Recent PhDs

Łukasz Tychoniec

Leiden University, 2021

PhD advisors: Ewine van Dishoeck, Michiel Hogerheijde, John Tobin

Protostellar jets and planet-forming disks: witnessing the formation of Solar System analogues with interferometry

This thesis is focusing on characterizing components of young protostellar systems, most notably their jets and disks. Using observations with the ALMA and VLA interferometers, we observed the environments where the first stages of star and planet formation occur. We revealed information on crucial chemical tracers of various protostellar systems components. With a particular focus on molecular jets, I show differentiation in chemical composition between the fast jet and the low-velocity outflow. For the first time, I was able to compare dust masses of young disks with older disks. By comparing this information with masses of the extrasolar planets detected so far, I showed that the solid cores of gas giants must form in the first 0.1 Myr of stellar life. That is an important time constraint that pushes the onset of planet formation earlier and highlights the importance of characterization of the youngest protostars in understanding the origin of Solar System and Earth.


Ugo Lebreuilly

ENS de Lyon, 2020

PhD advisor: Benoît Commerçon

Star formation : Dynamical study of interstellar dust

The interstellar medium is composed by approximately 1% of dust in terms of mass. Surprisingly, this tiny amount of dust already plays a very important role in stellar formation. The dynamics of dust grains may differ from that of the gas particles, leading to local variations in concentration. However, very few studies have focused on the gas and dust differential dynamics during star formation. My thesis aims to fill this gap and is divided into four parts. In the first part, I develop a module dealing efficiently with dust dynamics that can simultaneously include multiple grain species intended to the multidimensional adaptive grid code RAMSES (Teyssier 2002). I then carefully test my module by comparing my results with known analytical solutions. I also show that my implementation is robust, fast and accurate. Then I perform star formation simulations that consider multiple dust species. This study establishes that a decoupling between the dust and the gas appears for grains of sizes larger or equivalent to a hundred micrometers. I also find that this decoupling depends strongly on the initial properties of the prestellar core. Then, I develop an analytical formalism, similar to the non-ideal magnetohydrodynamics but that includes the dynamics of charged grains. This formalism allows to highlight different coupling regimes between the grains, the magnetic field and the gas as a function of the grain size, its charge and its environment. In parallel, I investigate the dynamics of dust in the weakly ionized zones of protoplanetary disks in order to study the formation of chondrules. Chondrules are dust grains found in most meteorites and are key to understand the formation of disks and planets.

Núria Miret Roig

Université de Bordeaux, 2020

PhD advisors: Hervé Bouy & Javier Olivares Romero

COSMIC DANCE: A comprehensive census of nearby star forming regions

Understanding how stars form is one of the fundamental questions which astronomy aims to answer. Currently, it is well accepted that the majority of stars form in groups and that their predominant mechanism of formation is the core-collapse. However, several mechanisms have been suggested to explain the formation of substellar objects and their contribution is still under debate.

The main goal of this thesis is to determine the initial mass function, the mass distribution of stars at birth time, in different associations and star-forming regions. The mass function constitutes a fundamental observational parameter to constrain stellar and substellar formation theories since different formation mechanisms predict a different fraction of stellar and substellar objects. We used the Gaia Data Release 2 catalogue together with ground-based observations from the COSMIC-DANCe project to look for high probability members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. We applied this method to the 30 Myr open cluster IC 4665 and the 1-10 Myr star-forming region Upper Scorpius (USC) and rho Ophiuchi (rho Oph). We found very rich populations of substellar objects which largely exceed the numbers predicted by core-collapse models. In USC, where our sensitivity is best, we found a large number of free-floating planets and we suggest that ejection from planetary systems must have a similar contribution as core-collapse in their formation.

Age is a fundamental parameter to study the formation and evolution of stars and is essential to accurately convert luminosities to masses. For that, we also presented a strategy to study the dynamical traceback age of young local associations through an orbital traceback analysis. We applied this method to determine the age of the beta Pictoris moving group and in the future, we plan to apply it to other regions such as USC.

The members we identified with the membership analysis are excellent targets for follow-up studies such as a search for discs, exoplanets, characterisation of brown dwarfs, and free-floating planets. I this thesis, we presented a search for discs hosted by members of IC 4665 and we found six excellent candidates to be imaged with ALMA or the JWST. The tools we developed, are ready to be used in other regions such as USC and rho Oph, where we expect to find a larger number of disc-host stars.

Catherine Zucker

Harvard University, 2020

PhD advisors: Alyssa Goodman & Doug Finkbeiner

Charting our Uncharted Milky Way

Our position in the Milky Way, buried within its disk, makes it extraordinarily difficult to piece together the structure of our home Galaxy. We know the Milky Way is a barred spiral but many questions, including the precise number, location, and prominence of spiral features remain debated. Towards the goal of better understanding star formation and the structure of our Milky Way, we present four avenues of research developed to map the molecular gas and dust in the Galaxy. Specifically, using a combination of extraordinarily elongated gaseous filaments, numerical simulations of Milky Way analogs, 3D dust mapping of our solar neighborhood, and 4D spatial-kinematic views of individual star-forming regions, we are beginning to build new models of our Milky Way's interstellar medium both locally and towards the inner Galaxy.

Towards the inner Galaxy, we systematically characterize the physical properties of the largest-scale filaments in the interstellar medium. We find that the diversity in their physical properties likely reflects different formation mechanisms and evolutionary histories, with the longest and densest filaments most likely to trace the Galaxy's gross spiral structure in position-position-velocity space. By producing synthetic observations of comparable filaments forming in an AREPO simulation of a Milky Way-like galaxy, we find that while large-scale filaments preferentially form in the mid-plane of the galaxy, additional physics (stellar feedback, magnetic fields) is needed to reproduce the range of observations.

Within the solar neighborhood, we use 3D dust mapping techniques in combination with stellar distances from Gaia DR2 to produce the largest uniform catalog of accurate distances to local molecular clouds. Comparison with "gold-standard" maser distances obtained from VLBI observations indicate agreement to within 10%, with no systematic offsets out to 2.5 kpc. Using this new catalog, we present the discovery of a 2.7 kpc long coherent arrangement of stellar nurseries, which undulates about the Galactic plane with an amplitude of 160 pc and appears to be the Local Arm of our Galaxy nearby. Extensions of the 3D dust mapping technique applied to a single cloud in this structure demonstrate that 3D spatial views of dust can be knitted together with kinematic information from gas to create 4D views of the local interstellar medium. Ultimately, we plan to build on these and complementary techniques to produce an integrated 3D model of our Milky Way's stars, gas, and dust out to 6 kpc in the coming years.