Astronomers peer into stellar sandstorms

An international team of astronomers has found a potential explanation of how red-giant stars loose the bulk of their mass towards the end of their lives – a process currently not fully understood. Using new observational techniques, the researchers looked at the dust shells surrounding these dying stars, which gave them information about what causes the powerful "superwind" of dust grains that leads stars to lose their mass. Much of this star dust comprises of silicates, which go on to form planets such as the Earth.

      An artist's impression of the sandstorm surrounding one of the red-giant stars observed.

Celestial lifestyles

Towards the end of their lives, intermediate-mass stars – stars with masses ranging from 0.6–10 solar masses – eject the bulk of their outer envelope in a slow, dense wind. This "superwind" occurs over a period of 10,000 years, is a 100 million times stronger than the solar wind and removes almost half the mass of the star, leaving behind a fading stellar remnant.

The problem with this model, though, is explaining just how so much mass is lost in the "superwind". This is because it is difficult to observe gas and dust that is very close to its parent star. Current theory suggests that the "superwinds" occur because of the acceleration of the minute dust grains in the shells surrounding the stars. These grains are said to absorb starlight, which transfers momentum to them and causes the dust to blow away from the star. The problem with this model is that at the grain size estimated, light from the star would cause the dust to sublimate before it could be pushed away.

Unmasking stellar dust

In the new work published in Nature, the team, led by Barnaby Norris from the University of Sydney, Australia, looked at three red-giant stars and their dust shells using the European Southern Observatory's Very Large Telescope in Chile. The researchers used a technique known as "aperture-masking polarimetric interferometry" to look at the red giants plus other dust-free stars to verify their detection methods. Team member Albert Zijlstra, of Manchester University's Jodrell Bank Observatory in the UK, explains that using an "aperture mask" inside an infrared instrument along with a polarimeter is a combination that had not been previously used for this purpose. "The aperture mask turns a single telescope into a collection of much smaller telescopes, which can then be used as an interferometer. This gives excellent, reproducible image quality at the cost of reduced sensitivity," he explains. Using the observed data, the team developed a model to determine the dust-shell radius and the amount of light scattered by the shell at each wavelength.

A simulated image produced from the data of the dust shell around the red-giant star W Hydrae, shown in two polarizations of light: horizontal and vertical. This is equivalent to viewing the dust shell through polarized sunglasses, with the glasses held horizontally (left image) and vertically (right image). The image of the star in the centre of the dust shell is not from these observations and is for illustrative purposes only. The Sun (a single dot) is included for scale. 

Closer and bigger

The researchers found that the dust exists a lot closer to the stars than previously thought – less than two stellar radii. They also found that the grains are much larger in size than expected, being almost a micrometre across or about 300 nm in radius – this is quite large for stellar-wind particles. At these sizes, the grains are transparent to starlight, and so would not be sublimated by the intense radiation from the light. Although transparency suggests that the grains would again not be propelled away to form the wind, the researchers say that the acceleration occurs as a result of photon scattering rather than absorption. These large grains are driven out at speeds of 10 km s^–1, creating a virtual "stellar sandstorm".

How to hide from a magnetic field

Researchers in Europe have built a magnetic cloak that, in theory, is reasonably practical to manufacture. An object concealed by the new cloak, the researchers claim, is magnetically undetectable, while the cloak itself is made from materials available in many physics labs the world over. This means that it is, in principle, the first cloak that should be reasonably practical to manufacture.

Cloaks and shields

In 2011 Alvaro Sanchez and colleagues at Universitat Autònoma de Barcelona, Spain, developed a theory for a type of magnetic cloak they called an "antimagnet" that would have two crucial properties. One is that any magnetic field created within the cloak would not leak outside the cloaked region and the other is that the cloak and the cloaked region would be undetectable by an external magnetic field; that is, the field would not be distorted by the cloak. Now, Sanchez along with Fedor Gömöry and colleagues from the Slovak Academy of Sciences, has designed and demonstrated a modified version of the cloak proposed last year.

The new cloak is a simple bi-layer cloak made up of two common materials – an inner superconducting layer made up of a high-temperature superconducting tape and an outer ferromagnetic layer composed of a few turns of a thick FeNiCr commercial alloy sheet. "The cloak we proposed last year was more of an ideal cloak," explains Sanchez. "But it was complicated with 10 layers and included superconducting plates. This new cloak, while not perfect, is a much simpler design for achieving similar results using a static uniform magnetic field." He adds that it is fair to say that this is the first cloak that is an exact cloak that can be feasibly implemented in practice.

The superconducting layer on its own repels the magnetic field, while a ferromagnetic layer on its own attracts the magnetic field lines; so both independent layers distort the field. The cloak is the accurate combination of the two layers, determined by a specific radius, which adjusts for the permittivity (μ) such that there is no external field distortion at all. This radius is calculated using Maxwell equations. "It is quite amazing that almost 160 years after Maxwell equations were first developed, we are still finding new solutions based solely on them!" says Sanchez.

Artist's impression of how the magnetic cloak works. The ferromagnet attracts magnetic field lines (left), the superconductor repels magnetic field lines (centre), and the superconductor-ferromagnetic bilayer cloaks a magnetic field (right). The result is that an object inside the cloak is magnetically undetectable.

Perfection problems

Sanchez tells physicsworld.com that the entire team is highly inspired by the initial work on building invisibility cloaks using transformation optics carried out by John Pendry and colleagues at Imperial College London since 2006. "There are generally two ways of achieving a cloak – either using transformation optics or using plasmonics. The problem with the first is that, while it is theoretically the perfect cloak, it is nearly impossible to physically create. With plasmonics, while the materials are available, you get a slight shadowing or scattering effect, not a complete cloak at all. This is the first time that you get both using commercially viable materials," Sanchez explains.

Sanchez points out that an advantage in developing a cloak for a static magnetic field is that, for such a field, the magnetic and electric effects decouple and the researchers only have to consider the magnetic permeability. The team tested its cloak using a static field of 40 mT – which is greater than the Earth's magnetic field. Currently, the cloak has been built on a small but reasonable scale – 12.5 × 12 mm. Sanchez explains that another advantage is that, for a static magnetic field, the cloak can work on any length scale – from microns to metres – as there is no intrinsic cut-off, unlike other cloaks that work at fixed wavelengths.

Because the cloak is capable of running under relatively strong magnetic fields and relatively warm liquid-nitrogen temperatures, and as it is made from commercially available materials, it could be readily put to practical use, the researchers say. The team is also looking at other methods to manipulate and control magnetic fields into different "shapes", for purposes other than cloaking, in the coming months