§1.3 Neutron stars and x-ray bursters

If our bodies were packed as solidly as the atomic nuclei they contain, our masses would increase by ten trillion times, and each of us would have more mass than Mount Everest.

If the remnant of stellar collapse has a mass greater than the Chandrasekhar limit of 1.4 M_☉, the degeneracy pressure from the electrons is not sufficient to prevent further gravitational collapse. Collapse resumes until a density of approximately 10¹⁵ g⋅cm^-3 is reached, at which point the electrons and nuclei are packed so closely and collide so frequently that the electrons fuse with protons an “neutronize” the matter. For stellar masses in the range 1.4 M_☉ < M < 1.8 M_☉, the remnant at roughly nuclear density is held from further collapse by neutron degeneracy pressure. If the mass exceeds 1.8 M_☉, however, this neutron degeneracy pressure is insufficient to prevent the star from collapsing into a black hole [Rolfs and Rodney 1988].

In the collapse of a pre-supernova star (with r ≈ 1.5⋅10⁶ km and a rotational period of ≈ 30 days) to a neutron star (with r ≈ 150 km), the conservation of angular momentum leaves the remnant with a period of only ≈ 30 msec. (If the neutron star is accreting material from a companion, it may be further spun up by the additional angular momentum gained with the infalling material.) These rapidly spinning neutron stars are observed as pulsars in the visible and/or radio wavelengths. A famous example is the pulsar at the center of the Crab Nebula (SN1054), which pulses at 30 Hz in both radio and visible light.

The large amount of energy imparted to the infalling material by the strong gravitational field also causes x-ray emission. Many point sources of x-rays in the spiral arms of our galaxy are thought to be neutron stars. Neutron stars in close binary systems may occasionally be sites for thermonuclear explosions similar in principle to novae. These explosions are thought to be what we observe as “x-ray bursters”. Astronomers observe sudden (≈ 1 sec) increases by an order of magnitude or more in x-ray intensity, which take seconds or minutes to decay back down to normal levels.

The gravitational field of a neutron star is thought to be too strong for x-ray bursters to enrich the interstellar medium with their ashes. Astrophysicists are, however, interested in being able to accurately model the observable properties of x-ray bursters, such as the observed x-ray flux as a function of time. The risetime and peak luminosity of the light curve are dependent on the energy generation of the x-ray burster, which is dependent in large part on nuclear reactions. Energy generation is limited to approximately 10¹⁴ erg⋅g^-1⋅s^-1 in the CNO cycles by waiting point nuclei, independent of temperature. In order for the observed luminosities to be reproduced, greater amounts of energy are needed. One possible path to greater energy generation is “breakout” through ¹⁵O(α,γ) [Weischer Görres et al. 1999].

The purpose, then, of the measurement made in this thesis, is to better understand the reaction rates of ¹⁵O(α,γ), ¹⁸F(p,γ) and ¹⁸F(p,α). The next chapter will introduce the mathematical formalism for calculating the dependencies of these reaction rates on the properties of isolated, narrow resonances in ¹⁹Ne. Then it will review the recent literature to give the latest information on the known or calculated resonance properties. The rest of the chapters will be devoted to describing the experiments which we have recently carried out and their results, and to interpreting those results.