Evidence for localized onset of episodic mass loss in Mira

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.

The OGLE Collection of Variable Stars: Nearly 66,000 Mira Stars in the Milky Way

“…A relatively short stage in a star's life, the Mira phase is characterized by large bolometric luminosities (ranging from several hundred to tens of thousands of the solar luminosity; Iwanek et al 2021b) and low average densities (about 7-8 orders of magnitude lower than the solar density; Berlioz-Arthaud 2003;Catelan & Smith 2015). High-amplitude pulsations cause mass-loss phenomena via the stellar wind (Perrin et al 2020), which leads to the enrichment of the interstellar medium with elements heavier than nickel produced by the slow neutron-capture process. Therefore, Miras, like other LPVs, are important tracers of galactic chemistry (Whitelock et al 1997;Battistini & Bensby 2016;Kraemer et al 2019;Yu et al 2021).…”

Water vapour masers in long-period variable stars

“…The asymmetric structures may be an additional component to the underlying framework of a spherically symmetric expanding shell. Their origin might be directional differences in the mass-loss due to convection cells (Freytag et al 2017;Perrin et al 2020) or deviations from spherical mass loss due to the presence of a companion (Chen et al 2017). We expect that the long-living patterns in the H 2 O maser profiles should change on the timescales of the shell-crossing times ( < ∼ 25 yr).…”

The Symbiotic X-ray binaries Sct X-1, 4U 1700+24 and IGR J17329-2731

“…There are indications, indeed, that atmospheric structures of red giants could change on the time scale of weeks as confirmed by both observations and hydrodynamic models (see Höfner & Freytag 2019, and references therein). Furthermore, their stellar winds are permeated by shock fronts which are induced by stellar pulsations and/or convection (see, e.g., Perrin et al 2020). The presence of a NS embedded in the wind further perturbs the atmospheric morphology with its orbital motion as well as with the effect of wind ionization by the X-ray radiation (see, e.g., Bozzo et al 2021, and references therein).…”