A variabilidade de estrelas Be vista por espectroscopia no ultravioleta

Abstract

Be stars are a type of main-sequence star that, at some point, have undergone mass ejection into a circumstellar environment. They are also known for their high rotational velocities and the presence of emission lines in their spectra, the latter caused by the circumstellar disk formed from the ejected material. However, the mechanism responsible for this ejection is still not well understood. The International Ultraviolet Explorer (IUE) was a space telescope that operated from 1978 to 1996, collecting data in the ultraviolet region and observing, among other objects, Solar System bodies, active galactic nuclei, and hot stars, including Be stars. The present work aims to quantify the spectral variations in IUE data for a large sample of Be stars in the ultraviolet, through an empirical analysis of their spectral properties, which are directly linked to circumstellar mass ejection. For this purpose, 94 Be stars were selected from the Database of Be Star Spectra (BeSS), filtering those objects with IUE spectra that have at least three observations in both the near- and far-ultraviolet regions. Temporal variations in flux levels and spectral indices were analyzed, as well as the correlations between these quantities. Additionally, for each object, the mean and standard deviation of the measured quantities were calculated. Finally, possible correlations between the observed variations and both spectral subtype and projected rotational velocity were investigated. The results show that earlier-type Be stars display greater variability than later-type ones. Furthermore, there is a significant contribution from the disk in the ultraviolet spectrum, with a behavior similar to that observed in other regions of the electromagnetic spectrum.