title: A study on the time evolution of Be stars
Creators: name: Castanon Esteban, A
Publisher: name: Liverpool John Moores University
Year: 2024

description: The work presented in this thesis shows the results from an in-depth study of a sample of 58 Be stars observed between the years 1998 and 2022. The spectroscopy of this sample has been studied, starting from the raw observations and converting them into data that could be analyzed. A classification based on the morphology of the Hα and Hβ emission was done, and this classification has been used for the rest of the study, showing repeated times the differences between objects with different classifications. A big part of the posterior study has focused on the difference between single and double peak emission stars. With this classification, a study of the separation between peaks has been done, showing that the emission of the Hβ line happens at a different part of the disk than the Hα line, and it hints at the fact that single peak lines might be formed from two very close emission peaks that have merged into one. The projected rotational velocities and the critical velocities of the central stars have been calculated, and it has been proven that the populations of the classification, have different projected rotational velocities trends. Some objects also show variability between emission and no emission, with a rate of change between 0.73% and 0.78% of the sample changing every year on average. Some basic polarimetry results have also been obtained from an observation taken with MOPTOP in the year 2022. After calculating the values of the Stokes parameters from the observations, and the polarization degree and angle from the Stokes parameters, the intrinsic and interstellar values of the polarization degree and angle have been separated using a big sample of nearby stars. After this, using the polarization degree, the angle dependency has been removed from the projected rotational velocity and finding that after this the single and double peak populations could be the same, meaning that the only difference between these two type of objects is the angle of inclination. After this, iii no direct relation has been found between the interstellar polarization degree and the distance or the reddening, meaning that other processes might be involved in increasing or decreasing the interstellar polarization degree. Finally, a study of the variability of the sample has been done. The V/R variability has been studied, showing period of T¯ = 0.86 ± 0.32 years for Hα and T¯ = 1.07 ± 0.39 years for Hβ. These values are much shorter than the usual estimated value of T ∼ 7 years that are considered for the V/R variability, and could indicate that the process is in the mid-term timescale instead of the long-term timescale. After that, a study of the emission variability and how it relates to other observables has been done, finding a higher value of the emission and the variability for stars around a stellar class of B1.5; however, no other relation was found with the polarization degree, the projected rotational velocity of the star, or the distance between peaks. Finally, the change in the values of the emission and the polarization of the sample were analyzed, finding no apparent correlation between them, which could mean that there is no relation between the radius and the density of the disk.