Main research area
of
Jan-Uwe Ness

1. Stellar coronae - what is that all about?

The term corona is derived from Latin, meaning crown. This crown shows up when, during a total solar eclipse, no direct sunlight can reach us and the outermost regions of the solar atmosphere become visible that are otherwise outshined by the light from the bright surface.
The solar corona is extremely hot, much hotter than the surface, which is very puzzling. For example, the Earth's atmosphere is much colder than the surface. The heating is to this day not understood. At these high temperatures the corona shines bright in X-rays, and since the surface is too cold to do that, all X-rays from the Sun come from the corona. The same applies to other stars, and X-ray observations are thus the perfect means to study the properties of stellar coronae with the aim to get a general understanding of the phenomenon.
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2. X-ray plasma diagnostics

The corona consists of highly ionized elements and free electrons. These electrons travel with extremely high velocities (corresponding to a high kinetic temperature). The density of the plasma is very low, such that the rate of collisions is low, but any collisions that do take place are very violent, i.e., high amounts of energy can be transferred from a projectile to a target. The projectiles are electrons and the targets are ions, and the collisions can (depending on their energy) cause further ionization or an excitation. An excitation lifts an electron of the ion into a higher energetic state leaving the ion behind as in an excited state. The energy deposited in this way will be released by radiation (light) which can be measured from Earth. Any transitions between two different energy levels can be identified from the emitted light and thus, the abundance of the element and the ionization stage (and thus average temperature) can be found. Also, the average density can be found when looking at forbidden transitions (more in 2.1), which is one of the main subjects of my work.
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2.1 Density measurements with He-like ions

The density measurements are more tricky, because they can only be done with observations that are capable of resolving individual emission lines. Such high spectral resolution in X-rays is quite challenging, but the missions CHANDRA and XMM-Newton have accomplished this. The He-like ions have two lines arising from forbidden transitions (violating one or more selection rules). One transition has to violate two rules and other one only one rule. In cases of increasing densities, the population of the excited state that has to break two rules shifts into the other level by the increasing number of collisions, thus making that line stronger, while becoming itself weaker. The measurement of these emission lines allows quantitative conclusions of density.
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3. Scientific application

Many properties about the solar corona have been found out, but detailed processes leading to the production of a corona are still not understood. Also, very little is knwon about the status of the solar corona 1000 years ago, or even only 300 years ago (during the so-called Maunder Minimum). An approach to better understand coronal processes is to study nearby solar-like stars. Since the corona has its strongest radiation output in the X-ray regime, and the X-radiation from stellar coronae is not disturbed by other X-ray sources, e.g. from the surface, it is most reasonable to perform these studies in the X-ray regime.

Spectra (display of radiation in the form of an intensity value for each wavelength) supply most valuable information, when measured in high resolution. Such spectra are provided by the latest missions CHANDRA and XMM-Newton. For the first time it is possible to determine coronal plasma densities for stars other than the Sun.

The density information can be used in order to derive structural information for stellar coronae, which cannot be resolved spatially, due to their tremendous distances. Of course, only typical structural information can be derived, since we only measure the radiation from all sorts of different regions with different properties.

The grating spectra obtained with Chandra or XMM provide a resolution that is unprecedented. In fact, individual emission lines can be resolved which was never before possible for stellar coronae in the range 5-100 A. However, the number of photons collected in individual emission lines is very small, such that special efforts have to be undertaken in order to obtain lines intensities. For measuring these weak lines, careful statistics are required. Conventional methods assume Gaussian statistics for fluctuations, but for small numbers of counts, this assumption does not hold. In collaboration with Rainer Wichmann I developed a program in C that takes the special challenges of X-ray spectra into account. I called it Cora 3.4 (actual version); the naming is derived from the original background: CORonal Activity, but the program can be applied to all sorts of other spectra as well.

Equipped with these tools, the X-ray spectra of various cool stars can be analyzed which are obtained with the LETG (Low Energy Transmission Grating) onboard CHANDRA. The aim is to measure coronal densities, plasma temperatures and emission measures (measures for the amount of radiation per volume). From these measurements structural information can be derived. It is reasonable to assume that the plasma is confined to loop-like structures as observed in the Sun. These loops are formed along the magnetic field lines. For the Sun, scaling laws have been developed which can now be applied to other stars. The scaling laws correlate physical properties (temperature, density and brightness) with geometrical properties (size). At the time before densities could be measured, no scaling laws could be applied, because the set of physical properties was incomplete. Now, the densities are available, and rough average statements about the sizes can be made.

4. Results

To be continued

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