Defense
Student: Amanda Caveagna Rubio
Program: Astronomy
Title: “Hello, Goodbye: Accretion and Decretion Processes in Interacting and Post-Interaction Binary
Systems"
Interativos e Pós-Interativos”
Advisor: Prof. Dr. Alex Cavalieri Carciofi - IAG/USP
Judging Comitee:
- Prof. Dr. Alex Cavaliéri Carciofi – Presidente e Orientador – IAG/USP
- Profa. Dra. Jane Gregorio-Hetem - IAG/USP
- Prof. Dr. Reinaldo Santos de Lima – IAG/USP
- Profa. Dra. Rebecca Martin - University of Nevada
- Dr. Jorick Sandor Vink - Armagh Observatory & Planetarium
- Dr. Stephen Justham - Max Planck Institute for Astrophysics
Abstract:
Mass is the main characteristic that defines the fate of a star. If a considerable amount of
mass is lost or gained during their lifetime, the entire evolution of a star will change
accordingly. One of the most effective ways a star can lose or exchange mass is in binary
systems, where one star fills its Roche lobe and transfers mass to its companion. The
conditions for mass transfer stability and effectiveness are still subject to intense discussion
in the field of binary stars, and with good reason. These parameters dictate the outcome of
binary evolution, a process of indisputable importance for the creation of exotic stars and
binaries, such as blue stragglers and double compact object systems, affecting supernova
rates, and the enrichment of the interstellar environment. Post-interaction systems are not
always easily identifiable, however. For instance, a binary system composed of a white dwarf
and a main sequence star can be formed via stable mass transfer, common envelope
evolution, or with no mass transfer at all. Be stars, the most rapid non-degenerate rotators,
might require mass transfer to acquire their fast rotations, but might also be formed in
isolation. In these Be stars, the rapid rotation works along with some undefined internal
mechanism to cause (usually) explosive and episodic events of mass loss, which lead to the
formation of a disk around the star. If the Be star is in a binary system, we circle right back
to mass loss and mass transfer, with the companion now accreting material from the Be disk.
In this thesis, I travel the long and winding road of mass loss and mass transfer along four
projects. I compare binary population synthesis models to a large dataset of white dwarf +
main sequence binaries to calibrate the mass transfer parameters used in these codes,
developing a methodology that can be applied to future datasets and surveys. I also find that
the prescriptions and criteria for stability and common envelope ejection should be revised. I
then shift the focus to Be stars, where I use smoothed particle hydrodynamics and radiative
transfer codes to explore their mass ejection events. My results provide important constraints
to the geometry and dynamics required to create outbursts that are comparable to data,
which any theory aiming to explain the Be phenomenon from first principles must account
for. I also present BeAtlas, a grid of synthetic observables of Be stars, that can be a powerful
tool in the study of Be, as well as B and Bn, stars as a population. Finally, I use smoothed
particle hydrodynamics and radiative transfer simulations to describe in detail the behaviour
of Be disks in close binary systems, including the expected observational consequences of
this interaction. I also detail how such observational characteristics can be used to detect
otherwise unseen companions, and to plan and interpret observations. As an example of the
complex behaviour of binary Be stars, I present an overview of the last 30 years of
spectroscopic observations of pi Aqr, which can be used as a basis of comparison for these
simulations. This thesis is a significant step forward in our understanding of Be stars as a
class, laying down the foundation for improved models of Be outbursts and binary Be stars.
The methodology I present for comparing binary population synthesis models to data
facilitates systematic interpretation of observations of post-interaction systems, which will
provide the much needed constraints to our mass transfer models.
Palavras- chave: Be stars, binary systems, mass transfer, mass loss