Study of candidate Be stars in the Magellanic Clouds using NIR photometry and optical spectroscopy

TL;DR

This study investigates the infrared and spectroscopic properties of candidate Be stars in the Magellanic Clouds, revealing new subclasses and differences in star populations related to metallicity and location.

Contribution

It provides a detailed analysis of NIR and spectroscopic features of Be star candidates, identifying a new subclass in the LMC and comparing star populations across the Magellanic Clouds.

Findings

Type 4 stars dominate the sample and include a new subclass not found in the Galaxy or SMC.

Type 3 stars are unlikely to be CBe stars due to their properties.

SMC has a higher fraction of certain CBe star types compared to the LMC, possibly due to metallicity effects.

Abstract

Mennickent et al.and Sabogal et al.identified a large number of Classical Be (CBe) candidates in the L&SMC based on their photometric variability using the OGLEII database. They classified these stars into four different groups based on the appearance of their variability. We studied the infrared properties of the sample as well as the spectroscopic properties of a subsample. We cross-correlated the optical sample with the IRSF catalog to obtain the J, H, Ks magnitudes of all the four types of stars in the L&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 is the dominant group. The NIR colour-colour diagrams suggest that the type 4 stars in the LMC have a subclass, which is not found in our Galaxy or in the SMC. The main type 4 sample which is \sim 49% 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) \sim 0.75, they are not located close to regions with high reddening. The type 3 stars (\sim 6% & 7.3% in the L&SMC) are found to have large H{\alpha} EW in the SMC and some are found to have large NIR excess. This small fraction of stars are unlikely to be CBe stars. The type 2 stars are found in larger fraction in the SMC, when compared to the LMC. The spectroscopic and the NIR properties suggest that these could be CBe stars. The spectroscopic sample of type 1 stars which show H{\alpha} in emission and 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.