Paper Spotlight: “Determinations of the 12C/13C Ratio for the Secondary Stars of AE Aquarii, SS Cygni, and RU Pegasi”

A Type Ia supernova is the complete explosion of a white dwarf that was being orbited by a secondary star prior to the explosion. Their high luminosity allows them to be used as the primary cosmological distance indicator, yet their progenitors remain unknown. It is almost certain that the progenitors of Type Ia supernovae are a subclass of Cataclysmic Variable (CV); however, to prove this, more analysis on various details of the secondary star in the CV system must be done.

This was done in a recent publication in the Astrophysical Journal by Dr. Thomas E. Harrison and second-year graduate student Rachel Marra in their paper “Determinations of the 12C/13C Ratio for the Secondary Stars of AE Aquarii, SS Cygni, and RU Pegasi”. The full paper can be found here.

Based on the main theory of CV formation (binary star system with secondary star accreting material onto the white dwarf), the secondary star should not have had sufficient time to start the evolutionary process of moving off the main sequence into the red giant phase. This is because there is not enough time for this to happen since the secondary star is a low mass star, and therefore takes longer to evolve off of the main sequence. This suggests that the composition of the secondary star is expected to be fairly ‘normal’ compared to a main sequence star of a similar mass. However, this is shown to be incorrect by looking at spectroscopic data from Gemini North, which showed a higher than expected abundance of 13C and a lower abundance of 12C. In addition, enhanced levels of sodium and deficits of magnesium were found in the secondary stars of all three systems.

An explanation for these weird chemical abundances could be that the secondary star has started evolving (it has already started the process of evolving off the main sequence and becoming red giant) meaning that it has undergone the CNO cycle. The CNO cycle destroys 12C and creates 13C, making it the simplest explanation for the ratio of carbon isotopes seen in these secondary stars.

While this paper doesn’t definitively show that CVs are the progenitors of Type Ia supernovae, it does shed light on the unusual and unexpected abundances of the secondary stars in these objects.

Image Credit: NASA/CXC/M. Weiss

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