Institute for Computational Astrophysics

When we talk about the temperature of a star, we’re usually referring to what’s known as the star’s effective temperature.  For a star in local thermodynamic equilibrium (LTE), its effective temperature (Teff) is defined as the kinetic temperature (Tkin) of the star's atmosphere at the point where the Rosseland mean optical depth (τR) has a value of 2/3. While this value can be calculated directly for any given star, it takes time and computer resources to do so.

Using the model stellar atmosphere and synthetic spectrum code PHOENIX running on ACEnet’s Mahone cluster, Master’s student Christopher Cooke, in collaboration with ICA faculty member Dr. Ian Short, has created a grid of model stellar atmospheres to use as a reference for estimating the value of Tkin at τR = 2/3 for any arbitrary star with a Teff between= 5600 and 5900 K and a surface gravity (log (g)) between 3.5 and 5.0.

Because the Sun is the best understood star in terms of atmospheric properties, it was used to test how the resolution of the grid of reference models affects the accuracy of estimates calculated from the grid. The above picture shows a comparison of computed values of  Tkin throughout the Sun (Teff = 5780 K, log(g) = 4.44) to estimates of these values using two different sets of reference model atmospheres, plotted as ‘residuals’ – that is, the difference between the actual computed values and the estimated values. The blue line shows the residual values of Tkin that is found when the separation between the reference stars is ΔTeff  = 50 k and Δlog(g) = 0.5, while the red line shows the residual values of Tkin that is found when the separation between the reference stars is ΔTeff  = 300 k and Δlog(g) = 1.5. The vertical green line represents τR = 2/3, and so the points where the blue and red lines intercept the green vertical line are estimates of Tkin = Teff for each of the estimation methods.  Values to the right of the green line become chaotic due to convection in the lower parts of the stellar atmospheres.

 As one might expect, choosing a finer grid of references produces more accurate results. The difference between the estimated and modelled temperatures at τR = 2/3 for the finer grid is approximately 0.5 K, whereas for the coarser grid it’s over 2.0 K. These differences are very small relative to the effective temperatures of stars, though for very low mass stars the accuracy to which we can calculate Teff becomes important.  As one looks to larger – and therefore hotter – stars, however, coarser and coarser grids become very effective, and can save many hours or days of computing time.

The Institute for Computational Astrophysics (ICA) was formed to expand an area of expertise within the Department of Astronomy and Physics at Saint Mary's University. Since its creation in late 2001, the ICA has grown to six full time faculty members, and at any time there are on the order of eight graduate students and two post doctoral fellows amongst the ICA membership. This does not include the other faculty, graduate students, and post doctoral fellows in the Department of Astronomy and Physics. The ICA promotes research in computational astrophysics through the research and publication activities of the individual members, by hosting visitors and colloquium speakers, and by having all its members participate in national and international conferences.

The ICA is also responsible for providing its members with access to high performance computing resources. The ICA has its own small cluster for code development, but most computing is done through ACEnet, the academic research high performance computing provider for Atlantic Canada. ACEnet also provides other collaboration and visualization tools on the Saint Mary's campus. The ICA has played a significant role in the formation and direction of ACEnet, with one ICA member serving as the Principal Investigator of ACEnet since the project was funded, two others having served or currently serving on the Research Directorate, and one member on the Resource Allocation Committee. ACEnet also employs two full time technical support personnel stationed at Saint Mary's, and ICA members have been major beneficiaries of their expertise.

Data Cave Art Show and Lecture: 3D art exhibition presented by Lisa Frank         

Ph.D. student James Wurster has accepted a postdoctoral position at Monash University in Australia

Former ICA post-doc Dr Pascal Elahi has accepted a position at University of Sydney in Australia.

Chris Geroux successfully defended his Ph. D. thesis

Dave Williamson successfully defended his Ph. D. thesis

Anneya Golob successfully defended her M.Sc. thesis

Dr. Mike Casey has accepted a research position at the Bedford Institute of Oceanography.

Stephen Condran, ACEnet System Administrator for the  Brasdor cluster, has joined SMU ACEnet staff.

Ph. D. student Chris Geroux has accepted a postdoctoral position with Dr. Isabelle Baraffe at the University of Exeter.