Aperture synthesis imaging of the carbon AGB star R Sculptoris: Detection of a complex structure and a dominating spot on the stellar disk
Journal article, 2017
Aims. We present near-infrared interferometry of the carbon-rich asymptotic giant branch (AGB) star R Sculptoris (R Scl). Methods. We employ medium spectral resolution K-band interferometry obtained with the instrument AMBER at the Very Large Telescope Interferometer (VLTI) and H-band low spectral resolution interferometric imaging observations obtained with the VLTI instrument PIONIER. We compare our data to a recent grid of dynamic atmosphere and wind models. We compare derived fundamental parameters to stellar evolution models. Results. The visibility data indicate a broadly circular resolved stellar disk with a complex substructure. The observed AMBER squared visibility values show drops at the positions of CO and CN bands, indicating that these lines form in extended layers above the photosphere. The AMBER visibility values are best fit by a model without a wind. The PIONIER data are consistent with the same model. We obtain a Rosseland angular diameter of 8.9 ± 0.3 mas, corresponding to a Rosseland radius of 355 ± 55 R?, an effective temperature of 2640 ± 80 K, and a luminosity of log L/L? = 3.74 ± 0.18. These parameters match evolutionary tracks of initial mass 1.5 ± 0.5 M? and current mass 1.3 ± 0.7 M?. The reconstructed PIONIER images exhibit a complex structure within the stellar disk including a dominant bright spot located at the western part of the stellar disk. The spot has an H-band peak intensity of 40% to 60% above the average intensity of the limb-darkening-corrected stellar disk. The contrast between the minimum and maximum intensity on the stellar disk is about 1:2.5. Conclusions. Our observations are broadly consistent with predictions by dynamic atmosphere and wind models, although models with wind appear to have a circumstellar envelope that is too extended compared to our observations. The detected complex structure within the stellar disk is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere at distances of 2-3 stellar radii. © ESO, 2017.
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stars: mass-loss
techniques: interferometric
stars: AGB and post-AGB
stars: fundamental parameters
stars: individual: R Scl
stars: atmospheres
Author
M. Wittkowski
European Southern Observatory (ESO)
K. H. Hofmann
Max Planck Society
S. Höfner
Uppsala University
J. B. Le Bouquin
Grenoble Alpes University
W. Nowotny
University of Vienna
C. Paladini
Université libre de Bruxelles (ULB)
J. Young
University of Cambridge
J. P. Berger
Grenoble Alpes University
M. Brunner
University of Vienna
I. de Gregorio-Monsalvo
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
K. Eriksson
Uppsala University
J. Hron
University of Vienna
E. M. Humphreys
European Southern Observatory (ESO)
Michael Lindqvist
Chalmers, Earth and Space Sciences, Onsala Space Observatory
Matthias Maercker
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
Chalmers, Earth and Space Sciences, Onsala Space Observatory
S. Mohamed
University of Cape Town
South African Astronomical Observatory
National Institute for Theoretical Physics
Hans Olofsson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
Chalmers, Earth and Space Sciences, Onsala Space Observatory
S. Ramstedt
Uppsala University
G. Weigelt
Max Planck Society
Published in
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 601 p. -Article number A3 art. no A3Categorizing
Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
Infrastructure
Onsala Space Observatory
Identifiers
DOI
10.1051/0004-6361/201630214