Variability of massive stars with known spectral types in the Small Magellanic Cloud using 8 years of OGLE-III data

Abstract

We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in the I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ∼80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity.

Photometric Variability of OB-type stars as a New Window on Massive Stars

“…We distinguished 4 modes of variability similar to the modes presented in Keller et al (2002) for classifying the blue variables in the LMC, that consist of high-amplitude, long-term outbursts ("bumper" events), short-term and low-amplitude, sudden outbursts ("flicker" events), monotonic trends across the 8-year time domain ("monotonic") and decline of magnitude that lasts up to 1000 days ("fading" events) (see Fig. 9, Kourniotis et al 2014). Of our 443 irregular variables, ∼76% are newly reported as variables in the present work.…”

“…We then flagged variables as "periodic with extra variability" when their periodic signal varies in amplitude or baseline by more than 0.05 mag (see Fig. 8, Kourniotis et al 2014). The remaining periodic variables consist of 126 classical Cepheids and 50 stars flagged as "rotating variables" being sinusoidally periodic variables with unequal minima which implies a strong indication of tidal distortion effects for an ellipsoidal star or starspot modulation.…”

“…In addition, we explore the occurrence of the Oe/Be phenomenon and provide a large number of candidate non-radial pulsators, which can be further studied via asteroseismology. The results of this work (Kourniotis et al 2014) will contribute to a better understanding of the role of metallicity in triggering processes associated to matter ejections and/or disk formation, which in turn affect mass loss and stellar rotation. …”

Photometric identification of the periods of the first candidate extragalactic magnetic massive stars

“…The data appear significantly variable (the star was in fact reported as an "irregular variable" by Kourniotis et al 2014); a clear long-term modulation is readily detected by eye in OGLE data (Fig. 1).…”

Identification of red supergiants in nearby galaxies with mid-IR photometry

“…Kourniotis et al 2014). Szczygieł et al (2010) showed that the majority of the identified variable massive stars in the LMC are RSGs, while the RSGs and AGB stars in M 33 have been the subject of variability studies in the near-IR (Javadi et al 2011) andmid-IR (McQuinn et al 2007).…”