Semiregular variables with periods lying between the period-luminosity sequences C', C and D

TL;DR

This study investigates the distribution and pulsation modes of semiregular variables and Miras in the Large Magellanic Cloud, revealing new sequences and raising questions about the origins of certain long-period variabilities.

Contribution

It identifies a new sequence (F) between known sequences and analyzes the pulsation modes and period ratios of long-period variables in the LMC.

Findings

Most stars with periods between sequences C and C' are multi-periodic.

Stars between sequences C and C' tend to oscillate in the fundamental radial mode.

A new sequence F is identified, possibly related to long secondary periods.

Abstract

We analyze the distribution of semiregular variables and Mira stars in the period-luminosity plane. Our sample consists of 6169 oxygen-rich long-period variables in the Large Magellanic Cloud included in the OGLE-III Catalog of Variable Stars. There are many stars with periods that lie between the well known sequences C and C'. Most of these stars are multi-periodic and the period ratios suggest that these stars oscillate in the same mode as the sequence C stars. Models suggest that this mode is the fundamental radial pulsation mode. The stars with primary periods between sequences C and C' preferentially lie on an additional sequence (named F), and a large fraction of these stars also have long secondary periods that lie between sequences C and D. There are also a small number of stars with primary periods lying between sequences C and D. The origin of this long period variability is unknown, as is the cause of sequence D variability. In addition, the origin of sequence F is unknown but we speculate that sequence F variability may be excited by the same phenomenon that causes the long secondary periods.