Variable stars in the Magellanic Clouds: results from OGLE and SIRIUS

We have performed a cross-identification between Optical Gravitational Lensing Experiment II (OGLE-II) data and single-epoch Simultaneous three-colour Infrared Imager for Unbiased Surveys (SIRIUS) near-infrared (NIR) JHK survey data in the Large and Small Magellanic Clouds (LMC and SMC, respectively). After eliminating obvious spurious variables, variables with too few good data and variables that seem to have periods longer than the available baseline of the OGLE-II data, we determined the pulsation periods for 8852 and 2927 variables in the LMC and SMC, respectively. Based on these homogeneous data, we studied the pulsation properties and metallicity effects on period–K magnitude (PK) relations by comparing the variable stars in the LMC and SMC. The sample analysed here is much larger than the previous studies, and we found the following new features in the PK diagram. (1) Variable red giants in the SMC form parallel sequences on the PK plane, just like those found by Wood in the LMC. (2) Both sequences A and B of Wood have discontinuities, and they occur at the K-band luminosity of the tip of the red giant branch. (3) The sequence B of Wood separates into three independent sequences B± and C′. (4) A comparison between the theoretical pulsation models and observational data suggests that the variable red giants on sequences C and newly discovered C′ are pulsating in the fundamental and first overtone modes, respectively. (5) The theory cannot explain the pulsation mode of sequences A± and B±, and they are unlikely to be the sequences for the first and second overtone pulsators, as was previously suggested. (6) The zero-points of PK relations of Cepheids in the metal deficient SMC are fainter than those of the LMC by ≈0.1 mag but those of SMC Miras are brighter than those of the LMC by ≈0.13 mag (adopting the distance modulus offset between the LMC and SMC to be 0.49 mag and assuming the slopes of the PK relations are the same in the two galaxies), which are probably due to metallicity effects.