 mas. UD diameters at the CO 2-0 bandhead, instead, are correlated with the visual lightcurve and anti-correlated with the near-continuum UD diameters, with values between 12.3 mas and 11.7 mas. Conclusions. The observed anti-correlation between continuum radius and visual lightcurve is consistent with an earlier study of the oxygen-rich Mira S Lac, and with recent 1D CODEX dynamic model atmosphere predictions. The amplitude of the variation is comparable to the earlier observations of S Lac, and smaller than predicted by CODEX models. The wavelength-dependent visibility variations at our epochs can be reproduced by a set of CODEX models at model phases between 0.3 and 0.6. The anti-correlation of water vapour and CO contributions at our epochs suggests that these molecules undergo different processes in the extended atmosphere along the stellar cycle. The newly available GRAVITY instrument is suited to conducting longer time series observations, which are needed to provide strong constraints on the model-predicted intra- and inter-cycle variability. techniques; interferometric - stars; AGB and post-AGB - stars; atmospheres - stars; mass-loss - stars; variables; general - stars; individual; R Peg 1. Introduction Low- to intermediate-mass stars evolve to red giant and asymptotic giant branch (AGB) stars. Mass loss increases during the AGB evolution. The AGB mass loss is driven by an interplay between pulsations, which extend the atmosphere, dust models) by Ireland et al. (2008, 2011) predicted a regular sinusoidal variation of the photospheric radius and an irregular chaotic variability of the outer molecular layers. The latest 3D radiation hydrodynamic (RHD) simulations of AGB stars (CO5BOLD) by Freytag & Höfner (2008) and Freytag et al. (2017) show non-radial structures such as long-lasting giant convection cells and short-lived surface granules. These dynamical phenomena trigger large-scale atmospheric shock waves that expand roughly spherically and are similar to those of 1D models, except they do not cover the full surface at a given instance. Thompson et al. (2002) conducted long-term narrow-band interferometric monitoring over 2–3 stellar cycles of the oxygenrich Mira S Lac and the carbon-rich Mira RZ Peg. Their data shows the expected sinusoidal variation in the continuum angular radius and different phase lags in the continuum and molecular bands. For the oxygen-rich Mira S Lac, the continuum minimum size tracked the visual maximum brightness, i.e. t (...truncated)


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M. Wittkowski, G. Rau, A. Chiavassa, S. Höfner, M. Scholz, P. R. Wood, W. J. de Wit, F. Eisenhauer, X. Haubois, T. Paumard. VLTI-GRAVITY measurements of cool evolved stars - I. Variable photosphere and extended atmosphere of the Mira star R Peg, Astronomy & Astrophysics, 2018, pp. L7, 613, DOI: 10.1051/0004-6361/201833029