Tracing a billion photons with Condor

 Champaign 26 Jan 00 The Limber Observatory in Texas used Condor Flock to trace a billion photons to solve the physics of Be star disks. The Wisconson Condor system was used for twenty runs, on 50 computers that each needed between 12 to 48 hours computing time for each run.

The "Alliance Data link news letter" reports that staff at the Limber Observatory in south Texas are exploring new territory: the computational territory of the physics of Be star disks. So-called Be stars are non-supergiant B-type stars whose spectra currently have, or had at one time, one or more Balmer lines in emission (bright spectral features from energetic hydrogen atoms, unusual for normal stars).

David McDavid , a researcher at the Limber Observatory , and his colleagues are exploring the source of the Be phenomenon by calculating the polarization of the light from a Be star due to electron scattering in the circumstellar disk. McDavid received an Alliance allocation in 1998 on the University of Wisconsin's Condor Flock . Runs made in early 1999 on Wisconsin's Flock produced a significant advancement in the area of Be star research.

The Condor concept was applied in its simplest form by running identical copies of the program on multiple machines and combining the output. The program was configured to read an initialization file (no change required except to use a unique random number seed for each machine), trace the assigned number of photons (no change required), dump the final photon counts binned by direction of escape (by inserting 5 new lines of code at the appropriate point), and terminate. A shell script was written to create 50 working directories and generate the individual input data files.

Approximately 20 runs were needed to test the models and refine the parameters. Each test run took between 12 and 48 hours of real time for the 50 machines to trace a total of 10^9 (one billion) photons.

From their runs on the Condor Flock, McDavid and his colleagues concluded that density wave models that explain the line profiles are consistent with the polarization data. Thus the spiral density wave model passes the "polarization test." This significant advancement was presented at International Astronomical Union Colloquium 175 in Alicante, Spain in June 1999.

 

Ad Emmen

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