Study of the transient nature of classical Be stars using multi-epoch optical spectroscopy

TL;DR

This study monitors nine classical Be stars over five years to analyze their disc-loss and formation episodes through Hα line profile variations, revealing transient disc behaviors and timescales of disc evolution.

Contribution

It provides a detailed multi-epoch spectroscopic analysis of Be star disc variability, identifying phases of disc loss, formation, and stability over several years.

Findings

Four stars are undergoing disc-loss episodes.

One star is likely forming a new disc.

Four stars have stable discs in recent observations.

Abstract

Variability is a commonly observed property of classical Be stars (CBe) stars. In extreme cases, complete disappearance of the H{\alpha} emission line occurs, indicating a disc-less state in CBe stars. The disc-loss and reappearing phases can be identified by studying the H{\alpha} line profiles of CBe stars on a regular basis. In this paper, we present the study of a set of selected 9 bright CBe stars, in the wavelength range of 6200 - 6700 {\AA}, to better understand their disc transient nature through continuous monitoring of their H{\alpha} line profile variations for 5 consecutive years (2015 -- 2019). Based on our observations, we suggest that 4 of the program stars (HD 4180, HD 142926, HD 164447 and HD 171780) are possibly undergoing disc-loss episodes, whereas one other star (HD 23302) might be passing through disc formation phase. The remaining 4 stars (HD 237056, HD 33357, HD 38708 and HD 60855) have shown signs of hosting a stable disc in recent epochs. Through visual inspection of the overall variation observed in the H{\alpha} EW for these stars, we classified them into groups of growing, stable and dissipating discs, respectively. Moreover, our comparative analysis using the BeSS database points out that the star HD 60855 has passed through a disc-less episode in 2008, with its disc formation happening probably over a timescale of only 2 months, between January and March 2008.