Astronomical job: Ironing out ions

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  • Published: Sep 15, 2013
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Astronomical job: Ironing out ions

Stellar insights

New insights into the role of highly charged ions present in astrophysical plasmas, such as radiation transport within stars has emerged from research carried out by scientists at the Heidelberg Max Planck Institute for Nuclear Physics (MPIK) in cooperation with DESY in Hamburg, Germany.

New insights into the role of highly charged ions present in astrophysical plasmas, such as radiation transport within stars has emerged from research carried out by scientists at the Heidelberg Max Planck Institute for Nuclear Physics (MPIK) in cooperation with DESY in Hamburg, Germany. Using the synchrotron PETRA III instrument, the scientists have for the first time investigated the X-ray absorption of highly charged iron ions using a transportable ion trap developed at MPIK to generate and store the ions ready for high-precision measurements.

Highly charged ions play an important role in astrophysics, indeed charged, as opposed to neutral, is the common state within luminous matter in space, such as stellar atmospheres and interiors, around neutron stars and black holes. Charged matter can convert gravitational potential energy into extremely intense X-ray emissions, which can be observed from Earth, offering clues as to how energy is transported within the Sun, for instance.

Solar compost heap

At the core of the Sun, temperatures are up to about 16 million Kelvin as this vast natural nuclear fusion system producing 4 x 1026 Watts of power churns through its hydrogen fuel. Paradoxically, the power density within the Sun is a mere 200 Watts per cubic metre, about as high as a compost heap in a garden. Of course, despite this if all the power were released at once the Sun would burn out in a single blaze of X-ray glory. Fortunately for life on Earth, radiation transport to the exterior is severely inhibited, which maintains the high core temperature and sustains the fusion reactions within. Convection, the heat transport by turbulent upstream flows of hot matter, only takes place further outward, starting at about 70% of the solar radius. This good insulation reduces hydrogen consumption and has extended the life of the Sun to billions of years and will continue to do so.

One measure of this radiation inhibition is the "opacity" of the solar matter, a term describing how efficiently radiation is absorbed by it. Although the Sun consists mainly of hydrogen and helium, these elements only play a secondary role for the opacity. Their share of it diminishes from about 50% in the outer core to below 20% in the radiation zone. In that zone, more relevant are the relatively tiny amounts of impurities (a mere 1.6% of the Sun's mass), the heavier elements, such as oxygen and iron.

If carbon dioxide is the greenhouse gas for Earth's atmosphere trapping heat and keeping the planet warmer than it would otherwise be, then solar iron, despite its mass fraction of just 0.14% plays a similar role for X-rays and contributes to about a quarter of the total opacity.

Evolution of the stars

To better understand iron's X-ray greenhouse role physicists including José Crespo López-Urrutia from MPIK and colleagues have worked with the DESY team and eight other institutions worldwide to investigate the highly charged state of iron ions systematically. A mobile electron beam ion trap generates and stores the ions at the PETRA III storage ring where they can be exposed to powerful X-ray beams so that the absorption can be measured precisely.

This new laboratory-based astrophysical data show good agreement with the latest theoretical calculations. In addition to the characteristic energies of the absorption lines found in the spectra, their natural line width (for the first time measured in this experiment) is also very important, because it determines the maximum radiant power that a single iron ion can handle.

The data suggest that this amounts to almost one Watt of power per ion for the observed X-ray transitions. Even within the solar core, iron ions are not yet saturated with respect to radiation transport, because they can absorb and emit X-ray photons a million times faster than can normal atoms do with the much less energetic visible photons. This combination of high rates and high photon energy crucially determines the dominance of iron in the solar radiation balance. The data will allow astrophysicists to improve the stellar models they use to explain the characteristics and evolution of stars.

Related Links

Phys Rev Lett 2013, 111, 103002: "X-Ray Resonant Photoexcitation: Linewidths and Energies of K-alpha Transitions in Highly Charged Fe Ions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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