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

TL;DR

This study analyzes pulsation modes in small amplitude red giants in the LMC, identifying multiple radial and nonradial modes and linking them to stellar properties and stochastic excitation mechanisms.

Contribution

It provides a detailed identification of pulsation modes in OSARGs and connects observed sequences to specific radial and nonradial oscillations, advancing understanding of their pulsation behavior.

Findings

Sequences correspond to radial first to third overtones

Sequences also include nonradial dipole p4 and quadrupole p2 modes

Stochastic excitation likely causes pulsations in OSARGs

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 $\sim0.9 - 1.4M_\odot$ which corresponds to a luminosity range of $\log L/L_\odot \simeq 2.8 - 3.4$. With these parameters, the scaled optimal frequency $\nu_{\rm max}$ 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.