A new interpretation of the period-luminosity sequences of long-period variables

TL;DR

This paper reinterprets the period-luminosity sequences of long-period variables in the Large Magellanic Cloud, proposing a new mode assignment based on combined theoretical modeling and observational analysis, clarifying the pulsation modes involved.

Contribution

It introduces a novel combined approach to assign pulsation modes to PL sequences, challenging previous interpretations and providing a clearer understanding of the pulsation modes in LPVs.

Findings

Sequences B and C' both correspond to first overtone pulsation.

Sequences A', A, and C correspond to third overtone, second overtone, and fundamental modes.

The splitting of the first overtone sequence is due to the presence of long secondary periods.

Abstract

Period-luminosity (PL) sequences of long period variables (LPVs) are commonly interpreted as different pulsation modes, but there is disagreement on the modal assignment. Here, we re-examine the observed PL sequences in the Large Magellanic Cloud, including the sequence of long secondary periods (LSPs), and their associated pulsation modes. Firstly, we theoretically model the sequences using linear, radial, non-adiabatic pulsation models and a population synthesis model of the LMC red giants. Then, we use a semi-empirical approach to assign modes to the pulsation sequences by exploiting observed multi-mode pulsators. As a result of the combined approaches, we consistently find that sequences B and C$^{\prime}$ both correspond to first overtone pulsation, although there are some fundamental mode pulsators at low luminosities on both sequences. The masses of these fundamental mode pulsators are larger at a given luminosity than the mass of the first overtone pulsators. These two sequences B and C$^{\prime}$ are separated by a small period interval in which large amplitude pulsation in a long secondary period (sequence D variability) occurs, meaning that the first overtone pulsation is not seen as the primary mode of pulsation. Observationally, this leads to the splitting of the first overtone pulsation sequence into the two observed sequences B and C$^{\prime}$. Our two independent examinations also show that sequences A$^{\prime}$, A and C correspond to third overtone, second overtone and fundamental mode pulsation, respectively.