Masses of subgiant stars from asteroseismology using the coupling strengths of mixed modes

For a few decades now, asteroseismology, the study of stellar oscillations, has enabled us to probe the interiors of stars with great precision. It allows stringent tests of stellar models and can provide accurate radii, masses, and ages for individual stars. Of particular interest are the mixed modes that occur in subgiant solar-like stars since they can place very strong constraints on stellar ages. Here, we measure the characteristics of the mixed modes, particularly the coupling strength, using a grid of stellar models for stars with masses between 0.9 and 1.5 M _⊙. We show that the coupling strength of the ℓ = 1 mixed modes is predominantly a function of stellar mass and appears to be independent of metallicity. This should allow an accurate mass evaluation, further increasing the usefulness of mixed modes in subgiants as asteroseismic tools.