Stellar winds
Dynamics and 3D Structure
Most stars, especially the more luminous ones, show continuous outflows of gas from their surfaces in a process known as stellar winds, with the solar wind being a well-known example. Understanding what drives these gases away from stars is an area of active research, combining cutting-edge computer simulations with observations from the latest ground- and space-based telescopes.
A prime area of our research is on the winds of cool, luminous giants known as Asymptotic Giant Branch (AGB) stars. These winds are primarily driven by the radiative acceleration of tiny solid particles referred to as dust grains, that form in the extended atmospheres of these stars. Cool giant stars emit radiation at about 5,000 to 10,000 times the brightness of the Sun, and the dust grains efficiently absorb and scatter this light. As the photons are mostly directed away from the star, the dust is pushed outward when interacting with the stellar radiation. Since the dust grains are embedded in the surrounding gas, they drag the gas along with them, driving the bulk material it into space. This process enriches the surrounding interstellar medium with newly formed chemical elements that are essential for the creation of Earth-like planets and life.
Recent studies, including advanced 3D radiation-hydrodynamical (RHD) models as simulated with the CO5BOLD code, have contributed significantly to our understanding on the dynamics of these stellar winds. These models now track the flow of material from the turbulent, pulsating interior of an AGB star, through its atmosphere and dust formation zone, and into the region where the wind is accelerated by radiation pressure on dust. Unlike earlier 1D models, which assumed purely radial pulsations, these 3D simulations reveal that convection and pulsations create a patchy, irregular structure in the atmosphere, resulting in a clumpy and complex wind-acceleration zone.
Recently our 3D RHD simulations have been adapted for use with the radiative transfer code RADMC-3D, allowing the creation of synthetic observables that mimic the effects of an AGB star's non-spherical shape on its surrounding dusty envelope. The findings suggest that the clumpiness of circumstellar dust and the angle-dependent illumination caused by temperature variations on the star's surface can significantly influence the observed spectral energy distributions (SEDs). This indicates that not all variability observed in AGB stars should be interpreted as global changes, as suggested by spherical models, but rather as the result of complex, localized processes.
3D star-in-a-box model of a cool giant. The figure shows a snapshot of the gas density, plotted for a slice centred on the stellar core (brighter colours indicate higher densities). The width of the box is about 4 stellar radii, the stellar radius is about 400 solar radii. The movie shows how giant convection cells trigger shock waves in the stellar atmosphere, lifting material far above the stellar surface (bright red blobs moving outwards), to distances where dust grains can form. Click on the image to open the video (opens in a new tab). Image: B. Freytag
Properties of stardust
The dust grains that play a crucial role in triggering the stellar winds of cool giants reveal their presence by affecting the light emitted by the star. These grains create distinctive bands in the infrared spectrum, which help us identify the materials they consist of. Depending on the star's chemical makeup, two types of grains are likely responsible for accelerating the winds: carbon grains and silicate grains. The former are composed of material similar to coal or soot, making them highly effective at absorbing light. The latter are more transparent but excel at scattering photons, especially when they grow to sizes of 0.1 to 1.0 micrometers in the stellar atmosphere. These materials are not only found in stellar atmospheres and winds but also throughout the solar system: in the dusty tails of comets, in meteorites, and in the crusts of terrestrial planets. In some instances, it has even been possible to identify original grains of stardust, known as pre-solar grains, in meteorites.
The properties and evolution of AGB stars are heavily influenced by the mass loss they experience through stellar winds. This process is driven by radiation pressure from the absorption and scattering of stellar radiation by dust grains formed in the atmosphere. Detailed RHD simulations have shown that using size-dependent opacities for grains of amorphous carbon leads to wind models that are more variable and dominated by gusts. While the average mass-loss rates and outflow speeds do not change significantly, the increased radiative pressure results in smaller grains and higher gas-to-dust ratios, affecting the photometric properties of these stars.
In M-type AGB stars, where oxygen is abundant in the atmosphere, the winds are driven by silicate grains, whose composition and properties change as they interact with stellar radiation. The gradual enrichment of these grains with iron alters their optical properties and the heating effect of stellar radiation, leading to distinct silicate features in the infrared spectrum. New DARWIN models, which take into account the variable Fe/Mg ratio in silicate grains, predict that while the mass-loss rates remain largely unchanged, the wind velocities tend to be higher than in models with Fe-free silicate dust. This enrichment process is key to understanding the specific spectral features and the dynamic behavior of AGB star winds.
Studying the properties of stardust in AGB stars is crucial for understanding the mass-loss mechanisms that enrich the interstellar medium with newly produced elements, contributing to the formation of new stars and planetary systems. By integrating detailed RHD simulations and advanced modeling techniques, we continue to extend our understanding of complex interactions between dust grains and stellar radiation that drive these massive stellar winds of AGB stars.
ALMA image of the CO gas around the well-known binary cool giant star Mira (o Cet) and its companion (the positions of the two stars are marked by crosses). The heart shape is created by the fast wind of the companion star (to the left) blowing a bubble in the dense slower wind of the giant star (to the right). Image: S. Ramstedt et al. 2014