Asteroseismology and mass loss in Be stars. Study with CoRoT

Abstract

The interiors of the stars are among the most difficult parts of the Universe to observe. Essentially, asteroseismology tries to make use of the oscillations to prove the stellar interiors, which are not directly observable. The basic principles of asteroseismology are, to a certain extent, similar to those developed and employed by Earth seismologists. Asteroseismology relies on advanced mathematical descriptions of oscillations in a three-dimensional body and numerical modeling. It is therefore a prominent example of interdisciplinary science. The general aim of this work is the study of Be stars with the CoRoT space mission. Classical Be stars are B-type stars that exhibit line emission over the pho- tospheric spectrum. The excess is attributed to a circumstellar gaseous component that is commonly accepted to be in the form of an equatorial disk. The mechanisms responsible for the production and dynamics of the circumstellar gas are still not constrained. Observations of non-radial pulsation beating phenomena connected to outbursts point toward a relevance of pulsation, but this mechanism cannot be gene- ralized. In this regard, the observation of classical Be stars with the high-precision CoRoT satellite is providing important keys to understand the physics of these objects and the nature of the Be phenomenon. In order to study the light variations of the selected stars we use photometric and spectroscopic observations. These observations allow us to extract frequencies, amplitudes and phases of these variations. As we will show, these light variations can be connected with pulsations on the stellar surface. For carrying out the frequency analysis we have developed a new code based on standard Fourier analysis. The point is that this code, called pasper, allows the frequency analysis of large sets of light curves in an automatic mode. This Ph.D. thesis is arranged as follows: In the first three Chapters we describe the scientific framework of this project, giving a brief description on Asteroseismology, presenting the current status of Be stars, and describing the basics of the Fourier analysis and the rudiments of the time series analysis. At the early begin of this Ph.D. thesis, the CoRoT satellite was still on ground getting ready for the launch. In this context, we perform a search for shortperiod B and Be star variables in the low metallicity environment of the Magellanic Clouds. This study constitutes the Part I of this Ph.D. thesis. This Part has a double goal: i) to test the frequency analysis codes; and ii) to detect observationally β Cephei and SPB-like B-type pulsators in low metallicity environments, actually not predicted by the pulsational theory and models. This constitutes the Part I. Part II is devoted to the study of Be stars with the CoRoT space mission. Here we depict a complete review on the CoRoT mission. We also describe the results on the analysis of three Be stars from the CoRoT exoplanet field. Finally, we present the results on the frequency analysis of the late Be star HD 50 209, observed in the seismology field of the CoRoT satellite. The analysis of this Be star has revealed up to sixty frequencies, grouped in six different and separated sets, attributed to g-mode pulsations. Finally, we resume the main conclusions of the whole project, including prospects and future work to be done. An addendum with all the published results derived from this project has been added at the end of this Part II. Part III encloses the Appendixes, providing a brief summary of this work in Spanish, a complete description on basic equations of non-radial oscillation, the user guide of the PASPER code and the user guide of the KURTZ_BOS code.

El objetivo de la asterosismología es describir el interior de las estrellas a partir de las oscilaciones. El interior de las estrellas es, posiblemente, el lugar más difícil de estudiar de todo el Universo, ya que las condiciones especiales que allí se dan no pueden reproducirse en ningún laboratorio terrestre. Así, la importancia de esta ciencia en el estudio global de la física estelar es crucial. Los principios básicos de la asterosismología son, en esencia, similares a los que los geólogos usan en el estudio de terremotos terrestres. La asterosismología hace uso de descripciones matemáticas de las oscilaciones en cuerpos tridimensionales, creando para ello sofisticadas simulaciones numéricas. Es, por tanto, un claro ejemplo de ciencia multidisciplinar. El objetivo general de esta Tesis Doctoral es el estudio de las estrellas Be con las misión espacial CoRoT. Las estrellas Be son objetos de secuencia principal, con alta velocidad de rotación, que presentan un exceso infrarrojo y emisión en las líneas de Balmer, debido a la presencia de una envoltura circumestelar concentrada en el ecuador y generada por eyecciones discretas de materia originado por mecanismos que todavía no son bien conocidos. Los mecanismos propuestos para explicar la eyección de materia son hasta ahora, las pulsaciones no radiales combinadas con la alta velocidad de rotación característica de este tipo de estrellas. La observación de estrellas Be con el satélite de alta precisión CoRoT está aportando claves importantísimas para comprender la física de estos objetos y la naturaleza del fenómeno Be. Para la detección de señales periódicas en las estrellas seleccionadas utilizamos tanto observaciones fotométricas como espectroscópicas. A partir de las observaciones obtenemos las frecuencias, amplitudes y fases de las variaciones. Como se muestra en la tesis, las variaciones en las curvas de luz estudiadas podrían estar estrechamente relacionadas con los mecanismos de pulsación estelar. Para realizar el análisis de frecuencias hemos desarrollado un código basado en técnicas estándar de análisis de Fourier. Este código, llamado Pasper, nos permite analizar grandes conjuntos de datos de forma casi automática.