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The asymptotic giant branch (AGB) star HR3126, associated with thearcminute-scale bipolar Toby Jug Nebula, provides a rare opportunity to studythe emergence of bipolar structures at the end of the AGB phase. We carried outlong-baseline interferometric observations with AMBER and GRAVITY (2--2.45micron) at the Very Large Telescope Interferometer, speckle interferometricobservations with VLT/NACO (2.24 micron), and imaging with SPHERE-ZIMPOL (0.55micron) and VISIR (7.9--19.5 micron). The images reconstructed in the continuumat 2.1--2.29 micron from the AMBER+GRAVITY data reveal the central starsurrounded by an elliptical ring-like structure with a semimajor and semiminoraxis of 5.3 and 3.5 mas, respectively. The ring is interpreted as the inner rimof an equatorial dust disk viewed from an inclination angle of ~50 degrees, andits axis is approximately aligned with the bipolar nebula. The disk issurprisingly compact, with an inner radius of a mere 3.5 Rstar (2 au). Our 2-Dradiative transfer modeling shows that an optically thick flared disk withsilicate grains as large as ~4 micron can reproduce the observed continuumimages and the spectral energy distribution. The images obtained in the COfirst overtone bands reveal elongated extended emission around the centralstar, suggesting the oblateness of the star's atmosphere or the presence of aCO gas disk inside the dust cavity. The object is unresolved withSPHERE-ZIMPOL, NACO, and VISIR. If the disk formed together with the bipolarnebula, the grain growth from sub-micron to a few microns should have takenplace over the nebula's dynamical age of ~3900 yrs. The non-detection of acompanion in the reconstructed images implies that either its 2.2 micronbrightness is more than ~30 times lower than that of the red giant or it mighthave been shredded due to binary interaction.

Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula