Frequency dependence of Delta_nu of solar-like oscillators investigated:   Influence of HeII ionization zone
  Oscillations in solar-like oscillators tend to follow an approximately regular pattern in which oscillation modes of a certain degree and consecutive order appear at regular intervals in frequency, i.e. the so-called large frequency separation. This is true to first order approximation for acoustic modes. However, to a second order approximation it is evident that the large frequency separation changes as a function of frequency. This frequency dependence has been seen in the Sun and in other main-sequence stars. However, from observations of giant stars, this effect seemed to be less pronounced. We investigate the difference in frequency dependence of the large frequency separation between main-sequence and giant stars using YREC evolutionary models. 
S. Hekker, Sarbani Basu, Y. Elsworth, W. J. Chaplin
astro-ph.SR