Long Secondary Periods in luminous red giant variables

Abstract

The origin of long secondary periods (LSPs) in red giant variables is unknown. We investigate whether stellar pulsations in red giants can explain the properties of the LSP variability. VIJHKs light curves obtained by OGLE and the IRSF/SIRIUS survey in the Small Magellanic Cloud are examined. The sample of oxygen-rich LSP stars show evidence of a phase lag between the light curves of optical and near- IR band. The change in radius contributes the bolometric change roughly half as much as the change in temperature, implying that the change in effective temperature plays an important role in the luminosity change associated with the LSPs. We have created numerical models based on the spherical harmonics to calculate the light amplitudes of dipole mode variability and have found that the models can roughly reproduce the amplitude - amplitude relations (e.g. ($\Delta I$, $\Delta H$)). The LSP variability can be reproduced by the dipole mode oscillations with temperature amplitude of $\lesssim$ 100 K and $\lesssim$ 150 K for oxygen-rich stars and most carbon stars, respectively. Radial pulsation models are also examined and can reproduce the observed colour change of the LSPs. However, there is still an inconsistency in length between the LSP and periods of radial fundamental mode. On the other hand, theoretical PL relations of the dipole mode corresponding to so-called oscillatory convective mode were roughly consistent with observation. Hence our result suggests that the observations can be consistent with stellar pulsations corresponding to oscillatory convective modes.