Title: Aperture synthesis imaging of the carbon AGB star R Sculptoris: Detection of a complex structure and a dominating spot on the stellar disk,
Abstract: We present near-infrared interferometry of the carbon-rich asymptotic giant
branch (AGB) star R Sculptoris.
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 Rsun, an effective temperature of 2640+-80 K, and a
luminosity of log L/Lsun=3.74+-0.18. These parameters match evolutionary tracks
of initial mass 1.5+-0.5 Msun and current mass 1.3+-0.7 Msun. 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.
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.