Helium-burning Puzzler

The Triple-Alpha fusion process “burns” helium-4 into carbon-12 and powers stars on the Helium Main Sequence, the short-lived and bright After-Life of a star when it has exhausted its Core hydrogen and is reborn through the Phoenix-like pyre of the Red Giant Branch. A standard Triple-alpha process code, NACRE, reproduces the Red-Giant Branch neatly and has enjoyed a lot of support, but it is only approximate. This prompted Kazuyuki Ogata, Masataka Kan, and Masayasu Kamimura (OKK) to try to compute the Triple-alpha process over a range of relevant temperatures using more exact computational tools, thus producing a new temperature-reaction rate profile for the process. Now Aaron Dotter and Bill Paxton have given the new profile a test-run by using it to compute the evolution of low-mass stars, with surprising results… basically the ‘disappearance’ of the Red-Giant Branch (RGB) and an exceptionally long-lived, cooler Helium Main Sequence (HMS).

This is a seriously wrong result because the RGB and HMS are observationally very well circumscribed stages in stellar evolution – we would’ve noticed a cooler, longer HMS by ten times more stars actually in it, and we would’ve certainly have noticed a dearth of stars in the RGB. Looking deeper at the OKK profile the authors noticed it allowed a much earlier and cooler onset of helium burning at 30 million K rather than the NACRE derived 80 million K. This has dramatic consequences as helium burning is exquisitely sensitive to the temperature, which drives the RGB stage – basically the star is in a runaway nuclear reaction as its core collapses and heats up, ending only in the “Helium Flash”. The OKK profile still produces the Flash, but with a lower peak output and milder entry onto the HMS.

So the audience are now wondering: Can the discrepancy be resolved? Can theory and observation ever meet? Stay tuned…

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