Talks, Seminars, Events

Abstract
Understanding the first steps in the formation of stars and protoplanetary disks is a great unsolved problem of modern astrophysics. Observationally, the key to constraining theoretical models lies in high-resolution studies of the youngest protostars. \;
I will present the state of the art regarding observations and modeling of the youngest protostellar disks, observed less than 0.1 Myrs after the onset of protostellar formation. \;
In a first part, I will show how our millimeter dust continuum interferometric data suggests that most (>\;75%) protostellar disks observed less than 0.1 Myrs after the onset of protostellar formation are only found at very small radii <\;60 au, which favors magnetized models for protostellar disk formation. I will also present our ALMA observations of a very young solar-type protostar suggesting a disk is currently forming in counter-rotation with respect to the protostellar core rotation, and discuss potential scenarii to understand this oddity. I will show our SMA and ALMA observations of the magnetic field topology in a sample of young protostars and compare all observed protostellar properties to the typical outcome of models for protostellar formation. I will argue that our observations of small disks, counter-rotating disks and organized magnetic fields in the youngest star-forming cores question the established paradigm of disk formation as a simple consequence of angular momentum conservation during the main accretion phase: they instead highlight the need to investigate magnetized models in order to unveil the mechanisms responsible for protostellar disk properties.
In a second part, I will show both our observations and models of the dust continuum emission call for significant grain growth (grains larger than 10 microns and up to submillimeter sizes) at radii 100-1000 au in the protostellar envelopes observed less than tenth of a million years after the onset of collapse. I will describe new avenues to explore dust pristine properties and describe better in the future, for example, the initial conditions for the formation of planetesimals.

Abstract
Saturn’s icy moon Enceladus harbours a global ocean, which lies under an ice crust of just a few kilometres thickness and above a rocky core. Through warm cracks in the crust a cryo-volcanic plume ejects ice grains and vapour into space providing access to materials originating from the ocean. The ocean is 30–55 km deep and provides an environment of mild salinity and alkaline pH. Hydrothermal activity is suspected to be occurring at the bottom of the ocean and also deep inside the water-percolated porous core. The energy is delivered by tidal dissipation. Two mass spectrometers aboard the Cassini spacecraft, the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Gas Spectrometer (INMS) frequently carried out compositional in situ measurements of plume material emerging from the subsurface of Enceladus. Our latest results now show that, in addition to volatile organic compounds, some emitted ice grains contain concentrated macromolecular organic material with molecular masses clearly above 200u. Moreover, the mass spectra of the two instruments provide key constraints on the macromolecular structure. We suggest that the detected organic compounds and other materials found in the plume originate from Enceladus' hydrothermally active rocky core. Thermal ocean convection together with bubbles of volatile gases, transports these materials from the moon’s hydrothermal core up to the ocean surface. There, a spray of salty water together with droplets of solid organic nucleation cores - generated by bubble bursting and subsequently coated with ice from vapor freezing - are ejected into space.

Abstract
Despite the extensive amount of stellar elemental abundance data, the chemical evolution of stars is not that well understood. Given progress in the determination of stellar ages, the constraining power of chemical abundance data should be much better than it currently is. The main limitation from the galactic chemical evolution (GCE) modeller's point of view is our current understanding of elemental enrichment from nucleosynthetic processes, such as type 1a or core-collapse supernovae (SN). By using a simple one-zone GCE model within a sophisticated statistical framework, capable of exploiting the elemental abundances of hundreds of stars simultaneously, we are now able to score yield tables from literature by their ability to reproduce elemental abundance data. This is important to reduce the model uncertainty and also to make informed choices for costly hydrodynamical simulations that include GCE. Our results also suggest that this method is very sensitive to global GCE parameters, like the high-mass slope of the IMF. Since the project is open source and has many possible applications, feel free to contact me if you want to get involved.

Abstract
Drawing from the archaeological, astronomical, and symbolical record, recurring patterns in cupule arrangements, as well as two literally central motifs from Neolithic funerary contexts in the Morbihan region of Brittany, France, are introduced and interpreted. In this region anthropogenic cupules/cupmarks (round depressions in rock surfaces, often between 4 and 10 cm diametre) occur mainly on megalithic monuments, rarely on rock outcrops. The “sperm-whale” (fr. “cachalot”) and the “crozier” (fr. “crosse”) occur on menhirs/steles (“standing-stones”) hitherto dated to around the middle of the 5th millennium BC, as well as on central-capstones and orthostats in slightly later passage-graves commonly dated to ca. 4200 - 3900 BC. Here, the often combined and seemingly disparate representations of both motifs are associated for the first time with the phenomenon of precession of the earth´s polar axis. “With respect to spatial orientation, navigation, chronometry, as well as underlying worldviews and aspects of funerary culture (i.e. cosmologies), the apparent and changing centre of the cosmos, the Northern Celestial Pole (NCP), and conspicuous circumpolar asterisms played a pivotal role in Neolithic Europe also.” Since 2009 this working hypothesis proved valid in a variety of instances between Japan and Ireland. In 2017, two asterisms of a “sperm-whale” and “crozier“, closely matching the outline of four 5th millennium BC engravings from Locmariaquer, were identified as concrete and still easily verifiable pole-finders in 4600 BC (+/- 200 years). These not only provide a detailed explanation for accepted regularities concerning megalithic funerary and ritual monuments, but further elaborate most recently published results in support of a “maritime diffusion model for megaliths in Europe”. The presentation is intended as a probe into the largely neglected perspectives of a systematic cooperation between the next generation of astronomers and archaeologists, concerning the pre-history of astronomical observations and their application for navigational and ritual purposes.

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Stars are complex objects involving multi-dimensional processes like convection, rotation and magnetic fields. Ideally, we would like to model stars with three-dimensional (3D) magneto-hydrodynamic (MHD) simulations but it is unfortunately not feasible to simulate their entire evolution in 3D. Stars, including massive stars, have thus mostly been modelled in 1D assuming spherical symmetry. Theoretical prescriptions are used to determine the 1D (time- and spherically-averaged) effects of the multi-D processes listed above into the 1D models. This has been quite successful over the past 50 years or so. The continuously improving observational constraints, however, have highlighted key deficiencies of the present 1D models. In particular asteroseismic constraints cannot be reproduced with stellar models including standard prescriptions of convection and rotation. With the increase in computing power with time, we now have the power to complement observational constraints with constraints from 3D (M)HD simulations of stellar interiors. In this talk, I will first give a brief introduction to 1D stellar evolution modelling and review some of its successes. I will then introduce a framework to develop synergy between 3D and 1D simulations of convection, the so-called RA-ILES framework. I will present recent results obtained by applying this framework to convective boundary mixing, which is one of the most important uncertainties in the evolution of stars of all masses. I will also discuss the topic of angular momentum transport and the related chemical mixing in stars. In this case, it is harder to use 3D simulations but we can use nucleosynthesis to complement other observational constraints. I will end the talk with a summary and outlook.

Abstract
In this talk, I review attempts to build a self-consistent model for the origin of turbulence of the interstellar medium (ISM) in star-forming galactic discs. Ideally such a model would incorporate all potential sources of turbulence: stellar feedback, gravitational and other instabilities, and driving by stellar gravity, and would be able to explain observed correlations between ISM turbulence and other properties of galaxies, such as their star formation rates. I summarise the various ways that theorists have attempted to fit together physical ingredients to reach this goal, the differing physical pictures behind these models, and the strengths and weaknesses of each when it comes to reproducing the observations. I then show that it is possible to combine the best elements of these models into a single, unified picture that explains the relative roles of the various sources of turbulence, and successfully reproduces most of the major observations. I suggest future observations and numerical experiments that can be used to test this unified model.

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KoCo Signature Speaker

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