Abstract
Context. Mass loss from long-period variable stars (LPV) is an important contributor to the evolution of galactic abundances. Dust formation is understood to play an essential role in mass loss. It has, however, proven difficult to develop measurements that strongly constrain the location and timing of dust nucleation and acceleration. Aims. Interferometric imaging has the potential to constrain the geometry and dynamics of mass loss. High angular resolution studies of various types have shown that LPVs have a distinct core-halo structure. These have also shown that LPV images commonly exhibit a non-circular shape. The nature of this shape and its implications are yet to be understood. Methods. Multi-telescope interferometric measurements taken with the Interferometric Optical Telescope Array (IOTA) provide imagery of the LPV Mira in the H-band. This wavelength region is well suited to studying mass loss given the low continuum opacity, which allows for emission to be observed over a very long path in the stellar atmosphere and envelope. Results. The observed visibilities are consistent with a simple core-halo model to represent the central object and the extended molecular layers but, in addition, they demonstrate a substantial asymmetry. An analysis with image reconstruction software shows that the asymmetry is consistent with a localized absorbing patch. The observed opacity is tentatively associated with small dust grains, which will grow substantially during a multi-year ejection process. Spatial information along with a deduced dust content of the cloud, known mass loss rates, and ejection velocities provide evidence for the pulsational pumping of the extended molecular layers. The cloud may be understood as a spatially local zone of enhanced dust formation, very near to the pulsating halo. The observed mass loss could be provided by several such active regions around the star. Conclusions. This result provides an additional clue for better understanding the clumpiness of dust production in the atmosphere of AGB stars. It is compatible with scenarios where the combination of pulsation and convection play a key role in the process of mass loss.
Highlights
All stars of low to intermediate mass evolve through the AGB phase, with a dredging up of synthesized elements, and undergo extensive mass loss as long-period variables (LPVs)
Expecting a comparable phenomenon in the similar LPV Mira, we suggest that the evidence favors dust formation, or at least critical grain growth, which is likely to occur in either the halo or the extended layers, where a combination of low ambient temperature and relatively enhanced density favors dust formation
The Mira image we obtained in H band at high spatial resolution offers supporting evidence for a possible outcome of pulsationconvection coupling and offers a possible direction for pursuing the evidence further
Summary
All stars of low to intermediate mass evolve through the AGB phase, with a dredging up of synthesized elements, and undergo extensive mass loss as long-period variables (LPVs). These data have allowed us to synthesize an image of the LPV Mira (omicron Ceti), presented, which is sufficiently indicative of the asymmetric structure to motivate inclusion of an asymmetric component in our models and to derive some related parameters in Sect. Description of IOTA observations The interferometric data presented were obtained using the IOTA interferometer (Traub et al 2003), which was a long baseline interferometer operating at near-infrared wavelengths until 2006 It consisted of three 0.45 m telescopes movable among 17 stations along two orthogonal linear arms. The flux gave a minimum value of 3000 K, and it was set to 3400 K, which is a rough average of the temperatures measured by Perrin et al (2004) at earlier phases
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