Seminars and Colloquia at ESO Garching and on the campus

At least a significant fraction of classical Be stars were formed by past mass transfer in interacting binaries, in which they acquired the necessary excess angular momentum to spin up and form the characteristic self-ejected circumstellar disks. Post-mass-transfer Be stars typically have stripped companions of the subdwarf OB-type (sdOB), which can further evolve into white dwarfs (WD) or if massive enough into neutron stars to become high-mass X-ray binaries. Although sdOB and WD companions are expected to be common, they are very hard to detect due to their low masses and luminosities compared to the Be star primaries. Only about two dozen hot sdO companions have been detected by (far-UV) spectroscopy, with sdB and WD companions being even more elusive. In a still ongoing interferometric program on the binarity of Be stars with the CHARA Array and the VLTI, we directly detected several bloated pre-subdwarf companions, several sdO companions and we confirmed the first sdB companion with temperature similar to that of the host Be star. We also obtained astrometric orbits of these binaries with further observations, enabling the determination of their dynamical masses, and providing a firm physical basis for the associated binary evolutionary models for the first time. We were also able to shed light on the highly debated Be stars with gamma-Cas-like X-rays, with evidence mounting that they have mass-accreting WD companions. The new interferometric data also enable studying the effects that close binarity has on the structure of the Be star disks, such as the presence of circumcompanion gas and circumbinary structures.

Historically, the search for the inter-galactic medium (IGM) motivated the search for  the Far Ultraviolet (<0.2 micron; FUV) background which in turn led to a number of experiments and missions.  Decades later the focus shifted to FUV as the primary heating and ionizing agent of the atomic phases (warm and cold neutral medium).  On the observational side, it was realized that at high Galactic latitudes, the diffuse FUV has three components: FUV light from hot stars in the Galactic plane reflected by dust grains (diffuse galactic light or DGL), FUV from other galaxies (extra-galactic background light, EBL) and a component of unknown origin.  This view has been amply confirmed by later GALEX observations. During the eighties, there was conisderable discussion that decaying dark matter particles produced FUV radiation.  In my talk I systematically investigate production of FUV photons from all major sources capable of producing FUV emission. I conclude that two thirds to perhaps all of the third component can be explained by the sum of Galactic Hot Ionized Medium (line emission), two photon emission from the  Warm Ionized Medium, low velocity shocks in the Galaxy and Lyman fluorescence in the Solar System (the interplanetary medium and  the exosphere of Earth).

I will present recent and ongoing explorations regarding cold clouds in the circumgalactic media of (simulated) z=0 Milky Way-like galaxies. We find that these CGMs are typically filled with >~100s-1000s of such cold gas structures, possibly analogs of high-velocity clouds (HVCs) observed in the Milky Way sky. These objects primarily originate as a result of cold gas outflows from the central galaxy and/or precipitation of the warm-hot phase of the CGM. Clouds arising as a result of stripping of cold gas from satellites are rare in our sample (<5%). Lastly, we find that properties of clouds are diverse, and may furthermore depend closely on their source of origin.

Stellar multiplicity is ubiquitous at early stages of star formation. This unavoidably modifies the initial conditions of protoplanetary discs and hence planet formation. The high level of stellar multiplicity translates into a high number of binaries and triple stellar systems in active star forming regions. In this lunch talk, we will briefly cover disc dynamics within multiple stellar systems, mainly binaries and triples. In addition, based on numerical simulations, we will discuss how to interpret complex disc morphologies such as spirals, warps, and streamers as signposts of ongoing gravitational interaction between young stars. We will illustrate these claims with representative examples of observed multiple stellar systems with discs (e.g. ALMA, VLT, GRAVITY, Gaia). To conclude, we will briefly discuss the current open question regarding planet formation in multiple stellar systems given the emergence of planetary architectures in such systems.