A note on the use of surface offset corrections in asteroseismic model fitting

We critically investigate the practice of adding a power-law surface offset correction f (ν) to the frequencies of stellar models prior to seeking best fit models to an observed star. We show that surface layer independent indicators of the internal structure, phase shifts and separation ratios, are displaced in frequency by f (ν) and are therefore not the same as those of the model. Consequently such best fit models do not have exactly the same interior structure as the observed star. Using results on the star HD 177153 we show that the difference between observed and model frequencies for best fit models obtained using surface layer independent procedures have a wide range of different offsets which do not in general follow a Kjeldsen-like power law, and further that best fit models obtained using the offset correction procedure do not necessarily satisfy surface layer independent constraints on the internal structure.