On the pulsation modes of OGLE small amplitude red giant variables in the LMC

We discuss the properties of pulsations in the Optical Gravitational Lensing Experiment small amplitude red giants (OSARGs) in the Large Magellanic Cloud. We consider stars below the red giant tip in this paper. They are multiperiodic and form three sequences in the period-luminosity plane. Comparing the periods and period ratios with our theoretical models, we have found that these sequences correspond to radial first to third overtones, and non-radial dipole p 4 and quadrupole p 2 modes. The red giant branch stars of OSARGs consist of stars have initial masses of ∼0.9-1.4 M which corresponds to a luminosity range of log L/L 2.8-3.4. With these parameters, the scaled optimal frequency νmax for solar-like oscillations goes through roughly the middle of the three sequences in the period-luminosity plane, suggesting that the stochastic excitation is likely the cause of the pulsations in OSARGs.