As early-type stars with a rotation speed close to their critical velocity, Be stars experience a kind of event called Be phenomenon. The material in their equator, during the Be phenomenon, is ejected to the outside space and forms a circumstellar disk. The mechanism triggering these events remains poorly understood, and observations of these events are limited since the duration of such events ranges from months to years. Long-term epoch photometry in the infrared bands is expected to be ideal for detecting Be phenomena since the brightness variation is larger than that in the optical, and the effect of interstellar extinction is also smaller. In this work, we conducted a systematic search for Be phenomena among Milky Way OBA stars in the mid-infrared. We examined the brightness and colour variations of known classical Be stars using the WISE W1 and W2 photometry bands to quantify their characteristics. Subsequently, we established a set of criteria to identify similar photometric variations in a large sample of OBA stars. 916 OBA stars have been found to show Be phenomena in the past 13 years, with 736 newly discovered. The peak-to-peak variations in magnitude and colour were found to be correlated, indicating the common presence of a decretion disk. The increase in colour was observed to be strongly correlated with the emission of the H{alpha} line, providing further evidence of the association with circumstellar disks. The brightness variation of a star with Be phenomena can be up to 1.5mag, and colour variations up to 0.4mag. The median durations for the disk build-up and decay phases are 474 and 524 days, respectively (while those less than 180 days are not sampled). The search for Be phenomena in the WISE bands greatly enlarge the number of stars showing disk variation, and makes multi-band photometry analysis of these events possible, with the help of current and future optical photometry surveys.

Cone search capability for table J/A+A/682/A59/table2 (Catalogue of BeT stars found in this study)