Dr. Jan-Uwe Ness:
Research activities in a nutshell

• Magnetically induced activity of cool stars
  

  Magnetic activity entails various observable phenomena such as Sun Spots, chromospheric utraviolet emission, highly structured coronal X-ray emission, and coronal mass ejections. Whether or not any of these phenomena we know from the Sun are typical can only be decided by taking samples of other stars.
  My PhD thesis has focused on the coronal X-ray emission from other stars that can be detected at Earth, albeit not spatially resolved. Plasma diagnostics from line-resolving X-ray spectrometers allow determinations of temperatures and densities.
  Highlights of my work:
  

  • On the sizes of stellar X-ray coronae
    Temperatures and density derived from line ratios of a sample of stars are correlated to the X-ray luminosity, an acitivity indicator. Scaling laws from the Sun allow us to derive a rough idea of sizes and filling factors of stellar coronae, giving us a better understanding how coronae of much more actives stars look like.
  • Modeling the Ne IX Triplet Spectral Region of Capella with the Chandra and XMM-Newton Gratings
    The Chandra High-Resolution Grating Spectrometer allows for the first time to resolve the complex 13-14Angstrom region that contains the NeIX triplet that is heavily blended by other lines. The NeIX triplet allow important density diagnostics. While wavelengths and intensities of lines were already known from theoretical calculations and laboratory measurements, this benchmark study presents an astrophysical measurement and discusses how to best deal with the problem of line blending.
  • Are stellar coronae optically thin in X-rays?. A systematic investigation of opacity effects
    The analysis of coronal X-ray spectra assumes that the emission lines are produced by collisions followed by radiative de-exciations without any further interaction. Thus each photon, once produced, will escape without further absorption. This work investigates whether this important assumption is actually true using ratios of resonance lines and forbidden lines. This technique makes use of the fact that forbidden lines are much less likely to be absorbed, and if any absorption takes place, it will first affect the resonance lines, leading to anomalous ratios of resonance-to-forbidden lines. No such anomalies were found such that the assumption of optically thin plasma is justified. However, if the plasma is perfectly symmetric such that absorption and re-emission balance out, then resonance scattering can still take place, but this can not be determined.
  • Helium-like triplet density diagnostics. Applications to CHANDRA-LETGS X-ray observations of Capella and Procyon
    First application of the density technique of He-like triplets applied to a coronal plasma from distant stars. Both stars were found with low densities. Effects of UV radiation from the surface were detectable in Procyon.
  • Television report of solar flare (in German)

• X-ray observations of Classical Novae
  

Classical Novae are thermonuclear explosions on the surface of White Dwarfs. Evolution of a solar-like star ends with a White Dwarf after central nuclear burning has ceased. Without the central energy source, the star can no longer remain its equilibrium between gravitational forces and radiation pressure from the centre and collapses to a dense object. The collapse stops when the Pauli Principle would have to be violated.
Since all the central hydrogen has been converted to helium during the stellar life time, White Dwarfs are hydrogen deficient, but if a companion star orbits close to a White Dwarf that hydrogen-rich matter from that star can be transferred to the White Dwarf, it gains a new energy source. The hydrogen accumulate on the surface of the White Dwarf until temperature and pressure have increased to the point allowing nuclear burning again. This happens in an uncontrolled way (thermonuclear runaway), and the explosion leads to the ejection of mass, driven by radiation pressure. The high-energy radiation, produced while nuclear burning continues, is obscured by the ejecta, and radiation transport transforms it into optical emission. For observers from Earth, they look like a new star with the spectral type of an F giant. As the expansion continues and the ejecta become thinner, they become more transparent to high-energy radiation, allowing successively hotter plasma layers to become visible.
My research focuses on studies of X-ray emission of novae that is visible once the ejecta have cleared to the point that nuclear burning on the surface of the White Dwarf can be seen. This phase is called SuperSoftSource (SSS) phase, and high-resolution SSS X-ray spectra are atmospheres with blackbody-like continuum emission and deep absorption lines.
Highlights of my work:


• Short-period X-ray oscillations in super-soft novae and persistent super-soft sources
  Transient periods between 33-67 seconds have been observed in a small number of novae. They could arise from nuclear burning instabilities.
• Obscuration effects in super-soft-source X-ray spectra
  Some SSS spectra are atmospheric with blackbody-like continuum and absorption lines, but some are dominated by emission lines on top of a blackbody like continuum. I have defined the SSa and SSe clases with 'a' for absorption lines and 'e' for emission lines and have shown that SSe are always seen if some type of obscuration can be expected. For example, all high-inclination systems emit SSe spectra.
• High-resolution spectroscopy and high-density monitoring in X-rays of novae
  Review article making intensive use of spectral time maps to show various phenomena of spectral changes during variability.
• Observational evidence for expansion in the SSS spectra of novae
  Absorption lines in SSa are systematically blue shifted in all observed novae. This means that expansion of the ejecta is still continuing during the late SSS phase.
• High-Resolution X-Ray Spectroscopy of the Evolving Shock in the 2006 Outburst of RS Ophiuchi
  RS Oph is a famous recurrent nova that exploded in 2006 and was observed with the high-resolution grating spectrometers on board XMM-Newton and Chandra. The companion star is a giant with a dense stellar wind that absorbs some of the kinetic energy of the ejecta, converting it into shock X-ray emission. Shock spectra resemble those of stellar coronae, thus bremsstrahlung continuum and emission lines.
  An exemplary full fledged emission measure analysis of the shock spectra is explained and discussed utilizing two techniques. Elemental abundancies could be derived revealing overabundance of alpha elements compared to iron. If these elements come from the White Dwarf, this means that RS Oph will not explode as a Supernova Ia, even if the Chandrakehar mass limit of 1.4 solar masses is reached. Nucelar fusion from alpha elements (e.g. Mg, Si) to Fe is not sufficient to discrupt a White Dwarf.
• X-ray emission from Saturn
  

  Since X-ray photons have much higher energies than optical light, their number is much smaller for a given energy budget. X-ray observations thus need to be much longer than optical observations. If observing a planet, significant motion of the planet over the chip complicates the analysis. Since X-ray observations rely on photon counting with information of position, time, and energy available for each photon, a transformation can be performed that concentrates all photons from the planet into a small region on the chip.
  Until 2000, no X-rays from Saturn had been observed, and a careful analysis of a ROSAT observation taken in 1992 revealed a weak but significant signal of 22 photons on a background of 8.
  →Detailed Project Description
  → Collection of Press Releases
  → Life Television Interview (in German)
  Highlights of my work:
  

  • A search for X-ray emission from Saturn, Uranus and Neptune
    First discovery of X-rays from Saturn
  • X-ray emission from Saturn
    Confirmation of X-ray emission from Saturn with Chandra. Enough counts were found to study the spatial distribution over the disk. The X-ray emission is concentrated towards the centre of the visible disk. Since the planet was inclined, we could conclude that the equator is not the centre of X-ray emission but instead solar X-rays are reflected. This was later confirmed by Bhardwaj et al. (2005). In contrast to Jupiter, Saturn has no X-ray emission from the poles, thus auroral emission plays a small role.
  • Detection of Saturnian X-ray emission with XMM-Newton
    Clear detection of X-rays with XMM-Newton with enough signal to perform rough spectral diagnostics
• Simulation of interacting galaxies with starformation

Our Galaxy consists of 100 billion stars which move about the Galactic Centre. Each star is characterized by it location relative to the Galactic Centre, its momentum (thus mass and velocity), and its angular momentum. Their orbits are determined by Newton's laws of motion and their gravitational attraction. In a computer simulation, the motion of each single star can be computed taking into account its own motion and the combined attraction by all other stars. This is computationally too expensive for 100 billion stars but sophisticated approximations have been developed to operate in a feasible domain.
My Diploma Thesis builds on previous work:

• Evolution code of stellar motion: Tree Code. Gravitational forces from stars that are far away are weak, and many stars can be combined to a single compound object to compute only one equation of motion.
• A start model of a stable disk galaxy like the Milky way. Evolving it over large time scales does not significantly change its structure
• In addition to the disk model, a spherical dwarf galaxy is added that will merge with the disk galaxy (=minor merger). The vertical kinetic energy that is released to align the inclination angle of the dwarf galaxy with the ecliptic plane of the disk is converted to a higher vertical velocity dispersion in the disk, making it thicker. From the face on direction, spiral arms are temporarily formed.
• The gas content is modeled with a Sticky Particle scheme. Clould particles have a radius, and if two such particles pass by a distance smaller than the sum of their respective radii, they merge to a new particle with the sum of their masses. The new radius depends on the mass. An artificial limit had to be set to avoid that clouds grow too large over long time scales.
The goal was to study whether a star burst would be triggered by a minor merger. To achieve that, I have implemented an inverse process of growth of cloud particles which can be interpreted as star formation. Depending on mass, an associated time scale for desintegration is computed via Jeans criterium; small clouds take longer. When two coulds merge, a new time scale is computed. Once the time is up, the clould will be split into a stellar particle (no radius) and many small cloud particles, preserving momentum and angular momentum. The velocity of the new particle corresponds to the kinetic energy that supernovae typicall insert into the system.
The thickening of the disk as a result of depositing vertical energy in the disk, first leads to a reduction of the star formation rate because the volume increases and thus lower collision probability. However, one model also revealed a short star burst.