Thesis defense
Student: André Luiz Figueiredo
Program: Astronomy
Título: "Be star demographycs: a comprehensive study of thousands of lightcurves in the Magellanic Clouds"
Advisor: Prof. Dr. Alex Cavaliéri Carciofi
Judging Committee
- Prof. Dr. Alex Cavaliéri Carciofi (IAG/USP - Orientador e Presidente da banca)
- Profa. Dra. Jane Cristina Gregorio Hetem (IAG/USP)
- Prof. Dr. Phillip Andreas Brenner Galli (IAG/USP)
- Prof. Dr. Armando Domiciano de Souza (Observatoire de la Côte d’Azur)
- Prof. Dr. Bruno Dias (Universidad Andrés Bello - Chile)
- Prof. Dr. Ronaldo Savarino Levenhagen (UNIFESP)
Abstract
Be stars are enigmatic objects, characterized by their high rotation rates and the ability to eject matter, forming a gaseous disk around them. Since the beginnings of observational astrophysics, about 2400 stars have been identified in the Milky Way. The present work is dedicated to the study of approximately 3000 multi-band light curves of Be stars from the Magellanic Clouds, with a coverage of more than twenty years each. After careful selection and filtering work, 1751 Be stars candidates were identified in both galaxies, a number that, compared to the above, gives an idea of the great scope of this work. This sample, coming from the OGLE survey, is certainly a more extensive comprehensive database on this type of object, covering a total of more than 60 millennia of photometry. The main aim of this work is to study the properties and variability of Be star disks in both galaxies, studying how mass and metallicity affect the manifestation of these disks and stellar activity. Each light curve was manually analyzed to distinguish between different dynamic scenarios such as disk formation, dissipation, inactivity, etc. The analysis is based on dynamic disk scenario, in addition to a grid of photospheric models, used to diagnose the inclination of these systems (which profoundly impacts the photometric signature associated with the emergence of a disk) and estimate the stellar mass. This allowed us to classify the activity of these stars, measuring, for the first time, quantities such as the fraction of time that the star actively loses mass (duty cycle), the fraction of time for which the star has a detectable disk (disk duty cycle), the rate of eruptions per year, among other quantities. The duty cycle has a wide distribution, with median values of 0.44 (LMC) and 0.60 (SMC). The disk duty cycle is high in both galaxies, with median values of 0.99 (LMC) and 1.0 (SMC), indicating that disks are almost always present for these stars. The rate of eruptions varies from approximately zero to twice per year, with median values of 0.31 (LMC) and 0.26 (SMC) eruptions per year. All aspects of variability are positively correlated with stellar mass. There are strong statistical differences in the behavior of the LMC and SMC populations, with low metallicity stars being more active in terms of their duty cycles and disk duty cycle. Although the SMC stars have less frequent eruptions, they last longer and produce discs with greater density, compared to the LMC sample. It is possible to conclude that both mass and metallicity are determining factors in the manifestation of Be stars, significantly influencing how the Be phenomenon appears in these stars. These results represent an important advance in identifying the physical mechanisms that determine the characteristics of Be stars. Furthermore, the present study leaves a relevant legacy for the literature, providing a valuable basis for future research with the stars in the sample, by providing several data on the characterization of the disks and an estimate of the mass of the central star, applicable to a comparable sample to all Be stars known in the Galaxy to date.
Keywords: photometry, circunstellar material, mass loss, Be stars, Magellanic Cloud