§1.1 Stellar nucleosynthesis

…a star is a gravitationally controlled thermonuclear reaction in the sky.

Many astronomical phenomena require detailed knowledge of nuclear reactions for adequate explanation. For example, it is well known that the sun, the source of nearly all the energy which fuels our society, is itself fueled by the fusing of hydrogen into helium. Observations of solar neutrinos at Kamioka, Japan [Fukuda, Fukuda et al. 2001] and in Sudbury, Ontario [Ahmad, Allen et al. 2001] have confirmed this fact beautifully.

Balancing gravitational pressure with radiative pressure produced by the heating of their interiors by nuclear reactions stabilizes stars like our sun. Put another way, the stellar matter (mostly hydrogen) is squeezed to such high temperatures (average energy) and densities that nuclei are able to overcome their electrostatic repulsion and fuse with one another. The most typical pathway for this is called the pp-I chain:

In our sun, two other sequences, the pp-II and pp-III chains, are responsible for another 14% of the fusions. The amounts of energy lost to the neutrinos, which escape without interacting, differ slightly among the chains.

Figure 1: Flow diagram for the CNO cycles. Purple boxes represent the valley of β-stability. CNO-I is the dominant cycle, while the other numbered CNO cycles are additional side cycles through which processing may occur. Hot CNO is a process that produces energy at a greater rate at higher temperatures away from the valley of β-stability. The rate of energy generation in hot CNO reaches its maximum at approximately 100 MK at which point it is limited by the waiting points [Weischer Görres at al. 1999].

A second important fusion process is burning through the CNO cycles. This process catalyzes the fusion using existing stable carbon and nitrogen in the star. Mostly it burns through the CNO-I cycle:

At low temperatures and densities (T < 50 MK, ρ < 100 g/cm³), the rate of this cycle is limited by the ¹⁴N(p,γ) reaction [Weischer Görres at al. 1999]. For approximately one cycle in 1000, ¹⁵N captures a proton instead of cycling back to ¹²C in the CNO-II cycle:

There is some additional leakage out of CNO-II to the nucleus ¹⁸F via the CNO-III cycle:

For about one cycle in 150, the ¹⁸O may capture a proton instead. If the resulting ¹⁹F cycles back to ¹⁶O via (p,α), this is called the CNO-IV cycle. Alternatively, the ¹⁹F may capture a proton into ²⁰Ne upon which protons may be converted into alphas by the faster Ne-Na cycle. The ratio of the ¹⁹F(p,γ) rate to the ¹⁹F(p,α) rate is small and very uncertain [Weischer Görres at al. 1999].

At high enough temperatures (e.g., 150 MK at densities of 100 g∙cm^-3), the CNO cycles are turned into the “hotCNO” cycle. In the hotCNO cycle, proton capture proceeds to the drip line [Weischer Görres at al. 1999]:

The hotCNO cycle is the primary energy generation process in novae. Note that the rate at which this cycle operates is limited by the β⁺-lifetimes of the ¹⁴O (τ = 1.70 minutes) and ¹⁵O (τ = 2.94 minutes) nuclei. These lifetimes, unlike the capture reaction rates, are independent of temperature. Any point in a nuclear burning process where β-decay is the dominant path is called a “waiting point”. The waiting point nuclei ¹⁴O, ¹⁵O and ¹⁸F shown in (1.2), (1.3), (1.4) and (1.5) may have important roles in explosive nucleosynthesis. We will focus on the latter two: ¹⁵O and ¹⁸F.

One proposed alternative path to the A>18 region is the alpha-capture reaction on ¹⁵O. One of the purposes of the measurement described in this document is to constrain the possible values for this reaction rate. The rate of ¹⁸F(p,α) is one factor determining the temperatures and densities at which the hotCNO cycle may operate. In novae, proton-induced reactions on ¹⁸F play an astronomically observable role described in more detail below [Weischer Görres at al. 1999].