Weighing Titanium in a Supernova express
When a massive star explodes as a supernova, heavy nuclei are forged in its interior within seconds. Among them is titanium-44, a radioactive isotope that later glows in gamma rays for years. How much of it remains depends on a single nuclear reaction: a proton is captured by a vanadium-45 nucleus, turning it into chromium-46. This is like a narrow doorway through which some titanium leaks out of the synthesis chain.
Recently, physicists measured this reaction directly for the first time, but the accuracy was limited by the mass of chromium-46. Now, with a new mass value that is four times more precise, the reaction rate at stellar explosion temperatures has increased to 69%. In a supernova model with a mass of 20 solar masses, this yields 26% more titanium-44 in the ejected material.
Interestingly, for supernova 1987A, the changes are almost negligible. It all comes down to the composition of the ejecta: if there are many free protons, the reaction proceeds actively, but if the material is richer in neutrons, protons are scarce and the door is nearly shut. So titanium-44 is sensitive to how 'proton-rich' the explosion was.
Original abstract
as written by the authors arXiv:2608.17757v1
The thermonuclear $$^{45}$$V($$p,\gamma$$)$$^{46}$$Cr reaction is the primary leakage pathway from the $$^{44}$$Ti--$$^{45}$$V quasi-equilibrium cluster during $$\alpha$$-rich freeze-out in core-collapse supernovae (CCSN), governing the final abundance of the $$\gamma$$-ray-emitting isotope $$^{44}$$Ti. A recent high-resolution $$\gamma$$-ray study [C. Cousins \textit{et al.}, Phys. Rev. Lett. 136, 252701 (2026)] identified ten previously unknown low-spin proton-unbound states in $$^{46}$$Cr, enabling the first experimentally constrained $$^{45}$$V($$p,\gamma$$)$$^{46}$$Cr reaction rate using the AME2020 mass excess, $$\text{ME}(^{46}\text{Cr}) = -29472(11)$$~keV. Here, we adopt the four-fold more precise CSRe mass excess $$\text{ME}(^{46}\text{Cr}) = -29477.2(2.6)$$~keV [M.~Wang \textit{et al.}, Phys. Rev. C \textbf{106}, L051301 (2022)] to recalculate the reaction rate. Including proton capture on the ground and first two excited states of $$^{45}$$V alongside new shell-model proton spectroscopic factors, we reduce mass-related rate uncertainties to a subdominant level. The revised rate is up to 69% higher than that of Cousins \textit{et al.} at $$\alpha$$-rich freeze-out temperatures ($$T \simeq 1.5$$--$$2$$~GK). CCSN nucleosynthesis calculations show this revised rate increases the ejected $$^{44}$$Ti yield by $$\sim$$26% in a $$20\,M_\odot$$ model compared to The \textit{et al.} [ApJ \textbf{504}, 500 (1998)], while causing negligible changes for the SN~1987A trajectory. We demonstrate that $$^{44}$$Ti production sensitivity is dictated by the ejecta electron fraction ($$Y_e$$): the reaction significantly affects proton-rich ejecta ($$Y_e \approx 0.50$$) but has little impact on neutron-rich ejecta ($$Y_e \approx 0.496$$), where lower free-proton abundances suppress reaction flow. This reconciles conflicting results from past sensitivity studies.
Titanium-44 decays with a half-life of about 60 years — a blink of an eye by astronomical standards, but enough for a supernova remnant to glow in gamma rays for decades.