3-D simulations of nova explosions

Scientists at the Universitat Politècnica de Catalunya. BarcelonaTech (UPC) have for the first time simulated critical phenomena that occur during nova explosions. Their work has made it possible to precisely characterise the physical properties and chemical composition of the material ejected in novae, and this has yielded the solution to an enigma that has puzzled experts for over 50 years: the origin of the irregular, inhomogeneous distribution of nova ejecta.

The paper, published recently in Nature, has facilitated analysis of the role these thermonuclear explosions play in the chemical enrichment of the galaxy.

As a result of the complex nuclear phenomena that take place inside stars, the universe has evolved from a chemically poor state -- dominated exclusively by the presence of hydrogen, helium, and traces of lithium -- to one containing nearly a hundred stable elements. Without the variety of elements now present in the cosmos, the formation of structures like planets and stars and the genesis of live forms -- including the calcium in our bones, the iron in our blood, and the uranium we use in our nuclear power plants -- would have been impossible. Most of chemical elements originated in supernovae and novae, titanic stellar explosions.

Novae are cataclysmic stellar phenomena that take place in binary systems consisting of a compact stellar object (a white dwarf star the size of a planet but with a mass of up to 1.4 times that of the Sun) and a low-mass star. The stars must be close enough for the intense gravitational field of the white dwarf to tear material away from the outer layers of its companion.

Novae, which are relatively frequent in our galaxy (some 30 to 35 nova-like explosions occur each year), are the third most energetic stellar explosions in the universe, after supernovae and gamma-ray bursts.

Novae and supernovae have been observed by humans for over two thousand years. The way these stars suddenly become much brighter -- a change sometimes observable to the naked eye -- has given rise to a great variety of conjectures about their origin.

With the advent of new, more precise observational techniques (such as photometry and spectroscopy) experts have been able to precisely characterise some physical properties of the material ejected during nova explosions, such as its chemical composition. Scientists know that the material transferred by the companion star is often of solar composition (i.e., close to 98% hydrogen and helium by mass). But other elements, in the range between carbon (C) and calcium (Ca) on the periodic table, can account for 30% to 50% of the material ejected during a nova explosion.

The origin of this peculiar pattern of chemical abundances and their irregular distribution in nova ejecta is an enigma in the field of stellar astrophysics that has eluded explanation for almost half a century. According to the most likely hypothesis, when the material transferred by the companion star piles up on the white dwarf, mixing episodes occur at the interface between the outer layers of the white dwarf and the envelope of transferred material. But the characterisation of this mixing process has provoked vivid discussions. One of the proposed mechanisms could not be rigorously simulated until sufficiently powerful computational tools were available.

Simulations with the MareNostrum supercomputer

Now, a team of researchers led by Jordi Casanova, a PhD student, Jordi José, a Professor of Physics at UPC's Department of Physics and Nuclear Engineering, and Enrique García-Berro, Professor of Physics at the Department of Applied Physics, has shown that the accumulation of material on the white dwarf via this mechanism is unstable. This instability results in mixing episodes involving material at the interface between the outer layers of the white dwarf (rich in elements like carbon and oxygen, or oxygen and neon) and the envelope of transferred material. The researchers were also able to establish the extent of heavy-element enhancement caused by this phenomenon, which up to now had been deduced from observational data.

This phenomenon was demonstrated based on 3-D simulations of the mixing process, which the team performed for the first time for nova explosions. The simulations were possible thanks to the use of sophisticated tools like the MareNostrum computer at the Barcelona Supercomputing Centre -- Centro Nacional de Supercomputación (BSC-CNS), after 150,000 hours of calculation.

The possibility of recreating 3-D physical phenomena like convection under the conditions that prevail during nova explosions also led to a solution to the second part of the puzzle. Scientists already knew how energy transport by convection works in stars but could offer only a formal account of this phenomenon. Now, thanks to the study conducted by the UPC researchers, it has been numerically shown that, under the conditions that exist during a nova explosion, the matter (plasma) is in a turbulent state; its movement is unstructured, almost chaotic.

The intermittent nature of this turbulence results in irregularities in the chemical distribution of material in ejected envelopes. Up until now, this phenomenon had never been numerically tested in nova explosions (in fact it had not even been proposed as a hypothesis to explain the origin of the observed chemical heterogeneity). The heterogeneity was attributed until now to a lack of precision in the measuring process. The UPC study shows that this phenomenon is real, and that it is caused by the intermittent nature of the turbulent phenomena that occur in stellar plasma as it undergoes a thermonuclear explosion.

Researchers Jordi Casanova and Jordi José are attached to the Barcelona College of Industrial Engineering (EUETIB) and the Space Studies Institute of Catalonia; Enrique García-Berro is attached to the Castelldefels School of Telecommunications and Aerospace Engineering (EETAC) and the Space Studies Institute of Catalonia.

Steven Shore of the University of Pisa (Italy) and Alan Calder of Stony Brook University (USA) also participated in the project, which has received support from the Spanish Ministry of Science and Innovation, the Government of Catalonia (through the Agency for the Management of University and Research Grants), the European Union's ERDF programme, and the European Science Foundation.