Abstract

Aims. The asymptotic giant branch (AGB) star HR3126, associated with the arcminute-scale bipolar Toby Jug Nebula, provides a rare opportunity to study the emergence of bipolar structures at the end of the AGB phase. Our goal is to image the central region of HR3126 with high spatial resolution. Methods. We carried out long-baseline interferometric observations with AMBER and GRAVITY (2-2.45 mu m) at the Very Large Telescope Interferometer with spectral resolutions of 1500 and 4500, speckle interferometric observations with VLT/NACO (2.24 mu m), and imaging with SPHERE-ZIMPOL (0.55 mu m) and VISIR (7.9-19.5 mu m). Results. The images reconstructed in the continuum at 2.1-2.29 mu m from the AMBER+GRAVITY data reveal the central star surrounded by an elliptical ring-like structure with a semimajor and semiminor axis of 5.3 and 3.5 mas, respectively. The ring is interpreted as the inner rim of an equatorial dust disk viewed from an inclination angle of similar to 50 degrees, and its axis is approximately aligned with the arcminute-scale bipolar nebula. The disk is surprisingly compact, with an inner radius of a mere 3.5 R-star (2 au). Our 2-D radiative transfer modeling shows that an optically thick flared disk with silicate grains as large as similar to 4 mu m can simultaneously reproduce the observed continuum images and the spectral energy distribution. The images reconstructed in the CO first overtone bands reveal elongated extended emission around the central star, suggesting the oblateness of the star's atmosphere or the presence of a CO gas disk inside the dust cavity. The object is unresolved with SPHERE-ZIMPOL, NACO, and VISIR. Conclusions. If the disk formed together with the bipolar nebula, the grain growth from sub-micron to a few microns should have taken place over the nebula's dynamical age of similar to 3900 yrs. The non-detection of a companion in the reconstructed images implies that either its 2.2 mu m brightness is more than similar to 30 times lower than that of the red giant or it might have been shredded due to binary interaction.