Density greyscale of an axisymmetric simulation (top half only shown) of a propagating jet (Mach number M=10, jet-to-ambient density ratio η=0.1) from left to right with: (top) passive magnetic field; (middle) strong toroidal field (plasma beta β=0.2); and (bottom) strong poloidal field (β=0.2) all after the same propagation time. In 2-D, a toroidal field enhances propagation speed, a poloidal field impedes it.

Credits: D.A. Clarke and N.R. MacDonald

The radiation from the Big Bang must travel almost all the way through the Universe before it reaches the Earth. While during early research much effort was spent on calculating the precise properties of this radiation, which is known as the Cosmic Microwave Background, today experimental measurements are sufficiently precise to actually measure the impact of all the material in the universe the radiation passes by and through. Aside from interacting with hot gas in clusters of galaxies, the radiation also feels the impact of the gravitational fields of galaxies and clusters of galaxies. This produces a gravitational lensing effect on the CMB, although as these two images above show, indicating the temperature of radiation, the effect is exceptionally subtle and one cannot expect to see the differences with the eye alone. Research carried out by Dr. Rob Thacker, in collaboration with Dr. Diego Saez at the University of Valencia and other collaborators in Spain, is looking to combine a number of different foreground effects after this initial lensing study. These simulations require not only cosmological evolution calculations, but must also trace the history of each individual ray of radiation as it traverses through the simulated virtual universe!

ICA graduate student Diego Castañeda is extending his work on matching the observed spectrum of the rapidly rotating star α Oph (see December, 2011 Image of the Month) by beginning to compare individual lines. The results of the comparison for the Mg II h and k lines are shown in this month’s image. The green curve is the IUE data, while the blue line is the computed flux profile. The computed flux comes from a weighted intgration of the intensity of the radiation emitted in the direction of the observed over the visible surface of the 2D stellar model with rotation. The model is the one used previously to match the entire spectral energy distribution for α Oph. The integration includes the Doppler broadening produced by the (assumed uniform) surface rotation with a surface equatorial velocity of 236 km/s. While not perfect, the agreement between model and data in this wavelength interval is encouraging.

Computing Oscillation Frequencies of Red Giants

The space observatories, Kepler, CoRoT, and MOST are being used to observe the natural frequencies of vibration on stars. The seismic data can be used by stellar modelers to study the interior structure of stars. Red giants are evolved stars with dense cores surrounded by a diffuse and extended envelope. Their highly concentrated cores pose a computational challenge to asteroseismologists trying to model the oscillation spectrum of these stars. The upper figure (normalized radial amplitude variation, Y1, versus radius fraction ) shows the eigenfunction of a typical oscillation mode in a red giant and the lower figure shows a blow up of just the inner 1% of the star. The rapid variations are real and, needless to say, difficult to resolve numerically. The oscillations in the core couple to oscillations near the surface of the star. Since it is the surface oscillations that are observable, there is the additional challenge of computing the observable manifestation of these mode couplings.

David Guenther, Professor