Evidence for localized onset of episodic mass loss in Mira

TL;DR

This study uses interferometric imaging to reveal localized dust formation regions near Mira, suggesting pulsation and convection-driven clumpy mass loss in AGB stars.

Contribution

It provides the first direct imaging evidence of localized dust formation zones associated with episodic mass loss in Mira, highlighting the role of pulsation and convection.

Findings

Detection of asymmetry consistent with a localized absorbing patch.

Evidence of dust formation near the pulsating stellar halo.

Mass loss may occur via multiple active dust-producing regions.

Abstract

We report Multi-telescope interferometric measurements taken with the Interferometric Optical Telescope Array (IOTA) to 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. 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. 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.