Peng (2006) proposed that the heavy-ion fusion reaction 14^{14}N+16^{16}O →\to 26^{26}Al+α\alpha ('reaction (A)'), proceeding through the 30^{30}P compound nucleus in explosively burning oxygen-rich zones, could provide an alternative, non-proton-capture source of the Galactic 26^{26}Al inventory traced by the 1.809 MeV gamma-ray line — operating in core-collapse supernovae (SNII), where violent convection would mix 14^{14}N from hydrogen-burning shells into oxygen-rich layers, and in thermonuclear supernovae (SNIa) from accreting O–Ne–Mg white dwarfs. The proposal rests on accelerator-mass-spectrometry cross-section measurements and order-of-magnitude yield estimates with unquantified assumptions about the branching fraction in the supernova Gamow window, the amount of 14^{14}N actually present in oxygen-burning zones, the density and duration of the high-temperature phase, the participating mass fraction, and the absence of any time-dependent hydrodynamic nucleosynthesis test. The open problem is to determine quantitatively whether this channel contributes appreciably to the Galactic 26^{26}Al budget; a quantitatively justified negative determination resolves the question equally.

Background

Radioactive 26^{26}Al (half-life 7.2×1057.2\times10^{5} yr) decays by β+\beta^+ to an excited state of stable 26^{26}Mg, emitting the 1.809 MeV gamma-ray line. Because the Galaxy is largely transparent to MeV photons, sky maps of this line (built first by the COMPTEL telescope on the Compton Gamma Ray Observatory, then by the SPI spectrometer on the INTEGRAL gamma-ray observatory) measure ongoing nucleosynthesis: the diffuse emission corresponds to a steady-state Galactic 26^{26}Al inventory of order 22–3 M⊙3\,M_\odot, i.e. a production rate of a few solar masses per Myr (Diehl et al. 2006 derive 2.8±0.8 M⊙2.8\pm0.8\,M_\odot). In the standard picture, 26^{26}Al is made in hydrostatic hydrogen burning through the Mg–Al chain — predominantly 25^{25}Mg(p,γ)26(p,\gamma)^{26}Al — and is expelled by massive stars, through the intense winds of Wolf–Rayet stars (a hydrogen-stripped late evolutionary stage of massive stars) and through core-collapse supernova (SNII) ejecta, with smaller contributions from classical novae (thermonuclear explosions on accreting white dwarfs) and asymptotic-giant-branch stars. Around 2005 this accounting was uncomfortable: core-collapse models of the time underproduced 26^{26}Al and predicted a 60^{60}Fe/26^{26}Al ratio higher than observed; Wolf–Rayet yields depended strongly on metallicity and, to reach the observed totals, on rapid stellar rotation; and revised low-energy nuclear rates (a smaller 27^{27}Al(p,α)(p,\alpha)/27^{27}Al(p,γ)(p,\gamma) branching in the Mg–Al chain, and the LUNA (Laboratory for Underground Nuclear Astrophysics) measurement reducing the 14^{14}N(p,γ)15(p,\gamma)^{15}O rate) all lowered predicted yields from proton-capture chains.

Peng (2006) proposed a production channel requiring no free protons at all: the heavy-ion fusion reaction

14N+16O→26Al+α^{14}\mathrm{N}+^{16}\mathrm{O}\rightarrow^{26}\mathrm{Al}+\alpha

(called reaction (A)), forming the 30^{30}P compound nucleus, which also decays through competing exit channels 28^{28}Si+pp+nn and 25^{25}Mg+pp+α\alpha. With the exit-channel branching fraction η\eta defined as the fraction of fusion events exiting through 26^{26}Al+α\alpha, the paper cites accelerator-mass-spectrometry (AMS) measurements by He et al. (2005) — which built on the sub-barrier 14^{14}N+16^{16}O fusion cross-section measurements of Switkowski et al. (1977) — as indicating η\eta of order 20% or more at low relative kinetic energies, together with a thermonuclear reaction rate from He, Yang and Peng (1992, Acta Astrophysica Sinica, in Chinese). At explosive oxygen-burning temperatures T9≈2T_9\approx2–33 (where T9T_9 denotes temperature in units of 10910^9 K, corresponding to Gamow-window center-of-mass energies of a few MeV for this ion pair), the paper's single-zone estimates give 26^{26}Al mass fractions from ∼10−8\sim10^{-8} at T9=2T_9=2 to ∼10−3\sim10^{-3} at T9=3T_9=3 (for density 10610^6 g cm−3^{-3}, initial 14^{14}N mass fraction 10−310^{-3}, η=0.2\eta=0.2, burning duration 10 s) — an extreme temperature sensitivity spanning about four orders of magnitude. The central astrophysical difficulty is compositional: 14^{14}N is the principal ash of hydrogen burning in the CNO cycle (the carbon–nitrogen–oxygen catalytic hydrogen-burning cycle of massive stars), but it is destroyed by α\alpha-captures during helium burning (ending mainly as 22^{22}Ne), so the oxygen-rich layers that reach T9∼2T_9\sim2–33 in an explosion normally contain essentially no 14^{14}N. The proposal therefore assumes either (i) violent convective mixing that transports 14^{14}N from hydrogen-burning shells into oxygen-rich burning zones during core-collapse explosions, or (ii) accretion of 14^{14}N-rich matter onto oxygen–neon–magnesium (O–Ne–Mg) white dwarfs whose thermonuclear burning (SNIa) reaches the required temperatures. With assumptions such as 1% of a 20 M⊙20\,M_\odot supernova participating and a Galactic supernova rate of about 3 per century, the paper argues the channel could contribute at the level of the observed inventory. A further nuclear subtlety is that only the ground state of 26^{26}Al feeds the 1.809 MeV line on Myr timescales (the low-lying isomer, a metastable nuclear excited state, decays within seconds), so the ground-state production cross section, measured separately by He et al. (2007), is the physically relevant quantity.

Problem Statement

Determine quantitatively whether the fusion reaction 14^{14}N+16^{16}O →\to 26^{26}Al+α\alpha ('reaction (A)'), operating in explosively burned oxygen-rich zones of core-collapse supernovae (SNII, where convection would have to mix 14^{14}N from hydrogen-burning shells into the oxygen layers) and of thermonuclear supernovae from accreting O–Ne–Mg white dwarfs (SNIa), can account for the Galactic 26^{26}Al inventory observed through the 1.809 MeV gamma-ray line. A quantitatively justified negative determination — that the channel's contribution is negligible — resolves the question equally. The physical ingredients and assumptions on which the proposal rests include: (1) the exit-channel branching fraction η\eta into 26^{26}Al+α\alpha — separately for the ground state and isomer — and its uncertainty as a function of energy across the Gamow window (the energy range within which nuclear reactions contribute at a given temperature) relevant to explosive oxygen burning at T9≈2T_9\approx2–33 (T9T_9 denoting temperature in units of 10910^9 K), anchored to the accelerator-mass-spectrometry (AMS) cross-section measurements; (2) the abundance of 14^{14}N actually available in the burning zones, i.e. the efficiency and extent of convective and hydrodynamic mixing during explosions, given that helium burning normally destroys 14^{14}N before oxygen burning; and (3) the characteristic densities, durations of the high-temperature phase, and fraction of progenitor mass participating in the reaction for the different supernova types. These ingredients are physical inputs that must be evaluated jointly in the single quantitative determination of this channel's share of the Galactic 26^{26}Al budget.

References

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