Dust-driven outflows from AGB stars

Cosmic dust comprises only a tiny fraction of the Universe’s matter but is has an important role to play in a wide range of astronomical studies. Dust can absorb, emit and scatter photons, drastically altering our view of the source we are trying to study. Rocky material is needed to form planets, dust is a key component in the chemical evolution of galaxies including our own Milky Way, and interstellar dust effects the light from distance galaxies which are used to study the early Universe.

The cosmic dust cycle begins with the formation of grains that condense in the cool outer envelopes of evolved stars. The dust is then ejected into the interstellar medium by strong stellar winds. Once in the ISM, dust grains act as seeds upon which molecules can form, which go on to form molecular clouds out of which future generations of stars will form – thus continuing the dust cycle.

While the details of dust-driven stellar outflows are understood in principle, the type of grains that drive the outflows are not well constrained. The general picture is that stellar pulsations lift the envelope gas to an altitude where it is cool enough  for dust to condense and then the newly formed dust grains are accelerated away from the star by stellar radiation pressure. The accelerating dust collides with the gas, causing it to accelerate and thus driving an outflowing wind of gas and dust. Up until now, however, observations of this tenuous gas and dust just a few tens of milliarcseconds from the star have not been possible.

PhD student Barnaby Norris of the University of Sydney and collaborators have used the ESO Very Large Telescope to observe three asymptotic giant branch (AGB) stars using aperture masked polarimetric interferometry. This technique allows them to study the dust shells around the AGB stars in scattered light at three near-IR wavelengths extremely close to the stars – less than two stellar radii. With dust modelling the team have determined the size of the dust grains and also the amount of dust responsible for the scattered light. The grains are surprisingly large – about 300 nm in radius – and their spectral signatures indicate a composition dominated by magnetism-iron silicates, either olivines and/or pyroxenes.  Further analysis demonstrates that the dust cannot comprise iron-rich silicates, as these would have formed further from the stars than two stellar radii. Instead the team suggest that the large grains must be  made of iron-free silicates like forsterite and enstatite. These silicates are almost transparent to stellar photons at 1 micron, and so despite their close proximity to the intense radiation field they do not heat to sublimation temperatures and hence they survive. However, being transparent to near-IR photons also means that the grains will not be effected by radiation pressure required to drive the outflows.  It turns out, however, that these large magnesium-rich silicate grains are effected by photon scattering which provides the radiative acceleration required to drives the winds.

Dust grains formed in the atmosphere of an AGB star are accelerated away from the star (white arrow) via absorption and emission or scattering of stellar photons.  Subsequent collisions with gas molecules cause them to gas accelerate and collide with other gas molecules, triggering an outflow or stellar wind. (Credit: Nature)

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