Can Gravitational-Wave detections tell us how stars fuse Oxygen out of Carbon? – Farmer et al. (2020)

1 Oct
Prediction of the location of the black hole mass gap as a function of the assumed reaction rate of C 12+ He4 -> O16 (Farmer et al. 2020)

Stars are cosmic factories that produce most elements heavier than helium in their deep interiors through nuclear fusion. The rate at which Carbon is fused into Oxygen is notoriously uncertain. It cannot be measured at any laboratory at earth, at least not under the conditions that occur in the interiors of stars.

Rob Farmer (postdoc at UvA) showed earlier that the location of the Pair Instability mass gap is a remarkably robust prediction, more solid than anything we know about the final stages of the most massive stars.

In this new paper, he shows that this prediction depends on the assumed rate for the rate at which Carbon is destroyed by alpha particles to make Oxygen. Why? High rates basically lead the star to consume all it’s carbon prematurely, already during the helium-burning phase. Being devoid of carbon, such a start skips the Carbon burning phase and has to resort to burning Oxygen immediately. In very massive stars this ignites explosively leading to a pair-instability supernova or pulsations.

Rob Farmer
Rob Farmer (UvA)

Instead, stellar models in which a lower rate is assumed for this reaction, still have carbon that they can burn. When burning Carbon in a shell they prevent premature contraction of the star holding up the outer layers. This can prevent the explosive ignition of Oxygen. Instead, the more gentle burning calmly depletes the core of its fuel leading to an inescapable final implosion to a black hole.

Long story short, assuming that the reaction is less efficient allows more massive stars to form black holes. Reducing the rate by 3 sigma moves the lower mass gap up to 100 solar masses. The hope is that gravitational wave detections will probe the location of the black hole mass gap and will eventually allow is to start to learn about their very massive brilliant progenitor stars.