Source: | January 3, 2020

University of Tennessee astrophysicist Eric Lentz. Image courtesy of Rachel Harken

Titan, the groundbreaking Cray XK7 supercomputer operated by the Oak Ridge Leadership Computing Facility (OLCF) at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL), was officially decommissioned on August 1.

The petascale machine ran countless simulations over its 7 years of service, and its sheer computational power was consistently in demand by researchers. But for a brief window, just prior to Titan’s decommissioning, only one simulation was running.

This simulation—the last to ever occupy the supercomputer—examined the final moments of a star’s life.

When stars like our Sun run out of fuel, they become red giants and then later white dwarfs. Larger stars, those at least 10 times more massive than the Sun, collapse into an extremely dense neutron star before launching a massive shockwave in the form of a supernova explosion.

University of Tennessee astrophysicist Eric Lentz, the last user of Titan, answered a few questions about the supercomputer’s impact on his project, as well as what the future holds for his research.

What were you simulating on Titan?

Lentz: We’ve been modeling core-collapse supernovae—explosions of massive stars. The state of an exploding star is consistently changing. It starts as a relatively low-density white dwarf-like iron core, and in the first second of collapse increases in density by about four orders of magnitude. These simulations are rather computationally expensive, as most things on Titan tend to be, and we don’t get a lot of opportunities to do a lot of runs; one or two full simulations in an allocation is the limit of what we’ve generally been able to do.

The simulations are part of a project to explore the variations and inputs that go into core collapse, which means the things that tend to influence the nature of stars like their composition and their mass. A lower-mass star like our Sun has a very different fate than a star that has, say, 10 or 20 times its mass at the beginning. The types of explosions we’re looking at, which have an important impact on developing galaxies by injecting newly made elements into them, come in a fairly wide variety of initial conditions themselves. Among the simulations we were running during the final month or so on Titan were models representing stars that had about 25, 15, and 10 times the mass of the Sun.

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