Seminars and Colloquia at ESO Garching and on the campus

White dwarfs are at the core of several research areas in modern astrophysics: one of the dominant low-frequency gravitational wave signals that LISA will detect originates from the galactic population of short-period white dwarf binaries; white dwarf binaries are also the progenitors of thermonuclear supernovae (the cosmic beacons that led to the discovery of dark energy); and white dwarfs are also excellent laboratories to study accretion physics. I will give an overview of our observational census of the extremely varied population of white dwarf binaries, and of our theoretical understanding of their evolution.

Polarimetry of Tidal Disruption Events

(TDEs) provides an invaluable diagnostic for the

geometry and physical mechanisms driving the

accretion flow, potentially distinguishing between

competing emission models. Recent observations

have revealed that even non-jetted TDEs can be

highly polarized (e.g. AT2020mot), raising

fundamental questions about emission mechanisms

during the early stages of accretion. While stellar

stream shocks offer an attractive interpretation, it is

not yet clear whether this is a common underlying

mechanism.I will present results from the Black hOle

Optical polarization TimE-domain Survey

(BOOTES): the first systematic optical polarization

survey of TDEs using the Nordic Optical Telescope,

the Calar Alto 2.2m telescope, the Skinakas

Observatory 1.3m telescope and the Very Large

Telescope. We find a variety of polarization

behaviors across our sample. Combined with

contemporaneous multi-wavelength data, these

trends constrain the geometry and dynamics of the

disrupted debris and the nascent accretion flow. In

particular, we report time-variable polarization

degree and position angle -an effect not previouslyadelerickerby@yahoo.com.au

reported in TDEs- pointing to more dynamic and

anisotropic systems than commonly assumed, as

well as the possible emergence of jets. Our results

showcase how polarimetric monitoring opens a new

window on the mechanisms powering supermassive-

black-hole accretion and on the earliest stages of

accretion-disk formation, and they provide testable

predictions to distinguish shock-powered and

reprocessing-dominated models.

Relativistic cosmic-rays created by stellar and black hole feedback may play an important role in many aspects of structure formation. Cosmic-rays can drive outflows from galaxies and groups, and heat diffuse gas in the circumgalactic and intergalactic medium.  In this talk I will describe some of the possible impacts of cosmic-rays on galaxy formation, observational probes of their impact, and the theoretical uncertainities in our understanding of the role of cosmic rays in structure formation.