An Asteroseismic H-R Diagram

Sarbani Basu

The mass and age of a main sequence (MS) star can be easily and accurately determined through asteroseismology. The oscillation frequencies of an MS star follow regular patterns on the whole. The so-called large separation, i.e., the difference in frequency of modes of successive radial order (n), but of the same degree (l), depends mostly on the mass and radius of a star. On the other hand, the small separation, i.e., the small difference in frequency of modes of nearly same order, but different degrees, is affected by the innermost layers of the star. The ratio of the small to the large separation is even more sensitive to the central conditions, and hence, the stellar age. When this ratio is plotted against the large separation for stellar models of different mass and age, we obtain a nearly orthogonal projection of the stellar mass and age. Since these separations can be directly derived from the observed frequency spectrum of a star, one can estimate the stellar mass and age from this diagram.