Helium abundance in the solar envelope

Abstract

The variations in the adiabatic index in the second helium ionization zone of the Sun allow us to infer the helium abundance in the solar envelope using the observed solar oscillation frequencies. These variations leave their signature on the sound-speed in this region, hence, techniques based on solar sound speed inversion can be used to determine the abundance of helium. These techniques are known to be sensitive to the equation of state used in the reference models. Sensitivity of the helium abundance measurements to the equation of state is studied using models constructed with MHD or OPAL equations of state. Recent observations of high degree solar pmodes yield helium abundance Y=0.246 and 0.249, respectively using reference models with MHD and OPAL equations of state. Further, the models constructed using OPAL equation of state are found to be in better agreement with the inferred sound speed in the Sun, particularly below the second helium ionization zone.