Light and colour variations of Mira variables in the Small Magellanic Cloud

TL;DR

This study characterizes the light and colour variations of Mira variables in the Small Magellanic Cloud using 15 years of optical and near-infrared data, revealing phase lags, colour inversions, and relations between period, magnitude, and colour.

Contribution

It provides new bolometric correction relations applicable across pulsation phases and analyzes phase lags and colour inversions in Mira variables, enhancing understanding of their variability.

Findings

Bolometric correction relations are consistent across pulsation phases.

Phase lags differ between O-rich and C-rich Mira variables.

Colour inversions occur at specific phases without clear triggers.

Abstract

The goal of this paper is to characterise the light variation properties of Mira variables in the Small Magellanic Cloud. We have investigated a combined optical and near infrared multi-epoch dataset of Mira variables based on our monitoring data obtained over 15 years. Bolometric correction relations are formulated for various near-infrared colours. We find that the same bolometric correction equation holds for both the bolometricly brightest and faintest pulsation phases. Period-bolometric magnitude relations and period-colour relations were derived using time-averaged values. Phase lags between bolometric phase and optical and near-infrared phases were detected from the O-rich (the surface C/O number ratio is below unity) Mira variables, while no significant systematic lags were observed in most of the C-rich (the C/O ratio is over unity) ones. Some Miras show colour phase inversions, e.g., $H-K_{\rm s}$ at its bluest while $J-H$ and $J-K_{\rm s}$ are at their reddest values at about the bolometricly brightest phase. Their occurrence conditions were studied but no clear direct or indirect trigger was found. A large NIR colour change unassociated with stellar pulsation was observed in Miras with long secondary periods, and its possible explanation is described.