When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of elements with atomic number greater than 2, which come from previous stellar explosion [1]. These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (∼5 MK) and establishes hydrostatic equilibrium.
Over its main sequence life, the star slowly converts the hydrogen in the core into helium: The proton-proton chain fuses four proton-protons to one helium. Meanwhile the CNO-cycle is also producing helium by a sequence of(p, γ)reactions and beta decays starting with12C. The cycle eventually arrives at a(p, γ) reaction in 15N, leading to 16O·. Because 16O is an alpha-particle nucleus, the excitation energy at which it is produced is∼MeVhigher than the alpha-particle threshold in this nucleus. 16O· nucleus predominantly de-excites by emission of an alpha-particle. This(p, α) reaction then completes the cycle. Four protons are again converted into one alpha particle. When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue.
The core begins to contract, because the radiation pressure produced by the hydrogen fusion can no longer compete against gravity. This brings additional hydrogen from the outer layer into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core. The outer layers of the star then expand greatly due to radiation pressure, thus beginning the red-giant phase of the star’s life.
For stars of less than about 2 M [2] the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly108K, hot enough to begin helium fusing via the triple-alpha process, where two alphas fuse to8Be and a third alpha then fuses8Be to 12C. 12C can further capture a alpha and produce16O. The helium fusion results in the build up of a carbon–oxygen core.
When the central helium is exhausted, the star collapses once again, causing helium in a outer shell to begin fusing. At the same time additional hydrogen may begin fusion in a shell just outside the burning helium shell. A star below∼8 M[3] will never start fusion in its degenerate carbon–oxygen core.
Instead, at the end of the burning phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf, with a size about our earth [4]
and a mass about0.6 M[5]. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy. As a result, it cannot support itself by the fusion generated radiation pressure against gravitational collapse. It is supported only by electron degeneracy pressure, causing it to be extremely dense.
Approximately30 %of main sequence stars in our galaxy are observed to be within binary star systems [6]. In such a system one has two stars orbiting each other. In most cases the masses are different.
1
2 CHAPTER 1. INTRODUCTION One star of the binary system will undergo the stellar evolution faster caused by its heavier mass: The gravitational force is larger and therefore more hydrogen has to be burned to hold up the radiation pressure. Thereby the core runs out of hydrogen faster and the stellar evolution is accelerated.
While one star has evolved to a white dwarf the other one is still in his red-giant phase. Material from the red giants outer shell (mostly composed of hydrogen) can now pass the so called Roche limit. It is the distance within which a celestial body will disintegrate due to a second celestial body’s tidal forces exceeding the first body’s gravitational self-attraction. The pulled material spirals down on the surface of the white dwarf. Conservation of angular momentum about the system’s center of mass causes the accreted material to form an accretion disk. The kinetic energy of the accreted material is thereby converted into heat. On the white dwarfs surface the material forms a hot, degenerate envelope surrounding the white dwarf [7]. An illustration and an observation of the Chandra telescope is shown in Figure1.1.
Figure 1.1: Left image [8]: Chandra image shows Mira A (right), a highly evolved red giant star, and Mira B (left), a white dwarf; right image [9]: an artist illustration of the same stellar object
As the white dwarf consists of degenerate matter the accreted hydrogen does not inflate for increasing temperatures. Eventually the temperature is high enough that proton capture reactions can occur in a thin radial layer at the interface between the base of the envelope and the surface. This will cause a drive up in temperatures in the burning zone. When the temperature reaches ∼50 MK, the nuclear energy generation is dominated entirely by CNO-cycle burning [10,11].
Now a thermonuclear runaway takes place: The CNO-cycle burning in combination with the degen-erate matter rapidly drives up the temperature of the burning shell, without subsequent expansion and cooling as would be the case with an ideal gas. The degenerate conditions prevent an expansion.
Resonant proton-capture reactions onto the seed nuclei, that are now produced in the surface, begin to occur. These are explained in more detail in Chapter2. Such reactions can produce a high abundance fraction of nuclei between20≤A≤40.
Some 100 s to 1000 s later the degeneracy of the envelope is lifted, using 400 MK as the maximum temperature reached by Oxygen-Neon novae models. The temperature within the envelope exceeds the Fermi temperature of the degenerate matter and the envelope luminosity exceeds the Eddington luminosity limit, which is the maximum luminosity a star can reach with balance between radiation pressure and the gravitational force. This then causes the now non-degenerate system to behave like a ideal gas and pressure becomes temperature depended. The envelope explosively ejects the freshly forged elements into space [10, 12]. This is called anova. A recently observed nova, GK Persei, can be seen in Figure 1.2.
Only up to five percent of the accreted mass is fused during the power outburst [14]. So a white dwarf can potentially generate multiple novae over time as additional hydrogen continues to accrete onto its surface from the companion star. Eventually, the white dwarf could explode as a type Ia supernova, if it approaches the Chandrasekhar mass limit.
The total mass of ejected material in nova is10−4−10−5M [15]. Compared to the mass of a white dwarf (0.6 M) this is quite small. But the material contributes to the chemical enrichment of the interstellar medium. Observations of elemental abundances in the ejected shells can be used to test
Technische Universität München 2 Physics Department
1.1. NOVAE: THERMONUCLEAR RUN AWAY SITES 3
Figure 1.2: GK Persei; image contains x-rays from Chandra X-Ray Telescope (blue), optical data from NASA’s Hubble Space Telescope (yellow), and radio data from the National Science Foundation’s Very Large Array (pink). The x-ray data show hot gas and the radio data show emissions from electrons that have been accelerated to extremely high energies by the nova shock wave. The optical data reveal clumps of material that were ejected in the nuclear explosion [13]
nova model predictions [7]. Such observations can be done by looking at the isotopic abundance fractions of presolar grains.
Physics Department 3 Technische Universität München
4 CHAPTER 1. INTRODUCTION