Mid-infrared Interferometry of Dust around Massive Evolved Stars

We report long-baseline interferometric measurements of circumstellar dust aroundmassive evolved stars with the MIDI instrument on theVeryLarge Telescope Interferometer and provide spectrally dispersed visibilities in the 8Y13 m wavelength band. We also present diffraction-limited observations at 10.7 m on the Keck Telescope with baselines up to 8.7 m, which explore larger scale structure. We have resolved the dust shells around the late-type WC stars WR 106 and WR 95 and the enigmatic NaSt 1 (formerly WR 122), suspected to have recently evolved from a luminous blue variable (LBV) stage. For AG Car, the prototypical LBV in our sample, we marginally resolve structure close to the star, distinct from the well-studied detached nebula. The dust shells around the twoWC stars show fairly constant size in the 8Y13 m MIDI band, with Gaussian half-widths of 25 to 40 mas, and the Keck observations reveal an additional extended structure around WR 106. The visibility profiles for NaSt 1 obtained from two MIDI baselines indicate a compact source embedded in an extended structure. The compact dust we detect aroundNaSt 1 and AGCar favors recent or ongoing dust formation. Using the measured visibilities, we build spherically symmetric radiative transfer models of theWC dust shells, which enable detailed comparison with existing SED-based models. Our results indicate that the inner radii of the shells are within a few tens of AU from the stars. In addition, our models favor grain size distributions with large ( 1 m) dust grains. This proximity of the inner dust to the hot central star emphasizes the difficulty faced by current theories in forming dust in the hostile environment aroundWR stars. Althoughwe detect no direct evidence for binarity for these objects, dust production in a colliding-wind interface in a binary system is a feasible mechanism in WR systems under these conditions. Subject headinggs: stars: individual (AG Car, NaSt 1, WR 95, WR 106) — techniques: interferometric