Realising efficient computation of individual frequencies for red-giant models

Context. In order to improve the asteroseismic modelling efforts for red-giant stars, the numerical computation of theoretical individual oscillation modes for evolved red-giant models has to be made feasible. Aims. We aim to derive a method for circumventing the computational cost of computing oscillation spectra for models of red-giant stars with an average large frequency separation Δν < 15 μHz, thereby allowing asteroseismic investigations of giants utilising individual frequencies. Methods. The proposed truncated scanning method serves as a novel method detailing how the observable individual l = 0, 1, 2 frequencies of red giants may be computed on realistic timescales through so-called model truncation. By removing the innermost region of the stellar models, the g-mode influence on the oscillation spectra may be avoided. This then allows us to estimate the observable regions from the resulting pure p-mode oscillations. The appropriate observable frequency regions are subsequently scanned for the complete and non-truncated stellar model. The observable regions are determined by considering the limitations on observability from the internal mode coupling and damping, yielding consistent frequency spectra obtained at a much reduced computational cost. Results. The truncated scanning method proves that the individual frequencies of red-giant models can be obtained for a wide range of applications and research. The computational efficiency is improved by a factor of 10 or better. This means that the inclusion of l = 1, 2 individual frequencies is now a possibility in future asteroseismic modelling efforts of red-giant stars. Further potential avenues for improvements of this method are outlined for future pursuits.