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

Abstract

Mennickent et al. and Sabogal et al. identified a large number of classical Be (CBe) candidates (∼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 (∼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 ∼60 and ∼65 per cent of the total sample in the LMC and SMC, respectively. The near‐infrared (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. This subclass is ∼18 per cent of the type 4 sample. The main type 4 sample which is ∼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) ∼ 0.75, they are not located close to regions with high reddening. The type 3 stars (∼6 per cent and 7.3 per cent in the LMC and SMC) are found to have large Hα 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 (∼14.5 per cent), when compared to the LMC (∼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 (∼24 per cent) when compared to the SMC (∼13 per cent). The SMC type 1 stars have relatively large Hα EW and this class has properties similar to CBe stars. The spectroscopic sample of type 1 stars which show Hα 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.

Optical spectroscopy of Galactic field classical Be stars

“…Subsequently, we estimated the emission strength ratio of Ca 8498:8542:8662 Å lines is 1:1:1 for nine cases which is quite different from the theoretically predicted value of 1:9:5 (Osterbrock & Ferland 2006;Polidan & Peters 1976). The theoretically predicted emission strength ratio 1:9:5 corresponds to an optically thin scenario whereas a deviation from this considers the line forming region as optically thick.…”

Proper motion separation of Be star candidates in the Magellanic Clouds and the Milky Way

“…Since SPM4 measured a V magnitude, a comparison with their OGLE-II's V could provide additional information to clarify these problematic matches and also to discard false matches, but the SPM4 photometry in the LMC and SMC areas comes mostly from photographic plates and has a poor quality compared to CCD photometry. We also noticed the J-H vs. H-K color-color diagram done with the JHKs 2MASS magnitudes listed by SPM4, suffered from a noticeable higher dispersion, as compared to those published by Paul et al (2012) using the InfraRed Survey Facility (IRSF) magnitudes by Kato et al (2007).…”

“…This last hypothesis was discarded in the study by Mennickent et al (2009). Paul et al (2012) showed that the photometric method used in the aforementioned works is effective in the selection of Be star candidates. Their spectroscopic analysis found that most of the stars studied from a sample of such candidates in both LMC and SMC, belong to early type stars with emission supporting circumstellar material.…”

Δa Photometric Survey of the Small Magellanic Cloud

“…There have also been detections of a number of bona fide Be and metal-weak stars (compared to the metallicity of the SMC). Three of those detections were already known as Be stars (Paul et al 2012), thereby constituting a valuable test for the reliability of our observations.…”