Recent PhDs

Giulia Perotti

Niels Bohr Institute - University of Copenhagen, 2021

PhD advisor: Jes Kristian Jørgensen

Ice and Gas - Linking Infrared and Millimetric Observations towards young Solar-type Stars

The interaction between interstellar dust, ice and gas plays a major role for the chemistry in regions where stars and planets form. Different reactions occur in the gas and on the ice mantles of dust grains in these regions, and consequently, the mutual exchange of matter between the two phases is what regulates the chemical evolution of newborn stars and planets. Methanol (CH3OH) is a key molecule in this process, as it predominantly forms through the sequential addition of hydrogen atoms to condensed CO molecules. Once present on the ice mantles, it is considered a fundamental precursor of more complex interstellar species.

To determine the importance of the various chemical processes governing this interplay, I conduct multi-wavelength observational studies and obtain relative abundances of solid- and gas-phase molecules, particularly of methanol. This methodology allows to calculate gas-to-ice ratios and directly access the efficiencies of condensation and desorption processes. The selected targets are the cold protostellar envelopes of low-mass stars belonging to three nearby star-forming regions, with distinct physical conditions and histories. By comparing these, it is possible to test the dependencies of the chemical evolution of protostars on the large-scale environment from which they form.

The calculated CH3OH gas-to-ice ratios of the order of ~10^-3-10^-4 validate previous experimental and theoretical predictions, consistent with a considerably efficient non-thermal desorption mechanism in cold envelopes. Similarities in the gas-to-ice ratios in different nearby star-forming regions suggest that the CH3OH-mediated chemistry in the outer protostellar envelopes is relatively independent on variations of the physical conditions. This might explain the ubiquitous presence of methanol in a variety of interstellar and circumstellar environments. The combination of millimetric and infrared observations presented in this thesis has proven to be an essential tool to cast light onto the small-scale variations in the ice chemistry and its relation to the physics on large scales in star-forming regions. Thereby, these studies serve as a critical pathfinder for future work with the James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array to constrain further the routes to the formation of complex molecules during the embedded stages of star formation.


Giovanni Sabatini

Co-tutorship agreement between: Università di Bologna & Universidad de Concepción, 2021

PhD advisors: Dr. Jan Brand, Prof. Stefano Bovino & Dr. Andrea Giannetti

Establishing a timeline for the high-mass star formation process

In this Thesis we aim to answer a long-standing astrophysical problem, quantifying the timescales of the evolutionary phases characterising the high-mass star formation process. Understanding the details of the formation of massive stars (i.e. M>8-10 Msun) is not trivial, since these objects are rare and at a relatively large distance. They also form and evolve very quickly and almost their entire formation takes place deeply embedded in their parental clumps. During the evolution, the chemical composition of massive clumps can be heavily affected by the changes in density and temperature induced by the presence of massive young stellar objects. Chemical tracers that show a relation between their observed abundances and the different phases of the star formation process are commonly called chemical clocks. In this Thesis, through the comparison of observations of a large sample of massive clumps in different evolutionary stages, and accurate time-dependent chemical models, we estimate the timescales of the different phases over the entire star formation process. In addition, we provide relevant information on the reliability of crucial chemical clocks, both for the early and the late stages, confirming that the chemistry is a powerful tool to establish a timeline for the high-mass star formation process.

Laurie Chu

University of Hawaii at Manoa, 2020

PhD advisors: Klaus Hodapp

From Molecular Clouds to Our Solar System: An Evolutionary Study of Ice and Dust in Preparation for the James Webb Space Telescope

Ice and dust play a key role in building the Solar System. During their life cycle, these primitive components are exposed to complex physical and chemical processes. Some of the earliest remnants from the formation of the early Solar nebula still remain in comets providing a way to probe these initial building blocks. As comets travel toward the sun, ices sublimate revealing much about their composition and history. However, the survival of ices in comets is poorly constrained. The comet 49P/Arend-Rigaux is a low-activity periodic comet and was suspected of losing its volatiles (ices) over time. Over several apparitions small tail and jet-like features were observed. Using dust dynamical models I determine the grain properties and the outgassing duration of these different displays of activity. By modeling the ice sublimation over time I show there is a clear decrease in activity over 30 years providing a strong example of a comet transitioning to a dormant state. Outside the Solar System, initial conditions promoting ice formation can be studied within small dense molecular cores where cold surfaces of dust grains become chemical factories for simple and complex ice molecules to form. However it is unclear if complex organic molecule (COM) formation requires energetic UV radiation from newborn stars. To test this, I measure the CO and CH3OH abundances for the first time with lines of sight toward background stars through molecular cores. I find a large abundance of CH3OH ice and a high conversion rate from CO into CH3OH during the pre-stellar phase, signifying that COMs can indeed form in cold environments and account for COMs observed at later stages of star formation. To constrain the local density of hydrogen (H and H2) in the cores where COMs can form, I create very high spatial resolution extinction maps and transform them into three dimensions using an inverse-Abel transformation. Only a small fraction (~<2%) of the volume of the cores have sufficient density for CH3OH and thus presumably other COMs to form. This work is in preparation for large-scale ice maps that will be obtained with the slitless spectroscopy mode of JWST-NIRCAM. I present simulations testing the feasibility of the observations and projected science return.

José Ignacio Añez López

Universitat de Barcelona, 2021

PhD advisors: Josep Miquel Girart and Gemma Busquet

Observational and theoretical perspective of massive star formation

(Abridged) Magnetic fields are important in the process of accretion disk formation and evolution. In the millimeter and submillimeter regime, the main way to infer the magnetic field is by observing polarized emission from dust grains that have been magnetically aligned. In this thesis, we are aimed to better understand the massive star formation process paying special attention to the role of the magnetic field. To do this, we will carry out a multi-scale analysis with a double approach, theoretical and observational. Specifically on one hand, we will investigate the accretion process through an accretion disk around a high-mass star. Our goal is to understand whether massive stars can be formed in a similar way to low-mass stars, that is, accreting material from the envelope to the protostar through an accretion disk that would avoid the radiation pressure problem. To do this, we modeled the 1.14 mm image taken with the “Atacama Large (Sub) Millimeter Array” (ALMA) with high angular resolution towards the accretion disk around GGD 27–MM1. To this end, we will use the models developed by D’Alessio et al. (2006) and which have been successfully applied to protoplanetary disks around low-mass stars. On the other hand, we will study the fragmentation process in an infrared dark cloud (IRDC G14.225- 0.506). We intend to understand the role of the magnetic field in this fragmentation process. To do this, we will study polarized emission observations taken with the “Caltech Submillimeter Observatory” (CSO) towards two physically identical hubs but with different levels of fragmentation.

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.