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Peculiar mixing in red giant stars

posted May 5, 2013, 6:55 PM by Jeremy Drake   [ updated Jun 3, 2013, 11:58 AM ]
Stars like the Sun are powered by nuclear fusion in their cores, essentially by conversion of  hydrogen into helium.  One way this is achieved is indirectly, through a cycle of nuclear reactions involving the elements carbon, nitrogen and oxygen - the so-called "CNO cycle".  In addition to converting H into He, this cycle also alters the amounts of carbon and nitrogen in the core, with carbon getting depleted and nitrogen enhanced.  As a result, the carbon isotope ratio 12C/13C is also changed from an initial value of 60 or so down to as low as 4.  
These reactions all occur beneath a layer of the star called the radiative zone that is stable and does not mix with the surface layers.  This means we see no change in the amounts of the different elements at the stellar surface.  Until the star becomes a red giant that is.  In becoming a red giant, a deep convection zone develops from the surface layers that reaches down and mixes with some of the core material.  This material is brought to the surface and we can see changes in the C and N abundances, and in the12C/13C ratio. These changes can provide key tests of our stellar evolution theory.  Theory has been failing in that carbon isotope ratios are often lower than model predictions, suggesting additional mixing is occurring not currently predicted by standard theory.  

We measured the C/N abundance in an interesting red giant star, lambda Andromeda, that is the only visible member of a close binary system with an orbital period of 20.5 days.  The orbital motion of the system enables a precise estimate of the mass of  lamda And, something that is very difficult to determine with any precision for a single star.  This enables us to compare observations with predictions of evolutionary models for a star of the same mass. We measured the C and N abundances using a special  method employing high resolution X-ray spectra obtained by NASA's flagship Chandra X-ray Observatory and ESA's XMM-Newton satellite.  The figure shows our measured C/N ratio vs the carbon isotope ratio found in an earlier study, together with model predictions as the coloured symbols.  Also shown are data from an earlier study of single but otherwise similar giants.  Like those stars, lambda And shows a C/N ratio unchanged by mixing, but a very low 12C/13C ratio.  This is puzzling because existing ideas about how extra mixing might work posit the mixing events to occur later than the evolutionary phase that lambda And is currently in.  This means we still do not fully understand the peculiar mixing of gas within red giants. This work was published in the 2011 November edition of the Astronomical Journal.