Helium abundance in the solar envelope

Abstract

The abundance of helium in the solar envelope can be determined using the variation of the adiabatic index of the stellar material in the second helium ionization zone. All techniques for inferring helium abundance from the observed frequencies of solar p modes are known to be sensitive to the equation of state used in the reference models. The sensitivity of inferred helium abundance to the equation of state is studied by using different reference models with MHD and OPAL equations of state. Recent observations of high-degree solar p-mode frequencies yield a helium abundance Y=0.246 when determined using reference models with the MHD equation of state and Y=0.249 using the OPAL equation of state. Further, the models constructed with the OPAL equation of state are found to be in better agreement with the inferred sound speed below the HeII ionization zone.