Var C: Long-term photometric and spectral variability of a luminous blue variable in M 33

Abstract

Aims. 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. Methods. Photometric and spectroscopic data were taken for the LBV Var C in M 33 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. Results. 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.

On the Social Traits of Luminous Blue Variables

“…Var C has been observed in several maximum light episodes (Burggraf et al 2015) since its initial discovery. It entered another maximum light phase in 2013 (Humphreys et al 2014a).…”

“…It entered another maximum light phase in 2013 (Humphreys et al 2014a). A period analysis of its light curve suggests possible semi-periodic behavior of 42.4 years (Burggraf et al 2015). Nearby stars include three hot supergiants and a possible RSG (Figure A5).…”

A Statistical Analysis of the Accuracy of the Digitized Magnitudes of Photometric Plates on the Timescale of Decades With an Application to the Century-Long Light Curve of Kic 8462852

“…This work is unprecedented and of the greatest importance for the astrophysical community. Recent studies have made use of these data in the fields of GRBs (Hudec et al 2013), RR Lyrae (Liška & Skarka 2015), LBVs (Burggraf et al 2015;Walborn et al 2014), AGN flares (Grindlay & Miller 2015), stellar activity cycles (Oláh 2014), novae (Tang et al 2012), dust accretion events (Tang et al 2011), long-period eclipsing binary systems (Rodriguez et al 2016), or long-term amplitudes in K giants (Tang et al 2010).…”

Luminous Blue Variables

“…The S Dor variability is the one and only clear distinction of LBVs from other massive evolved stars. An example of a long term lightcurve is given for the LBV Var B in M 33 in Figure 1, the analog version for Var C was published by Burggraf [16]. Also plotted here are the changes of the spectral type for the star, that mark an S Dor cycle.…”

“…LBVs is a combination of a photometric brightness and color change, caused and accompanied by changes in the stellar spectrum. During such a S Dor variability or S Dor cycle which lasts years or decades [15,16] the star varies from a optically fainter to a brighter star and back. This variability is therefore caused by the star changing from an early (hot) to a late (cool) spectral type, it implies also A more elaborate photometric classification, based on the duration of the S Dor cycle was made by van Genderen in 2001 [19].…”