Asteroseismology of red giant stars

Abstract

If appropriately excited, a star will oscillate like a giant spherical instrument. In most stars, including the Sun, surface convection provides the excitation mechanism (Goldreich and Keeley, 1977). With turbulent velocities reaching speeds comparable to the local sound speed near the surface of the star, the vigorous convective motions can excite standing acoustic waves. These are known as pressure or p modes because the restoring force arises from the pressure gradient. The broad frequency spectrum of this excitation mechanism gives rise to many oscillation modes, both radial and non-radial, excited simultaneously. These stochastically excited and intrinsically damped oscillations were first detected in the Sun (Leighton et al., 1962), and hence are commonly known as solar-like oscillations. When stars grow old and the supply of hydrogen fuel is exhausted in the core, their envelopes expand and cool: they become subgiants and eventually red giants. Like the Sun, low-mass red giants have convective outer envelopes but the much longer convective time scales drives oscillation modes at much lower frequencies. The expansion and contraction of different parts of the stellar surface when a star oscillates gives rise to variations in temperature -- and hence also luminosity -- across the stellar surface. In this chapter asteroseismology of low-mas (M<3M_Sun) is described.