Var C: Long-term photometric and spectral variability of an LBV in M33

TL;DR

This study analyzes over a century of photometric and spectral data of the LBV Var C in M33, revealing a potential 42.3-year semi-periodic cycle and providing insights into LBV variability and evolution.

Contribution

It presents the first long-term analysis of Var C's variability, suggesting a semi-periodic cycle and comparing its behavior to other LBVs, advancing understanding of LBV evolution.

Findings

Possible 42.3-year semi-periodic cycle identified

Historical data confirms secular variability over 100 years

Light curve similarity to LMC R71 suggests common evolutionary features

Abstract

So far the highly unstable phase of luminous blue variables (LBVs) has not been understood well. It is still uncertain why and which massive stars enter this phase. Investigating the variabilities by looking for a possible regular or even (semi-)periodic behaviour could give a hint at the underlying mechanism for these variations and might answer the question of where these variabilities originate. Finding out more about the LBV phase also means understanding massive stars better in general, which have (e.g. by enriching the ISM with heavy elements, providing ionising radiation and kinetic energy) a strong and significant influence on the ISM, hence also on their host galaxy. Photometric and spectroscopic data were taken for the LBV Var C in M33 to investigate its recent status. In addition, scanned historic plates, archival data, and data from the literature were gathered to trace Var C's behaviour in the past. Its long-term variability and periodicity was investigated. Our investigation of the variability indicates possible (semi-)periodic behaviour with a period of 42.3 years for Var C. That Var C's light curve covers a time span of more than 100 years means that more than two full periods of the cycle are visible. The critical historic maximum around 1905 is less strong but discernible even with the currently rare historic data. The semi-periodic and secular structure of the light curve is similar to the one of LMC R71. Both light curves hint at a new aspect in the evolution of LBVs.