Curriculum Vitæ Alessia Gualandris

• In addition to having large stellar densities, galactic nuclei are believed to contain
massive black holes. Stars on low angular momentum orbits passing close to the
black hole can receive significant velocity changes and be expelled to large
distances. Five stars with extremely large velocities (� 700 km s −1 ), called
hypervelocity stars (HVSs), have been discovered in the last year in the halo of the
Milky Way. In order to test existing theoretical models of these extremely
energetic stars, I performed numerical scattering experiments of encounters
between stellar binaries and the supermassive black hole in our Galaxy. I
investigated the origin of the first HVS discoverd in the halo of the Galaxy and, by
tracing its orbit backward in time in the Galactic potential, I predicted the value of
the proper motion of the star compatible with an ejection from the Galactic centre.
For this purpose, I developed a numerical integrator to solve the equations of
motion of a test particle in the potential of the Milky Way.
• I also studied an additional formation scenario for HVSs involving intermediate
mass black holes (IMBHs). If IMBHs are formed in the Galactic centre region,
they sink toward the centre due to dynamical friction. During the inspiral, the
black holes interact with the surrounding stars and eject some with high velocities.
I performed numerical simulations of the ejection of HVSs by infalling IMBHs and
of their travel in the Galaxy. I explored the possibility of distinguishing between
the population of HVSs created by stellar encounters and the one created by
infalling IMBHs. This might be very important for the search of dynamical
signatures of IMBHs in the Galaxy.
• Dynamical encounters in dense clusters can also involve compact objects: white
dwarfs, neutron stars and black holes. Numerical simulations of star clusters
indicate that most black holes eject each other from their parent cluster in the
early evolution of the system. Alternatively, black holes can receive large kick
velocities during the supernova explosion in which they form, both in a cluster or
in the Galactic disc. Motivated by these findings, I explored the origin of the high
velocity and high galactic latitude black hole X-ray binary XTEJ1118-480. Since
the black hole has a companion star, I combined a kinematic analysis and binary
evolution calculations to investigate the origin of the system. The dynamical
ejection scenario results incompatible with the age of the binary system. As a
consequence, the system must have acquired its large velocity in the supernova
explosion in which the black hole formed. An open question in the study of the
collapse of massive stars to compact objects is the occurrence of asymmetric natal
kicks in black hole systems, similarly to the case of neutron stars (for which there
is observational evidence in pulsars). In the case of XTEJ1118-480, I was able to
put constraints on the velocity of the system at the moment of the ejection from
the Galactic plane by tracing its trajectory backward in time in the Galactic
potential. We found that the standard recoil due to mass loss from a symmetric
supernova explosion cannot account for the large space velocity of the system and
we conclude that an additional asymmetric kick is required during the supernova.
This result has very interesting implications for the understanding of the physical
processes behind the formation of stellar mass black holes.