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

Abstract

We discuss the properties of pulsations in the OGLE Small Amplitude Red Giants (OSARGs) in the Large Magellanic Cloud (LMC). We consider stars below the red-giant tip in this paper. They are multi-periodic 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 nonradial dipole p${}_4$ and quadrupole p${}_2$ modes. The red-giant branch stars of OSARGs consist of stars have initial masses of $\sim 0.9-1.4M_{\odot }$ which corresponds to a luminosity range of $\log L/L_{\odot }\simeq 2.8-3.4$. With these parameters, the scaled optimal frequency ${\nu }_{\max }$ for solar-like oscillations goes through roughly the middle of the three sequences in the period-luminosity plane, suggesting the stochastic excitation is likely the cause of the pulsations in OSARGs.

8 pages, 13 figures, accepted for publication in MNRAS

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• Self-Excited Pulsations and the Instability Strip of Long-Period Variables: the Transition from Small-Amplitude Red Giants to Semi-Regular Variables
• Chronos --- Taking the pulse of our Galactic neighbourhood (ESA Voyage 2050 White Paper)
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