Asymmetries and Outflows in the Circumstellar Dust of Mira A

Chandler, A. A.; Tatebe, K.; Wishnow, Edward H.; Hale, David; Townes, C. H.

doi:10.1086/522109

Cited by 15 publications

(11 citation statements)

References 27 publications

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“…Lopez et al (1997) found a best fit to observations in the IR taken with the Infrared Spatial Interferometer (ISI) by models with inhomogeneities or clumps. Chandler et al (2007) presented a number of explanations for their ISI observations, among them shock waves and non-uniform dust formation. Recently, Stewart et al (2016b) produced images of Mira by analyzing occultations by the rings of Saturn observed with the Cassini spacecraft showing layers and asymmetries in the stellar atmosphere.…”

Section: Introductionmentioning

confidence: 99%

Global 3D radiation-hydrodynamics models of AGB stars

Freytag

Liljegren

Höfner

2017

A&A

130

194

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Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales. Aims. To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations. Methods. We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters. Results. The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features. Conclusions. Our models self-consistently describe convection, convectively generated acoustic noise, fundamental-mode radial pulsations, and atmospheric shocks of various scales, which give rise to complex changing structures in the atmospheres of AGB stars.

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Section: Introductionmentioning

confidence: 99%

Global 3D radiation-hydrodynamics models of AGB stars

Freytag

Liljegren

Höfner

2017

A&A

130

194

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“…Interferometry and high angular resolution imagining are also becoming important for studying both the structure and the dynamics of AGB star atmospheres, giving new insights into the shock propagation and dust condensation distances (see e.g. Chandler et al 2007;Karovicova et al 2013;Ohnaka et al 2012Ohnaka et al , 2016.…”

Section: Introductionmentioning

confidence: 99%

Pulsation-induced atmospheric dynamics in M-type AGB stars

Liljegren

Höfner

Eriksson

et al. 2017

A&A

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Context. Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to a two-step process. First, matter in the stellar atmosphere is levitated by shock waves, induced by stellar pulsation, and second, this matter is accelerated by radiation pressure on dust, resulting in a wind. In dynamical atmosphere and wind models the effects of the stellar pulsation are often simulated by a simplistic prescription at the inner boundary. Aims. We test a sample of dynamical models for M-type AGB stars, for which we kept the stellar parameters fixed to values characteristic of a typical Mira variable but varied the inner boundary condition. The aim was to evaluate the effect on the resulting atmosphere structure and wind properties. The results of the models are compared to observed mass-loss rates and wind velocities, photometry, and radial velocity curves, and to results from 1D radial pulsation models. The goal is to find boundary conditions which give realistic atmosphere and wind properties. Methods. Dynamical atmosphere models are calculated, using the DARWIN code for different combinations of photospheric velocities and luminosity variations. The inner boundary is changed by introducing an offset between maximum expansion of the stellar surface and the luminosity and/or by using an asymmetric shape for the luminosity variation. Ninety-nine different combinations of theses two changes are tested. Results. The model atmospheres are very sensitive to the inner boundary. Models that resulted in realistic wind velocities and massloss rates, when compared to observations, also produced realistic photometric variations. For the models to also reproduce the characteristic radial velocity curve present in Mira stars (derived from CO ∆v = 3 lines), an overall phase shift of 0.2 between the maxima of the luminosity and radial variation had to be introduced. This is a larger phase shift than is found by 1D radial pulsation models. Conclusions. We find that a group of models with different boundary conditions (29 models, including the model with standard boundary conditions) results in realistic velocities and mass-loss rates, and in photometric variations. To achieve the correct line splitting time variation a phase shift is needed.

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“…Thus, the elevation of a few percent of each of the mass of each of these "puffs" of material over each pulsation cycle, on average, could support the observed Mira mass loss rate. Chandler et al (2007) reported their detection of episodic mass loss around Mira at 11.15 µm with the ISI interferometer. Our detection in H band is compatible with their finding.…”

Section: Discussionmentioning

confidence: 99%

Evidence for localized onset of episodic mass loss in Mira

Perrin

Ridgway

Lacour

et al. 2020

A&A

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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.

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Asymmetries and Outflows in the Circumstellar Dust of Mira A

Cited by 15 publications

References 27 publications

Global 3D radiation-hydrodynamics models of AGB stars

Global 3D radiation-hydrodynamics models of AGB stars

Pulsation-induced atmospheric dynamics in M-type AGB stars

Evidence for localized onset of episodic mass loss in Mira

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