Nuclear Astrophysics - Novae

Classical novae are caused by a thermonuclear explosion on the surface of a white dwarf within a binary stellar system. For several reasons, classical novae are special laboratories for nuclear astrophysics. First, the nuclear physics input needed to model these objects includes a restricted number of nuclear reactions. Second, the nova Gamow window is accessible in the laboratory, without recourse to cross section extrapolations. Third, classical novae are presently the only stellar explosions for which the nuclear physics input is primarily based on experimental information as opposed to theoretical calculations. We have identified a number of elemental abundance ratios that are sensitive to the peak temperature of the explosion and the degree of mixing between the accreted material and the underlying white dwarf. Potentially, more information could be obtained from isotopic ratios in presolar dust grains. Such grains (stardust) have been found in primitive meteorites, but surprisingly, grains that can be unambiguously linked to classical novae have not been identified yet. 

An image of the classical Nova V1974 Cygni 1992 is shown below. It was one of the brightest classical novae in 20 years and is located about 10,000 light years from Earth. Classical novae are caused by matter accreted from a companion star onto the surface of a white dwarf. The resulting thermonuclear runaway produces a nearly spherical, expanding shell, containing significant amounts of the elements neon, magnesium, aluminum, and silicon. In fact, the high neon content of 5% by mass in this and similar “neon” novae supports the theory that these outbursts occur not on the common CO white dwarfs, but on the more massive ONe white dwarfs. The latter objects result from the prior evolution of intermediate-mass stars that undergo core carbon burning. 

Nova CygniThe modeling of thermonuclear explosions is a highly complex affair, and thus it is crucial to constrain some of the key stellar model parameters by comparing observed elemental abundances to predictions from computer simulations. Our group has extensive experience with nucleosynthesis simulations. In 2002, we published one of the very first reaction rate sensitivity studies for explosive nucleosynthesis. That work has been very successful at sparking many new reaction measurements at radioactive and stable ion-beam facilities worldwide. More recently, we have made significant progress on two fronts. First, we have identified pairs of observed elemental abundances whose ratio is sensitive to the peak temperature achieved during the nova explosion. We find that N/O, N/Al, O/S, S/Al, O/Na, O/P, and P/Al abundance ratios represent particularly sensitive “nuclear thermometers” of neon novae. Second, in a companion study, we identified pairs of observed elemental abundances whose ratio is sensitive to the degree of mixing between the accreted matter and the white dwarf matter during the explosion. We demonstrated that Ne/H, Mg/H, Al/H, and Si/H are particularly sensitive “nuclear mixing meters” for neon novae. 

A logical extension of these elemental abundance studies is the investigation of how reaction rate variations impact isotopic abundances. This question is crucial for identifying presolar (stardust) grains that originate from classical novae. Presolar grains represent pristine, micron-sized time-capsules that originated from circumstellar envelopes and stellar ejecta before the solar system was born. Today they are found embedded in primitive meteorites and they are identified based on isotopic ratios that differ by several orders of magnitude from solar values. Presolar grains originating from supernovae and AGB stars have already been identified in the laboratory, but no presolar grains from classical novae have been unambiguously identified yet.

Further Reading