Case study: Cartesius and the white dwarfs

Why is it that the surface of a white dwarf star contains so many heavy metals? To answer that question, professor of computational astrophysics Prof Simon Portegies Zwart and his PhD students had to simulate the entire solar system. The project is both an astronomical and computational challenge.

Asteroids in our solar system

Our solar system consists of a star (the sun), a small number of planets and a few hundred thousand debris: asteroids. "These are chunks of rock a few kilometres in diameter with their own orbit around the sun," says Portegies Zwart (Leiden University). "Because of gravity, those chunks interact with each other. Asteroids have very weak gravity, but in space it still plays a role. Especially if they have orbits that make them close to each other, that gravity is important."

"You have to include the entire solar system in your calculation"

Limits of the computer

Researchers wanting to calculate asteroid interactions face a huge problem of scale, both in space and time. "After all, you have to include the entire solar system in your calculation," Portegies Zwart explains. "At the same time, with collisions you are talking about a scale of several kilometres. In time, on the one hand you have the lifetime of the solar system: 4.5 billion years. But an encounter between two asteroids takes place on a time scale of about 10 seconds. With that, you run into the limits of what a computer can process."

Collision with star

With this research, Portegies Zwart's research group has two objectives, one astronomical and one computational: "We are looking at white dwarfs, embers of stars. It turns out that their surface is very rich in certain heavy metals, such as iron, tin and lead. These should actually have sunk into that star long ago. One possible explanation is a constant influx of heavy metals from space. Those asteroids could be responsible for that: either they collide directly with the star, or they collide with each other and the grit then rains down on the star. We are investigating that."

"We have been able to scale the code and run it efficiently on the supercomputer"

Calculating efficiently

The computational objective is that the researchers want to compute this as efficiently as possible using the power of a supercomputer. "We have been able to scale the code and run it very efficiently on the supercomputer," says Portegies Zwart. "That is now working, so now we can start working on the astronomical questions. That is long-term research, we are currently working on a calculation that has been running for almost a year."

All-round HPC centre

Portegies Zwart finds SURF a nice institution to work with: "They are fast, responsive and they have expertise on the equipment. I also rely a lot on big supercomputers abroad, but SURF is one of the nicest institutes to work with. It is an all-round high-performance computing centre. Precisely what surrounds it - the support - is very important to me. Just putting up a fast machine is not enough."