Study of candidate Be stars in the Magellanic Clouds using near-infrared photometry and optical spectroscopy

Mennickent et al. and Sabogal et al. identified a large number of classical Be (CBe) candidates (similar to 3500) in the Large and Small Magellanic Clouds (LMC and SMC) based on their photometric variability using the OGLE II data base. They classified these stars into four different groups based on the appearance of their variability. In order to refine and understand the nature of this large number of stars, we studied the infrared properties of the sample and the spectroscopic properties of a subsample. We cross-correlated the optical sample with the IRSF-MCPS catalogue to obtain the J, H, Ks magnitudes of all the four types of stars (similar to 2500) in the LMC and SMC. Spectra of 120 stars belonging to the types 1, 2 and 3 were analysed to study their spectral properties. Among the four types, the type 4 stars are the dominant group, with similar to 60 and similar to 65 per cent of the total sample in the LMC and SMC, respectively. The near-infrared (NIR) colourcolour diagrams suggest that the type 4 stars in the LMC have a subclass, which is not found in our Galaxy or in the SMC. This subclass is similar to 18 per cent of the type 4 sample. The main type 4 sample which is similar to 49 per cent of the total sample has NIR properties similar to the Galactic CBe stars and the SMC type 4 stars. Though the new subclass of type 4 stars have high E(B-V) similar to 0.75, they are not located close to regions with high reddening. The type 3 stars (similar to 6 per cent and 7.3 per cent in the LMC and SMC) are found to have large Ha equivalent width (EW) in the SMC and some are found to have large NIR excess. This small fraction of stars are unlikely to be CBe stars. Three stars among the type 3 stars in the LMC are found to be double periodic variables. The type 2 stars are found in larger fraction in the SMC (similar to 14.5 per cent), when compared to the LMC (similar to 6 per cent). The spectroscopic and the NIR properties suggest that these could be CBe stars. The type 1 stars are relatively more in the LMC (similar to 24 per cent) when compared to the SMC (similar to 13 per cent). The SMC type 1 stars have relatively large Ha EW and this class has properties similar to CBe stars. The spectroscopic sample of type 1 stars which show Ha in emission and are confirmed as CBe stars are more abundant in the SMC by a factor of 2.6. If the effect of metallicity is to cause more CBe stars in the SMC, when compared to the LMC, then type 1, type 2 and type 4 stars follow this rule, with an enhancement of 2.6, 2.4 and 1.3, respectively.