Seminars and Colloquia at ESO Garching and on the campus

Numerous protoplanetary disks exhibit shadows in scattered light observations. These shadows are typically cast by misaligned inner disks and are associated with observable structures in the outer disk such as bright arcs and spirals. Investigating the dynamics of the shadowed outer disk is therefore essential in understanding the formation and evolution of these structures. We carry out two-dimensional radiation hydrodynamics simulations that include radiative diffusion and dust–gas dynamics to study the formation of substructure in shadowed disks. We find that spiral arms are launched at shadow edges, permeating the entire disk. The local dissipation of these spirals results in an angular momentum flux, opening multiple gaps and leading to a series of concentric, regularly-spaced rings. We find that ring formation is favored in weakly turbulent disks where dust growth is taking place. These conditions are met for typical class-II disks, in which bright rings should form well within a fraction of their lifetime (~0.1–0.2 Myr). For hotter disks gap opening is more efficient, such that the gap edges quickly collapse into vortices that can appear as bright arcs in continuum emission before decaying into rings or merging into massive, long-lived structures. Synthetic observations show that these structures should be observable in scattered light and millimeter continuum emission, providing a new way to probe the presence of substructure in protoplanetary disks. Our results suggest that the formation of rings and gaps is a common process in shadowed disks, and can explain the rich radial substructure observed in several protoplanetary disks.

Our understanding of planet and star formation is mainly based on already formed planets around (sub-)solar-like stars. In this talk, I aim to broaden our perspective by focusing on protoplanetary disks around more massive stars, leveraging the special properties of their stellar interiors. In particular, convective sub-photospheric regions disappear during the pre-main-sequence evolution of stars with masses roughly between 1.5 and 4 Msun. In turn, the absence of convection influences the mixing of stellar material, the strength of the magnetic field, and consequently, the way disk material is accreted by the central stars. I will summarize our recent findings on the metallicity and accretion properties of intermediate-mass young stars in relation to disk structures, the potential presence of giant planets, and the size of their innermost orbits. I will conclude by presenting our ongoing efforts to reliably determine disk-to-star accretion rates of the most massive young stars with fully radiative envelopes.

Type Ia and other peculiar supernovae (SNe) are thought to originate from the thermonuclear explosions of white dwarfs (WDs). Some of the proposed channels involve the ejection of a partly exploded WD (e.g. Iax SN remnant) or the companion of an exploding WD at extremely high velocities (>400 km s^-1). Characterization of such hyper-runaway/hypervelocity (HVS) WDs might therefore shed light on the physics and origins of SNe. Here we analyse the Gaia DR3 data to search for HVS WDs candidates and peculiar sub-main-sequence (sub-MS) objects. We retrieve the previously identified HVSs and find 46 new HVS candidates. Among these we identify two new unbound WDs and two new unbound sub-MS candidates. The remaining stars are hyper-runaway WDs and hyper-runaway sub-MS stars. The numbers and properties of the HVS WD and sub-MS candidates suggest that extreme velocity ejections (>1000 km s^-1) can accompany at most a small fraction of type Ia SNe, disfavouring a significant contribution of the D6-scenario to the origin of Ia SNe. The rate of HVS ejections following the hybrid WD reverse-detonation channel could be consistent with the identified HVSs. The numbers of lower-velocity HVS WDs could be consistent with type Iax SNe origin and/or contribution from dynamical encounters. We also searched for HVS WDs related to known SN remnants but identified only one such candidate ( https://ui.adsabs.harvard.edu/abs/2023MNRAS.518.6223I/abstract).

The MHONGOOSE Large Survey Project is obtaining ultra-deep 21-cm neutral hydrogen (HI) observations with the MeerKAT radio telescope to map the distribution and kinematics of the low column-density gas in and around 30 nearby star-forming spiral and dwarf galaxies. These deepest resolved HI observations of nearby galaxies to date serve to put additional constraints on the role of accretion of cold gas in the replenishing of these galaxies' gas reservoirs. Observations for the survey have just completed and MHONGOOSE is routinely reaching its target HI column density sensitivity of a few times 10^17 atoms cm^-2, two orders of magnitude lower than the typical values found in galaxy HI disks. Our full-depth data show that the outskirts of our galaxies are complex and dynamic environments, with many potential accretion and interaction features visible in HI that only now become visible due to the excellent column density sensitivity.  We detect a significant number of uncatalogued low-mass dwarf galaxies, which enable "Local Group science" in environments at tens of Mpc distance. A first comparison of the MHONGOOSE observations with simulated HI maps from recent cosmological simulations show a marked difference in kinematics and morphology, indicating that cold gas accretion is likely happening in a more gentle way. The sensitive MHONGOOSE observations point the way to a better understanding of the role of gas accretion in galaxy evolution in the nearby universe and identifies opportunities for new HI surveys with the upcoming SKA-MID telescope.