Carbon burning in massive stars – new findings from the STELLA team
The STELLA experiment team, including former USIAS Fellows Sandrine Courtin and David Jenkins, recently published research in Physical Review Letters indicating that the fusion of carbon (12C + 12C reaction) in massive stars could be quite a bit slower than expected. The new measures show a variation of almost an order of magnitude from the reference values used by astrophysicists in their models of stellar evolution, and this may have profound implications.
Essential to the life cycle of massive stars, the 12C + 12C fusion is a nuclear fusion reaction that takes place in the core of the star and which combines carbon into other elements. It may occur at different stages of stellar evolution, and impacts whether the late stages of the life cycle of a star are relatively peaceful or more explosive. In order to calculate reliable scenarios, astrophysicists therefore need to know the fusion rate as precisely as possible. The models developed in the 1980s lacked the necessary precision, especially at low levels of energy where the probability for fusion is also low.
Nuclear reactions taking place in stars can be reproduced in the laboratory but it is very challenging to do so because the probability for fusion is so low at the relevant energies. The approach usually taken is to measure the fusion probability at higher energies and extrapolate down into the astrophysically relevant energy window. However, while most reactions involving fusion of heavy ions like carbon show a smooth variation in cross section as a function of energy and can be readily extrapolated, the 12C + 12C cross section has a whole series of resonances which makes meaningful extrapolation difficult.
Actual measurements of the 12C + 12C reaction at low energies made in the past relied on detecting either charged particles or gamma rays emitted following fusion; each technique suffers from complications and systematic error.
The STELLA system was conceived to measure charged particles and gamma rays from 12C + 12C fusion in coincidence, which removes all the systematic uncertainty in earlier measurements. The experiment was mounted at the Andromede accelerator at IPN Orsay and ran for a total of 12 weeks of beamtime. With these very long running periods, it was possible to make a meaningful measurement at an energy corresponding to the conditions in the most massive stars, which resulted in the new measures. The STELLA team now hopes to look at carbon fusion at even lower energy levels, and is considering a similar experiment for oxygen.
Professor Sandrine Courtin was USIAS Fellow in 2015, and is based at the Hubert Curien Pluridisciplinary Institute (IPHC) of the University of Strasbourg in France. She has a long-standing collaboration with Professor David Jenkins from the Nuclear Physics Group at the University of York in the United Kingdom – and 2013 USIAS Fellow.
More information:
- Advances in the Direct Study of Carbon Burning in Massive Stars (Open access article in Physical Review Letters, 12/05/20).
- 12C + 12C fusion in massive stars could be slower than expected (News, University of York)
- La fusion 12C+12C dans les étoiles massives serait plus faible qu’attendu (News, CNRS) – in French
