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Friday, April 13, 2012

Controversial Stellar Wind Theory

International team of researchers managed to identify a specific dust grains around cold giant stars.

Model image for W Hya with circumstellar shell viewed in horizontally and vertically polarized light
Model image for W Hya. (nature.com)
An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure, entraining the gas and driving the wind. Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star.

For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here they said that spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung–Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains through photon scattering rather than absorption a plausible mass loss mechanism for lower-amplitude pulsating stars.

“It is of course gratifying that my model of stellar winds is now supported by observation,” says Susanne Höfner, Professor of Astrophysics at Uppsala University. “The model previously attracted a great deal of scepticism.”

Solving the riddle of the stellar winds will help us to understand how atoms present in our environment and bodies long ago escaped the stars in which these atoms were formed. 

Towards the end of its life, a star typically transforms into a cool giant star with a luminosity thousands to tens of thousands times greater than that of the Sun. This developmental stage is characterised by massive gas outflows, or stellar winds, which transport newly formed elements like carbon away from the star at an increasing rate. Small solid particles, or dust grains, that form in the outer layers of giant stars likely represent the motive force behind stellar winds. By catching a portion of the radiation emitted by a star, as a sail catches the wind, dust grains are accelerated away from the star, drawing surrounding gases with them. But the radiation plausibly should cause such powerful heating of the dust grains as would vaporise most materials present in the star’s vicinity. 

Several years ago, Susanne Höfner proposed a model of how stellar winds might function given these conditions – a theory that until now has been regarded as controversial. The model requires the existence of dust grains that are just large enough to absorb the right amount of radiation. Thus would the greater part of a star’s radiation escape absorption, with the result that the grains did not overheat, with just enough being absorbed to accelerate the dust grains and, accordingly, the gas. 

Just the right sort of dust grains have now been identified around a number of cool giant stars by an Australian-European research team. The results were obtained using highly advanced methods that combine high resolution, making it possible to observe the immediate vicinity of a star, with radiation analysis that allows for the measurement of dust-grain size.

“The findings are very interesting and permit us to proceed with our research into how red giants develop into white dwarfs and the relevance of a specific type of supernova that serves as an important yardstick in connection with investigations into the evolution of the universe,” Susanne Höfner says.

This story has edited by author of threelas
Publication: nature

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