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Thèses soutenues

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  • the PhD defense of Victor de Souza Magalhães on Dec. 20th 2017 at IPAG, Grenoble

Title : The protosolar nebula heritage : The nitrogen isotopic ratio from interstellar clouds to planetary systems

Abstract : The existence of interstellar molecules raises the question,
are these molecules the same molecules we see on the Solar system
today ? This is still an open question with far reaching
consequences. Some light may be shed on this issue if we are able to
trace the heritage of a group of chemically linked molecules, a
so-called reservoir. The best tool to trace the heritage of reservoirs
are isotopic ratios. The element that shows the largest isotopic ratio
variations in the Solar system is nitrogen. Which is an indication that
the isotopic ratio of nitrogen is sensitive to the physical conditions
during star formation.

The main objective of this thesis is to identify the reservoirs of ni-
trogen at different stages of star and planet formation. The first step
in this endeavour was to identify the isotopic ratio of the bulk of ni-
trogen in the local ISM today. This was determined to be 323 ± 30 from
the CN/C15N ratio in the protoplanetary disk around TW Hya. Along with
it we also measured the HCN/HC15N= 128 ± 36 in the protoplanetary disk around MWC 480. This very distinct nitrogen iso- topic ratios on
protoplanetary disks are a clear indication that there are at least two
reservoirs of nitrogen in them. How these reservoirs get separated is
however unknown. This could possibly happen due to chemical
fractionation reactions taking place in prestellar cores. We therefore
aimed to obtain an accurate direct measurement of the nitrogen isotopic
ratio of HCN in the prestellar core L1498.

To obtain this measurement the most important hurdle to overcome were
the hyperfine anomalies of HCN. These hyperfine anomalies arise due to
the overlap of hyperfine components. They are especially sensitive to
the column density of HCN, but also to the velocity field and line
widths. Thus hyperfine anomalies are a tool to measure the abundance of
HCN and to probe the kinematics of prestellar cores.

To accurately reproduce the hyperfine anomalies, and thus mea- sure
accurate column densities for HCN, we needed to explore a degenerate
parameter space of 15 dimensions. To minimise the de- generacies we
have derived a density profile based on continuum maps of L1498. This
reduced the parameter space to 12 dimensions. The exploration of this
parameter space was done through the use of a MCMC minimisation
method. Through this exploration we ob- tained HCN/HC15N = 338 ± 28 and ;HCN/H13CN = 45 ± 3. The uncertainties on these values are calibration
limited and determined non-arbitrarily by the MCMC method. Implications
of these results are discussed in the concluding chapter, where we also
present some future perspectives.

  • the PhD defense of Hamza Labiad on Dec.19th 2017 at IPR, Rennes

Title : Rotational Energy Transfer in CO-Ar and CO-H2 Collisions at Very Low Temperatures For Astrophysical Applications

Abstract : Alma, Herschel and the James Webb Telescope (in the near
future), are powerful tools to understand molecular complexity, star
formation and many chemical and physical processes in the interstellar
medium. Explaining the observations made by these telescopes requires
robust models for radiative transfer calculations involving collisional
excitation. Furthermore, collisional processes lie at the heart of
physics and chemistry. Understanding intermolecular forces is of
fundamental interest forf chemical physics, and collision-induced
rotational energy transfer, RET, is an excellent tool to benchmark
theoretical calculations.

Experimental measurements are reported for rotational energy transfer
in the Ar-CO and H2-CO systems. Experiments were performed in cold
uniform supersonic flows of Ar and H2, using an infrared – vacuum
ultraviolet double resonance technique to measure absolute
state-to-state rate constants and total relaxation cross sections for
rotational energy transfer within the (v = 2) vibrational state of CO
in collision with Ar at temperatures from 30.5 to 293 K, and CO in
collision with H2 from 5.5 to 293 K.

The experimental results have been compared to close-coupling
calculations performed by T. Stoecklin and O. Denis-Alpazar using a
high-quality 3D potential energy surface (Y. Sumiyoshi and Y. Endo, J.
Chem. Phys. 142 (2015) 024314) for the Ar-CO system, and for the H2-CO system new close coupling calculations performed by A. Faure on a very
high-quality full-dimensional PES (A. Faure et al. Sci. Rep 6 (2016)
28449). Excellent agreement is obtained between measured and calculated
values for both collisional systems.

  • the PhD dedense of Basile Augé on Oct. 12th 2017 at Caen/Normandie

Titre : Effets du rayonnement cosmique galactique sur les petits corps glacés du système solaire externe : indices pour la formation de la matière organique des micrométéorites antarctiques ultracarbonées

Résumé : Les météorites et particules de poussière interplanétaire
apportent des contraintes sur la formation et l’évolution de la matière
dans le système solaire. Les micrométéorites, dont certaines
proviennent des régions externes du système solaire, représentent la
source dominante de matière extraterrestre arrivant sur Terre. Les
micrométéorites collectées dans les neiges antarctiques sont dans un
excellent état de conservation du fait de conditions géographiques et
météorologiques favorables à leur préservation. La collection
CONCORDIA/CSNSM de micrométéorites contient en particulier des
micrométéorites peu altérées thermiquement lors de leur entrée
atmosphérique. Certaines sont caractérisées par une très haute teneur
en matière organique, dépassant 50% en volume, très largement au dessus
des valeurs habituelles trouvées dans les météorites. Cette matière
organique présente de plus la spécificité d’être fortement enrichie en
deutérium et contient jusqu’à cinq fois plus d’azote celle extraite des
météorites. Les différents scénarios proposés pour expliquer la
formation de cette matière et satisfaisant à l’ensemble des
caractéristiques de ces micrométéorites impliquent des corps parents
orbitant au-delà de Neptune, dans la ceinture de Kuiper ou dans le
nuage de Oort. La température y est suffisamment basse pour condenser à
leur surface les molécules volatiles comme l’azote et le méthane tandis
qu’ils sont exposés à l’action radiochimique du rayonnement cosmique
galactique. Afin de contraindre ces scénarios, des expériences ont
été conduites en exposant différentes glaces N2-CH4 aux faisceaux
d’ions du GANIL simulant ce rayonnement. L’évolution chimique des
glaces au cours de l’irradiation et pendant le recuit des échantillons
a été suivie par spectroscopie infrarouge au moyen de deux dispositifs
disponibles au CIMAP : la chambre d’analyse CASIMIR et le nouvel
appareil IGLIAS. Des analyses complémentaires ex situ ont été menées
par spectrométrie de masse. Les résultats apportant des éléments de
réponse à l’origine de la matière organique des micrométéorites
ultracarbonées ainsi que sur l’origine de leur enrichissement
isotopique seront présentés et discutés.

  • The PhD defense of Fiorella Polles Sep. 29th 2017 at CEA/Saclay

Title : Properties of the interstellar medium of the star-forming galaxy, IC10, at various spatial scales

Abstract : The InterStellar Medium (ISM) provides the details of the
environment and processes that lead to star formation, which in turn,
drives the evolution of a galaxy. Moreover, it is the site of stellar
birth and the repository of stellar ejecta, hosting the signatures of
metal enrichment. How stars have been formed in galaxies in the early
universe, when heavy elements are deficient, is a fundamental issue in
astrophysics. While it is hard to analyze the ISM chemistry at high
redshifts, the Local Group of galaxies offers the opportunity to study
several ‘chemically-young’ dwarf galaxies in detail. These dwarfs are
perfect laboratory to investigate how the metal-poor ISM affects the
interplay between gas, dust and stars.

This thesis project investigates the properties of the multi-phase ISM
of the nearby dwarf galaxy IC10, which has a metallicity of 1/3
solar. In particular, it focuses on the analysis of the physical
properties of the compact HII regions and the diffuse ionized gas,
traced by the mid- and far-infrared cooling lines. The proximity of
this galaxy (d=700 kpc) enables the analysis of the ISM on different
spatial scales : from the compact, bright clumps (-25 pc) to the whole
star-forming body of the galaxy (-650pc). It has been observed with
the space telescopes Spitzer and Herschel, providing a set of -16
infrared emission lines. I complemented this dataset with new
observations made with the SOFIA airborne telescope.

I have studied the spatial distribution of the Far-InfraRed (FIR) lines
(linear resolution 40 pc). I found that [OIII] 88μm is the brightest
infrared cooling line everywhere along the body of the galaxy,
suggesting a high filling factor of diffuse ionized gas. The ISM is
very porous, allowing UV photons to leak from the HII regions and
traverse large distances, unique to low metallicity galaxies with low
dust abundance. [CII] 157 μm, which can arise from both ionized and
neutral gas, is the second brightest FIR cooling line. Despite the
prominent ionized gas phase, the ratio [CII]/[NII]122 μm shows that
most of the [CII] emission (at least 98%) comes from the neutral Photo-
Dissociation Regions (PDRs), not from the ionized gas.

To assess the physical properties (e.g., density, extinction) of the
ISM and the nature of the ionizing sources, the observed Mid-InfraRed
(MIR) and FIR lines were confronted by photoionization and
photodissociation models. Using Cloudy (Ferland et al. 2013), I have
built grids of models adapted to the specific case of IC10 and I have
tested two different strategies to compare the observations with the
model predictions : line ratios and absolute flux values. Since the
sensitivity and the spatial coverage of the observations were not
homogeneous, it was not possible to select the same combinations of
tracers at the different scales. Hence, exploring those methods was
necessary to extract the best possible constraints of the physical
properties. The investigation suggested that, based on the dataset, the
best choice was to compare all of the available tracers at each scale
using the line absolute values. The modeling enabled me to determine
the properties of the the brightest clumps within the galaxy and
demonstrate that the emission at large scales (-300 pc) is dominated
by that of the compact, bright clumps that lie in the region. Moreover,
at those large scales we found that a simple description of a single
component model, was not sufficient to reproduce all of the tracers. A
second ionized gas component, with lower density, filling the ISM
between the compact HII regions, is required.

In order to draw a complete resolved picture of the ISM composition in
metal-poor galaxies, it is important to extend the analysis to the
PDR/molecular phase. The next step following the PhD work, is the
application of the modelling method refined in this project, to the PDR
phase including the tracers [OI] 63μm, [OI] 145μm ,
[CII] 158μm, CO
lines as well as the LTIR.

  • The PhD defense of Benjamin Khiar on Sep. 26th 2017 at Jussieu

Title : Laboratory astrophysics with magnetized laser-produced plasmas

Abstract : We present a theoretical and numerical study of
astrophysically-relevant, scaled experiments of laser-produced plasmas
in a strong magnetic field. This work is a contribution to the
relatively recent field known as high energy density laboratory
astrophysics (HEDLA). The basic experimental setup studied involves
plasma generated by one or several solid targets embedded in magnetic
fields of strengths up to 40 T (0.4 MG), and irradiated with a
nano-second laser with intensities up to 1014 W cm-2. By changing the
relative orientation of the magnetic field and the laser-target,
different astrophysical phenomena can be investigated in the
laboratory. When the magnetic field is perpendicular, super-Alfvenic
jets are seen to emerge from a diamagnetic cavity, which collimates the
initially wide-angle expanding plasma flow (v 500 km/s, T 1 MK, n 
1018 cm-3) through a series of oblique shocks. The resulting jet has a
magnetic field topology similar to idealized magnetized accretion
columns, and we present a detailed characterization of the dynamics of
its impact onto a solid surface mimicking the generation of accretion
shocks in TTauri stars. In particular, we show how instabilities, not
captured in current models, can modify the dynamics and the envelope of
shock-heated plasma that surrounds the accretion column. Finally, when
the magnetic field is oriented parallel to the laser-target, the plasma
evolves into a thin, unstable slab, whose physics bears striking
similarities to magnetically confined slabs studied in solar
physics. Alongside the numerical work, we present new experimental
results obtained on the ELFIE laser facility in a collaboration with
the LULI laboratory.

  • The PhD defense of Jeremy Chastenet on Sep. 26th 2017 at Strasbourg

Title : Analyse de l’émission des poussières dans les galaxies proches : Impact des hypothèses et choix des modèles

Résumé : "Ma thèse s’est concentrée sur les conséquences qu’ont les choix de modélisation de l’émission de la poussière sur ses propriétés
déduites dans les galaxies proches. Une première étude (Chastenet et
al. 2017) a montré que certains modèles ne reproduisent pas des
observations des Nuages de Magellan de façon satisfaisante, bien que
tous reproduisent l’émission IR de la Voie Lactée. Cela a aussi mis en
évidence que la composition de la poussière est différente d’une galaxie
à l’autre, et différente de celle de la Voie Lactée. Le choix de
l’environnement des grains, à travers la description du champ de
rayonnement qui chauffe ces grains, impacte significativement les
résultats tels que la masse totale de poussières. Une deuxième étude
s’est concentrée sur les biais systématiques dus à l‘emploi de lois
empiriques de mélange pour décrire le chauffage de la poussière. J’ai
montré que les masses déduites peuvent être sous- ou surestimées, bien
que les ajustements aux données apparaissent raisonnables. Les
résultats de cette thèse montrent alors qu’il est nécessaire de prendre
en compte les choix de modélisation pour déterminer au mieux les
propriétés des poussières dans les galaxies proches."

  • The PhD defense of Alexandre Sauvé on Dec. 9th 2016 at IRAP

Abstact : Planck is an ESA spacecraft launched in 2009, it is the third
generation of spatial observatory in the era of precision
cosmology. Its mission goal was to map with an exquisite precision
(∆T/T < 10−5). the Cosmic Microwave Background (CMB).
The required
level of accuracy needs an unusually high level control of the
systematic effects introduced by the on-board instruments. However, an
unexpected conjunction of elements has enhanced the nonlinearity
introduced by the chip performing the digitization of science data by
the HFI instrument. It resulted in the most challenging systematic
effect to deal with. It is presented here the work performed to
characterize and correct for this nonlinear effect. First a detailed
modeling of the complex thermal response of the bolometer detectors
under AC biasing is presented. Second, the steps leading to the
in-flight accurate characterization of the digitization chip are
detailed. To correct for the nonlinear effect on science data, the full
electronics readout chain response is modeled, taking into account the
detector response under AC biasing and the nonlinear effect of the
digitization chip with the noise. Furthermore, the complex parasitic
signal originating from the 4 K cryogenic stage mechanical cooler has
also to be taken into account. The provided correction has been applied
with success to the HFI data 2015 release reducing the effect by an
order of magnitude.

  • The PhD defense of Quentin Rémy on Dec. 6th at CEA-Saclay

Titre de la thèse : "Traceurs de gaz et de poussières du milieu
interstellaire local"

Résumé : Cette thèse présente les résultats d ?une étude du milieu
interstellaire local basée sur les mesures de rougissement du aux
poussières (E(B-V)), de leur émission thermique à 353 GHz (et de
l’épaisseur optique tau(353)), du rayonnement gamma, de l ?émission
free-free et des raies d ?émissions HI et CO. Ces traceurs permettent
de sonder la quantité totale de gaz ainsi que celle des différentes
phases du milieu interstellaire. L ?objectif de cette étude est de
tester les limites de la capacité de ces traceurs à estimer les
quantités de gaz, de chercher des effets d ?environnement et de
comparer les tendances d’évolution observées aux prédictions des
modèles théoriques.

Nous avons étudié plus particulièrement la régions de l ?anticentre
Galactique. L ?information sur la vitesse du gaz apportée par les raies
d ?émission HI et CO a été utilisée pour séparer dans l ?espace
position-vitesse six complexes de nuages locaux de l ?arrière plan
Galactique. Ces complexes incluent les nuages bien connus de
Taurus-Auriga, California, et Perseus. La forme du spectre
d ?émissivité en gamma dans chaque nuage est compatible avec le spectre
interstellaire local moyen, ce qui démontre la capacité de l ?émission
à tracer le gaz total. Les cartes de densités de colonne de gaz
déduites de l ?émission de rayons gamma d ?origine interstellaire ont
été utilisées pour mesurer les rapports tau(353)/NH et E(B-V)/NH dans
les différents nuages. Ces rapports permettent de suivre les
propriétés d ?émission et d ?extinction des poussières. Les variations
observées apportent des contraintes sur les limites de la capacité des
traceurs de poussières à estimer la quantité totale de gaz dans les
différentes phases. Le rapport tau(353)/NH augmente d’un facteur 4
dans les régions moléculaires par rapport au milieu diffus. Les
variations du rapport E(B-V)/NH sont moins marquées mais on note des
fluctuations très locales à l ?intérieur des nuages moléculaires. Ces
changements dans les propriétés d ?émission et d ?extinction des
poussières sont liés à des évolutions structurelles et chimiques des
grains prédites par les modèles.

Le facteur XCO, permettant de convertir l ?intensité CO intégrée en
densité de colonne de H2, est mesuré indépendamment dans chaque nuage.
Les valeurs de XCO déduites de l ?épaisseur optique des poussières sont
systématiquement plus grandes que celles déduites du rougissement et
de l ?émission de rayons gamma, et ce d ?autant plus à mesure que
l ?opacité des grains tau(353)/NH augmente, cela révèle un biais du à
l ?évolution des propriétés d ?émission des grains. Nos mesures
indiquent que le XCO moyen d ?un nuage diminue pour des nuages plus
compacts, et brillants en CO, en accord avec une diminution prédite
par les modèles, depuis les enveloppes diffuses, plus exposées à la
photo-dissociation, jusqu ?aux coeurs moléculaires denses, mieux
écrantés. Les valeurs de XCO inférieures à 10**20 cm-2 K-1 km-1 s
mesurées pour les nuages locaux sont nettement plus faibles que celles
obtenues par les simulations, pour des extinctions AV et des densités
volumiques de gaz sembalbles aux mesures. Ces différences confirment
que les simulations prédisent généralement une émission CO trop peu
intense à faible densité de colonne.

L ?émission dans les raies HI et CO ne trace pas le gaz du DNM (Dark
Neutral Medium) composé de HI optiquement épais et de H2 diffus.
L ?information supplémentaire apportée conjointement par les traceurs
de poussière et l ?émission gamma permet d ?estimer les densités de
colonne de gaz dans le DNM. Le DNM représente environ 20 % de la masse
totale des nuages étudiés. On note une augmentation de la masse dans
les enveloppes de DMN proportionnelle à la racine carrée de la masse
de gaz moléculaire des nuages vus en CO. Cette relation pourrait
servir à estimer la quantité de gaz du DNM dans les parties distantes
de la Galaxie et les galaxies externes. Afin d ?étudier les seuils de
transition HI-H2 et H2-CO nous avons mesuré l ?évolution des fractions
de gaz dans les différentes phases et la fraction de gaz sombre dans
la phase moléculaire. Ces mesures peuvent servir à tester les modèles
de formation et de photo-dissociation de H2 et de CO.

  • The PhD defense of Maxime Ruaud on Oct. 3rd 2016 at Observatoire de Bordeaux

Title : Chemistry of the interstellar matter : from diffuse to dense
interstellar medium

Abstract : The chemical evolution from the most diffuse parts of the
interstellar medium to the formation of dense clouds is a continuous
process : the chemical composition in one phase depends on the chemical
composition in the previous one. However, most studies of the time
dependent chemistry in the cold and dense interstellar medium make
strong assumptions on the transition between diffuse and dense
medium. The goal of my thesis was to study the chemical evolution of
the interstellar medium from the most diffuse parts to the formation of
dense clouds in a continuous way. To do so, I used a time dependent
gas-grain model that I significantly contributed to improve for the
treatment of the surface chemistry. In a first part, I followed a
"classical" approach (i.e. : similar to most of the pre-existing
studies) to model cold dense clouds. This allowed me to study in
details the physical and the chemical mechanisms responsible for the
chemical complexity of dense clouds and to compare the model
predictions with the existing literature. In a second part, I applied
this model to follow the evolution of the chemical composition during
the formation process of dense clouds from the diffuse medium. I used
results from an hydrodynamical simulation of the interstellar medium at
galactic scales. This study allowed me to show that the past physical
history of each particles that form the dense clouds have a significant
impact on their chemical composition

  • The PhD defense of Quentin Salome on Sep. 29th 2016 at Obs. de Paris

Abstract : Massive galaxies are less abundant than predicted by the standard model of galaxy formation (the lambda-CDM model). This means that galaxies form less stars than expected. To explain this behaviour, it is commonly accepted that some processes are at play and quench star formation. For massive galaxies, it is explained by the feedback of the supermassive black holes. While accreting gas, the central black hole produces energy and momentum. When gas accretion becomes important, the black hole forms an active galactic nucleus, and the energy is expected to quench star formation, via gas heating, turbulence or gas removal (negative feedback). However, evidence is found of so-called AGN positive feedback that favours star formation by compressing the gas. In particular, a fraction of the AGN population produces jets of plasma that are observed in radio emission. These radio jets may interact with gas that is located along the direction of propagation. Such interactions are invoked to trigger star formation (jet-induced star formation). My PhD focused on the jet-gas interaction for nearby radio galaxies. I explored the effect of the jet on the star formation efficiency in such interactions at global (few kiloparsecs) and intermediate (few hundreds parsecs) scales. To do so, I searched and mapped the molecular gas (via CO emission lines) that is a key ingredient for star formation. This cold gas is observable using current radio astronomy ground-based facilities, like ALMA, APEX, NOEMA and the 30m telescope.

* The PhD defense of Jeremy Bourgalais on Sep. 23rd 2016 in Rennes

Title : Kinetics of formation of van der Waals clusters and product
detection of carbon atom reactions of interest for astrophysical and
atmospherical environments and for combustion

Abstact : This PhD was carried out in the Laboratory Astrophysics group
of the Institute of Physics of Rennes (IPR).

In this work, a first part presents the application of the CRESU
technique (french acronym for Cinétique de Réaction en Ecoulement
Supersonique Uniforme) to the study of the kinetics of formation of
water and propane van der Waals clusters. First, we observed
experimentally the formation of water clusters over a temperature range
of 22.9 to 69.4K, and then we modeled the first steps of clusterization
through a kinetic model including association rate coefficients
determined by theoretical calculations performed by S. Klippenstein at
Argonne National Laboratories (USA)1. We also performed the first
measurements of the association coefficients for the formation of
propane dimer over a temperature range of 22.9 to 49.1K.

The second part of this work is dedicated to the product detection of
carbon atom reactions with various molecules (C2H4, C2H6, C4H8 and NH3)
2,3,4,5 at room temperature. These studies were performed at the
synchrotron Advanced Light Source (ALS) of Berkeley (USA). In addition
we studied the kinetics and the product formation of the reaction
between carbon atoms and ammonia at low temperature (50-296K). The
CRESU device of the Institute of Molecular Science (ISM) of Bordeaux
was used for this purpose. Those data were incremented in a dense
interstellar cloud model to see their influence on the abundance of
nitrogen hydrides (NxHy)3.

The work of this thesis contributes to better understand the mechanisms
of elementary reactions leading to the formation and growth of
molecules and van der Waals clusters in gas phase in a large variety of
physical conditions. They provide data for modeling planetary
atmospheres, interstellar clouds and combustion reactors.

  • The PhD defense of Ruka MISAWA on Aug. 29, 2016 at IRAP

Title : Observation et analyse de l’émission polarisée de la poussière intersellaire avec l’expérience ballon PILOT

Abstract : PILOT, the Polarized Instrument for Long-wavelength Observation of the Tenuous interstellar medium is a balloon-borne astronomy experiment designed to study the linear polarization of far-infrared (FIR) emission in two photometric bands centred at wavelengths of 240 μm (1.2 THz) and 550 μm (545 GHz), with an angular resolution of a few arcminutes. The dominant source of this emission is dust grains in the diffuse interstellar medium (ISM) of our Galaxy and nearby galaxies.

PILOT measures polarization using a half-wave plate mechanism. It has a
large focal plane offering an instantaneous field-of-view of about 0.8°× 1°. Major science goals of PILOT are to map the structure of the magnetic field in the diffuse ISM, and to study the geometric and magnetic properties of interstellar dust grains. PILOT observations complement both Planck all-sky data at lower wavelengths ( < 850 μm ) and lower angular resolution, and high resolution studies of much smaller regions in polarization with large single dish instruments such as the James Clerk Maxwell Telescope (JCMT) or the NIKA2 instrument on the IRAM 30m telescope and interferometers such as the Atacama Large Millimeter Array (ALMA).

This thesis presents the results of the ground calibration tests of PILOT. It also includes an overview of PILOT’s first scientific flight and of the preliminary analysis of the in-flight data, in particular the comparison with expected performances. The ground tests were divided into two main sessions : tests of the PILOT photometer, and end-to-end tests. The photometer tests, which include measurements of the instrument transmission using the Fourier Transform Spectrometer tests, were performed during summer 2012 and spring 2013 at the IAS in Orsay. The end-to-end tests were performed during autumn 2013 and spring 2014 at CNES in Toulouse. The first scientific flight, which had a total duration of 24 hours, was launched from Timmins, Ontario, Canada on 20 September 2015.

  • The PhD defense of Viet Tiet PHUNG on Jul. 12, 2016 at ISMO

Title : Cavity ring-down spectroscopy of astrophysically relevant molecular species, toward quantitative and high resolution studies using spectro-temporal properties of high finesse cavities

Abstract : The main objective of this PhD was to develop spectroscopic
techniques using high finesse optical cavities. These were applied to
the measurement of quantitative spectroscopic data for neutral, radical
and ionic molecular species of astrophysical interest in the near
infrared and visible spectral range.

The first part was devoted to the measurement of the oscillator
strength of high vibrational overtone bands of the cyanoacetylene
(HC3N) molecule with the Cavity Ring Down Spectroscopy (CRDS)

The second part was devoted to the study of the spectroscopy of
transient neutral and ionic species. For that, an inductively radio
frequency (RF) discharged has been coupled to the CRDS set up. The
pertinence of this plasma to efficiently produce anions was demontrated
via the quantitative measurement of the C2-/C2 ratio in a wide variety
of conditions. A quantitative spectral analysis of the radical
isotopomers 14NH2 and 15NH2 was also performed for the first time.
This study provides experimental data that will allow to better
constraint the 15N/14N isotopic ratio in comets through the emission
lines of these two amino bearing isotopomers.

The third and last part of the work was devoted to the development of a
CRDS scheme called Broad Band Dual Etalon Frequency Comb Ring Down
Spectroscopy. This new heterodyne technique, based on the use of the
microsecond frequency combs generated by two high finesse optical
cavities, should allow performing molecular spectroscopy with ultrahigh
spectral resolution. First proof experiments were performed and
perspective’s for improvement of the method is provided.

  • The PhD defense of Lapo Fanciullio on Dec. 16th 2015 at IAS

Title : New insights on dust properties from PLANCK intensity and polarization data

Abstract : Interstellar dust is a key component of the interstellar medium (ISM),
both as an actor in the physico-chemistry of the ISM, and as a tracer
for the gas and magnetic field structure. Our understanding of
interstellar dust is currently undergoing a noticeable change : new and
more flexible models have been created using up-to-date laboratory
results ; furthermore, we have the data from the Planck HFI instrument
which has observed dust emission in intensity and in polarization,
marking a great advance in Galactic physics. In my presentation I will
show how the confrontation of dust models with a combination of Planck
and optical data improve our understanding of dust properties. Through
the comparison of dust extinction and emission I estimate the impact of
dust evolution in the variations of dust temperature through the
diffuse ISM. The comparison of polarization in extinction and in
emission, meanwhile, helps clarify the role of dust alignment in the
observed decrease of dust polarization in translucent clouds.

  • The PhD defense of Charlene Lefevre at IAP Nov. 30th, 2015

Title : Coreshine : un phénomène et un outil

Abstract : Even though dust grains contribute only to 1% of the interstellar
medium mass, their study is crucial to understand both the structure
and content of interstellar clouds. Dust grains leave their birth
places, spread out into the diffuse medium before being gathered
together again when dense molecular clouds form. During this last
stage, they grow, by coagulation especially, and they acquire ice
mantles composed mainly of water. These morphological changes also
modify their optical properties (absorption, scattering, and
emission). However, it remains a highly degenerate issue to determine
their composition, size, and shape from observations. In particular,
I will highlight that using wavelengths associated to dust emission is
not sufficient to investigate the dense cores, where stars and planets
will form. I will show that scattering can dominate the absorption at
3.6 and 4.5 μm, and that this phenomenon called coreshine is
a powerful
tool to investigate the densest parts of molecular clouds. The
coreshine detection in more than one hundred clouds of our Galaxy
allows us to eliminate a large number of dust models. Multi–wavelength
3D modeling is mandatory to characterize the balance between the
absorption and the scattering of the radiation field. While most of the
work about dust focus on absorption and re–emission of the radiation, I
will present how scattering, often neglected, brings a complete picture
of the radiative transfer inside dense clouds.

First, I will enumerate all the mecanisms that cause the coreshine to
appear. In particular, I will present the influence of the radiation
field with the help of a large sample of sources. Then, I will detail
the wavelengths that we used and what kind of information they bring in
the modeling context. Finally, I will show the impact of several
parameters linked to the dust on the coreshine strength : their shape
(compact grains versus aggregates), their porosity and the presence of
ice mantles.

In a second step, I will illustrate the relevant applications and
contri- butions of coreshine to the modeling. I will present how it can
be used to constrain the modeling of an interstellar cloud including a
prestellar core : L183. I will show thanks to this example that the
scattering contributes si- gnificantly to the observations up to 8
microns. This gives us insights about the dust properties from the edge
of the cloud to its core. The full modeling of L183, including from
1.25 to 500 μm (Spitzer+Herschel) including both the
scattering and the
emission, is compatible with grain growth by coa- gulation. Our results
confirm the existence of aggregates inside prestellar cores from
scattering. I will present that by constraining the density and dust
content from scattering, it is possible to obtain temperatures ranging
from 5 to 13 K. The obtained dust properties are fully compatible with
later stages of dust evolution, in particular inside protoplanetary

  • The PhD thesis defense of Aurelie Jallat in Orsay/IPN September 30th 2015

Title:Fragmentation de molécules carbonées d’intérêt astrophysique auprès des accélérateurs

Résumé : De nos jours environ 200 molécules ont été observées dans le
milieu interstellaire, environ 75% d’entre elles sont carbonées et 25%
sont des hydrocarbures. Les grains de poussières contiennent également
du carbone en grande quantité. La présence du carbone dans la majorité
des molécules et dans les grains de poussières s’explique par son
abondance et sa capacité à former des liaisons. Il est donc crucial de
les étudier d’un point de vue astrochimique. Dans une première partie,
ce travail présente les mesures des rapports de branchement et des
énergies dissipées sous forme d’énergie cinétique dans les fragments
neutres lors d’une collision à haute vitesse, des molécules carbonées
suivantes : SiC, AlC, AlCH, C2O, CN, CH2 et CH. Ces mesures ont été
obtenues grâce au dispositif expérimental AGAT. Ce dernier est installé
à demeure auprès de l’accélérateur Tandem d’Orsay. Il permet la
collision molécule-atome en cinématique inverse et la détection 100%
efficace de tous les fragments émis, y compris les neutres. Dans une
seconde partie, l’effet de l’ajout ou la correction de nouveaux
rapports de branchement d’hydrocarbures est discuté, sur la
modélisation de la chimie de deux objets célèbres : la région de
photo-dissociation de la nébuleuse de la Tête de Cheval et le nuage
moléculaire TMC-1. Ces deux objets sont bien connus pour leur richesse
en molécules observées. Ces nouveaux rapports de branchement diminuent
les abondances calculées des espèces de la phase gazeuse, déjà trop
basses par rapport aux observations. Une hypothèse stipule que des
hydrocarbures sont dégagés dans la phase gazeuse via les grains de
poussières carbonés. Suite à cette hypothèse, pour la première fois,
les effets de l’incorporation de réactions de photo-production
d’hydrocarbures par les grains de carbone amorphes hydrogénés ont été
étudiés, sur la chimie de la phase gazeuse de la nébuleuse de la Tête
de Cheval. L’ajout de ces réactions resserre l’écart entre la
modélisation et les observations.

  • The PhD thesis defense of Mikhail DORONIN in LERMA September. 28th, 2015

Title : Adsorption on Interstellar Analog Surfaces : from Atoms to Organic Molecules

Advisers : Jean-Hugues Fillion (LERMA) ad Alexis Markovits (LCT)

Abstract :Gas-grain interaction plays an important role in the
chemistry of the cold interstellar medium and protoplanetary disks. A
key parameter for modeling the exchange between grain surfaces and gas
phase is the adsorption energy, E_a . This work aims at developping a
reliable and systematic experimental/theoretical approach to determine
the adsorption energies of relevant atoms and molecules onmodels of
interstellar grain surfaces. The employed experimental technique is the
Temperature Programmed Desorption (TPD). The developed experimental
protocol and data treatment technique based on the distribution of
adsorption energies and use of a set of heating rates enables to
determine independently the coupled parameters of Polanyi-Wigner
equation : adsorption energy E_a and prefactor ?. Computational chemistry
approach, Density Functional Theory (DFT) as implemented in Vienna Ab
initio Simulation Package (VASP) is used to get the insight on the
behaviour of the surface-adsorbate systems at the atomic level. This
approach allows as well determining adsorption energies. A presence of
multiple adsorption sites with different adsorption energies is
predicted.The well-known Methanol CH_3 OH adsorption on graphite is
used as a benchmark for validating the experimental approach. Ar/Kr/Xe
adsorption on water ice is studied as a case relevant for
planetology. Acetonitrile CH_3 CN and methyl isocyanide CH_3 NC
adsorption on water ice, quartz and graphite, is investigated since
those two molecules are both detected in the interstellar medium.

  • The PhD defense of Anna Parikka in IAS on Sep. 28th, 2015

Title : Properties and evolution of dense structures in the interstellar

  • The PhD thesis defense of Valeska Valdivia in LERMA/ENS on Sep. 24th, 2015

Title : Impact of Radiative Transfer and Chemistry on the Formation of
Molecular Clouds

Advisor : Patrick Hennebelle.

Abstract : The interstellar medium (ISM) is a highly complex system. It
corresponds to an intermediate scale between stars and galaxies. The
interstellar gas is present throughout the galaxy, filling the volume
between stars. A wide variety of coupled processes, such as gravity,
magnetic fields, turbulence and chemistry, participate in its
evolution, making the modeling of the ISM a challenging problem. A
correct description of the ISM requires a good treatment of the
magnetohydrodynamics (MHD) equations, gravity, thermal balance, and
chemical evolution within the molecular clouds.

This thesis work aims at a better understanding of the formation and
evolution of molecular clouds, specially how they become "molecular",
paying particular attention to the transition HI-to-H2. We have
performed ideal MHD simulations of the formation of molecular clouds
and the formation of molecular hydrogen under the influence of gravity
and turbulence, using accurate estimates for the shielding effects from
dust and the self-shielding for H2, calculated with a Tree-based
method, able to provide fast estimates of column densities.

We find that H2 is formed faster than predicted by the usual estimates
due to local density enhancements created by the gas turbulent
motions. Molecular hydrogen, formed at higher densities, could then
migrate toward low density warmer regions.

Total H2 column densities show that the HI-to-H2 transition occurs at
total column densities of a few 10^20 cm−2. We have calculated the
populations of rotational levels of H2 at thermal equilibrium, and
integrated along several lines of sight. These two results reproduce
quite well the values observed by Copernicus and FUSE, suggesting that
the observed transition and the excited populations could arise as a
consequence of the multi-phase structure of molecular clouds. As H2
formation is prior to further molecule formation, warm H2 could
possibly allow the development of a warm chemistry, and eventually
explain some aspects of the molecular richness observed in the ISM.

  • Celia Verdugo defended her PhD thesis in LERMA on Tuesday, July 21st 2015

Title : Star formation in low gas density and low metallicity environments

Abstract : In nearby galaxies, an empirical relation has been established
between star formation and gas surface densities, the Kennicutt-Schmidt
relation (KS). The relation is nearly linear when molecular gas (H2) is
considered, while is less tight with atomic hydrogen (HI). These low gas
density regions are of a key importance in the field of star formation,
since the are also low metallicity environments, resembling the
conditions of a younger universe. This thesis summarizes the
observational work done with the iram 30m telescope in two kinds of
such regions : disk galaxies with extended ultra-violet emission (XUV),
and the interstellar medium going into the hot intra-cluster medium
(ICM) under ram-pressure stripping in the Virgo cluster. The GALEX
telescope has unveiled in far ultra-violet (FUV) star formation in the
outer parts of some disk galaxies that was not traced by Ha. To
determine the presence of H2 and analyze the KS relation in these
regions, CO observations were done in the outskirts of several XUV disk
galaxies, finding both detections and upper limits. These KS relations
showed a broken power law at low gas densities, below the HI-H2
threshold. In the Virgo cluster, similar CO observations were done
along the HI tidal arm connecting NGC4388 and M86, where no H2 is
expected. Two detections were found, showing very low star formation
efficiencies (depleting less than 0.1% of the gas reservoir per 10^8
yr), and showing again a discontinuity of the KS relation at low gas
densities, probing that the process of gas consumption into stars well
known at high densities cannot be extrapolated to lower densities, and
that H2 can survive a certain time in the hostile ICM.   

  • Solenn Vaupre defended his PhD thesis in IPAG July 10th 2015

Titre : Ionisation des nuages moléculaires par les rayons cosmiques

Resume : Les rayons cosmiques (RC) ont un rôle fondamental sur la
dynamique et l’évolution chimique des nuages moléculaires
interstellaires, qui sont le lieu de formation stellaire et planétaire.
Les RC sont probablement accélérés dans les enveloppes en expansion des
rémanents de supernova (SNR), ainsi les nuages moléculaires situés à
proximité peuvent être soumis à d’intenses flux de RC. Les protons
relativistes ont principalement deux effets sur les nuages moléculaires
 : 1) en rencontrant le milieu dense, les protons de haute énergie (>280
MeV) induisent via la désintégration des pions l’émission de photons
gamma. à cause de ce processus, les associations SNR-nuage moléculaires
sont des sources intenses d’émission GeV et/ou TeV présentant des
spectres similaires à celui des protons incidents. 2) à plus basse
énergie, les RC pénètrent le nuage et ionisent le gaz, induisant la
formation d’espèces moléculaires caractéristiques appelées traceurs de
l’ionisation. L’étude de ces traceurs permet de déduire des
informations sur les RC de basse énergie inaccessibles aux autres
méthodes d’observation. J’ai étudié l’ionisation des nuages
moléculaires par les RC près de trois SNR : W28, W51C et W44. Il existe
des preuves observationnelles d’interaction avec le nuage voisin pour
chaque SNR (présence de gaz choqué, masers OH, émission gamma). Mon
travail repose sur la comparaison d’observations millimétriques des
traceurs de l’ionisation à des modèles de chimie appliqués à ces nuages
denses. Dans chaque région, nous avons déterminé un taux d’ionisation
supérieur à la valeur standard, confortant l’hypothèse d’une origine
des RC dans l’enveloppe du SNR voisin. L’existence d’un gradient
d’ionisation en s’éloignant de l’onde de choc du SNR apporte des
contraintes précieuses sur les propriétés de propagation des RC de
basse énergie. La méthode utilisée repose sur l’observation des ions
moléculaires HCO+ et DCO+, qui montre des limitations importantes à
haute ionisation. C’est pourquoi j’ai également cherché à identifier
des traceurs alternatifs de l’ionisation, par un effort croisé de
modélisation et d’observation. En particulier, dans la région W44, les
observations de N2H+ ont permis de mieux contraindre les conditions
physiques, les abondances volatiles dans le nuage et l’état
d’ionisation du gaz. Ce projet de recherche a amené une meilleure
compréhension de la chimie induite par les RC dans les nuages
moléculaires. Il a également ouvert de nouvelles perspectives de
recherche interdisciplinaire vers la compréhension des RC, des
observations millimétriques aux observations gamma.

  • PhD thesis of Ichraf Ouestali defended on the 13th of April at Observatoire de Pars
    co-supervision UPMC and University of Tunis

Titre : "Etude théorique de la formation catalytique de petites molécules
sur des modèles de grains interstellaires"

  • PhD defense thesis of Giorgos Momferratos, on Friday Jan. 23rd at Observatoire de Paris

Tittle : The sites of extreme turbulent dissipation in the diffuse interstellar medium : structure and properties

  • PhD defense thesis of Uddhab Chaulagain on Janurary 22 at LERMA

Tittle : Radiative shocks : experiments, modelling and links to Astrophycs

Abstract : Radiative shocks are strong shocks which are characterized by a plasma
at high temperatures emitting an important fraction of its energy as
radiation. Radiative shocks are found in many astrophysical systems,
including stellar accretion shocks, supernovae remnants, jet driven
shocks, etc. Recently, radiative shocks have also been produced
experimentally using high energy lasers. Thus opening the way to
laboratory astrophysics studies of these universal phenomena.

In this thesis we discuss the results of an experiment performed on the
Prague Asterix Laser System facility. Shocks are generated by focusing
the PALS Infrared laser beam on millimetre-scale targets filled with
xenon gas at low pressure. The shock that is generated then propagates
in the gas with a sufficiently high velocity such that the shock is in
a radiative flux dominated regime. We used different diagnostics to
characterize these shocks. The two main ones include a radiography of
the whole shock structure using sub-nanosecond Zn X-ray laser at 21.2
nm, which is able to penetrate the dense post-shock layer, and a
space-and-time resolved plasma self-emission using high speed diodes.

The experimental results show, for the first time, an unambiguous shock
structure which includes both the post-shock and the precursor, and we
also obtained multiple shock velocity measurements from the different
diagnostics. The experimental results are compared to simulations, and
show good agreement with the numerical results. Résumé : L’hydrogène moléculaire est la molécule la plus abondante dans
l’Univers. Il est reconnu que cette molécule ne peut se former que par un
processus catalytique mettant en jeu la surface des grains de poussière
interstellaires. Les études théoriques ont été menées principalement sur
des surfaces de graphite et très récemment sur la forstérite [010]
cristalline et amorphe. C’est dans ce cadre que se situe notre étude qui
porte sur les réactions de formation de l’hydrogène moléculaire sur de
nouveaux modèles de surfaces silicées et silicatées.
Dans un premier temps, nous avons étudié la réaction d’abstraction par
l’hydrogène atomique à partir du tétramethylsilane (un prototype de grains
silicés) en phase gazeuse. On a utilisé les méthodes sophistiquées de la
chimie quantique basées sur la méthode perturbative à l’ordre 2 ainsi que
la méthode des clusters couplés. Les effets isotopiques primaire (KIE) et
secondaire (SKIE) ont été mis en évidence. La théorie canonique de l’état
de transition généralisée (CVT) incluant la correction de l’effet tunnel à
faible courbure (SCT) a été appliquée pour étudier la cinétique des
réactions dans un intervalle de températures allant de 180 K à 2000
K. Cette étude a montré que la réaction est colinéaire. Etant donnée
l’importance de l’effet tunnel à basse température, nous avons entrepris
des calculs de dynamique quantique. Pour ce faire, une surface d’énergie
potentielle en coordonnées hyper-sphériques V (ρ,δ) a été construite. Le
calcul quantique a été réalisé en utilisant une approche à dimensionnalité
réduite à deux dimensions appliquée à ce problème de collision réactive
colinéaire. Ce modèle traite explicitement les liaisons qui se rompent et
qui se forment lors du processus réactionnel. Les modes spectateurs sont
introduits sous forme de l’énergie de point zéro. La résolution des
équations couplées permet d’extraire la matrice S de diffusion conduisant
aux probabilités d’état à état ainsi qu’aux taux de réactions
quantiques. Les résultats montrent que H2 est principalement formé dans son
état vibrationnel fondamental. La comparaison avec les résultats
expérimentaux réalisés dans le domaine de températures 425-570 K montre un
accord satisfaisant. La comparaison des résultats quantiques avec les
résultats semi-classiques précédemment obtenus confirme l’importance de
l’effet tunnel aux basses températures.
Une étude de formation de H2 sur des nanosilicates, prototypes des surfaces
silicatées, a été menée. Les méthodes de la théorie de la fonctionnelle de
la densité (DFT) ont permis d’identifier les sites de physisorption et de
chimisorption d’atomes d’hydrogène, d’étudier la diffusion de l’atome H
physisorbé sur le cluster, de déterminer les caractéristiques énergétiques
de ces sites et les énergies d’activation pour la désorption et la
recombinaison de H2. Différents modes d’adsorption ont été identifiés et
plusieurs chemins exothermiques et endothermiques pour la recombinaison des
deux atomes H ont été obtenus pour ces nanoclusters. Les résultats trouvés
ont montré que les grains amorphes/poreux de la composition de la
forstérite ont tendance à dissocier H2 et que les grains de silicate pauvre
en Mg (e.g. de la composition de l’enstatite) ou les grains silicatés
cristallins/compacts ont tendance à catalyser la formation de H2.

  • PHD thesis of Mario Hernandez Vera (LOMC - Le Havre) defended in 2014

Title : Toward the understanding of cyanide/isocyanide abundances : Inelastic collisions and radiative transfer calculations

Cyanide and isocyanide species are ubiquitous in the interstellar medium
(ISM). We have studied the collisional excitation of three kind of
species : the metal cyanides/isocyanides which are the most common
metal-bearing molecules in the ISM ; the silicon cyanide/isocyanide, and
the simplest hydrogen cyanide molecule which is one of the best tracers
of the dense interstellar gas. Modelling of molecular emission spectra
of these species from interstellar clouds requires the calculation of
rate coefficients for excitation by collisions with H2, the most
abundant molecule in the cold ISM. Thus, we have used the coupled states
approximation to study the rotational (de-)excitation of AlCN(1Σ),
AlNC(1Σ), MgCN(2Σ), MgNC(2Σ), SiCN(2Π) and SiNC(2Π) molecules by
collisions with He, as a model of H2. We have also considered the
rotational (de-)excitation of HCN(1Σ) molecules by ortho- H2 and para-H2
molecules using the close coupling approach. In all cases, new highly
correlated potential energy surfaces have been employed. Significant
differ- ences between the rate coefficients of the isomers were
observed. These differences confirm that specific calculations have to
be performed for each isomer in order to obtain the necessary level of
details in astrophysical applications. We have also assessed the impact
of our collisional rates coefficients in the molecular emission
simulations using radiative transfer calculations.

  • PHD thesis of Romane Le Gal (IPAG, Grenoble) defended 2014

Title : The interstellar chemistry of nitrogen and
especially on the nitrogen hydrides chemistry in cold dense
interstellar regions.

The new spectroscopic window opened by the advent of the Herschel Space
Observatory, has enabled the detection of simple nitrogen species, the
nitrogen hydrides NH, NH2, and NH3, in cold envelopes of
protostars. These envelopes are made of dense cold gas characteristic
of the physico-chemical conditions of molecular clouds. The observation
of nitrogen hydrides in such environments has brought new constraints
on the interstellar chemistry of these kind of clouds, and gives, in
particular, the opportunity to revisit the chemistry of nitrogen.

The aim of my thesis was to comprehensively analyse the interstellar
chemistry of nitrogen, focussing on the gas-phase formation of the
nitrogen hydrides, in the light of recent theoretical and experimental
work. In addition, recent work about the conversion of the two spin
symmetries (ortho and para) of molecular hydrogen and new calculations
of nuclear spin branching ratios for the production pathways of
nitrogen hydrides in their ortho and para configurations conducted at
IPAG, enabled us to treat self-consistently the different spin
symmetries of the nitrogen hydrides together with the ortho and para
forms of molecular hydrogen. To this purpose, we develop a new
gaz-phase chemical network in which the kinetic rates of critical
reactions involved in the nitrogen chemistry have been updated.

This new network is used to model the time evolution of the nitrogen
species abundances in dense cold gas conditions. The steady-state
results are compared to observations of NH, NH2 and NH3 towards a
sample of low-mass protostars, performed thank to Herschel, with a
special emphasis on the influence of the overall amounts of gaseous
carbon, oxygen, and sulphur. Our chemical models reproduced the
nitrogen hydrides abundances and so constrain the gas-phase elemental
C/O ratio and the total amount. Moreover, our predicted ortho-to-para
ratios for NH2 and NH3 are in good agreement with other recent Herschel
observations performed towards cold diffuse clouds. Then, in dark gas
conditions, the nitrogen hydride abundances are consistent with a pure
gas-phase synthesis.

  • PHD Thesis of Lionel de Sá (Observatoire de Paris) defended in 2014

Title : Accretion onto young
stars - a radiation hydrodynamics model

Accretion columns connect young stars to the surrounding disk of gas
and dust. Numerous numerical studies have predicted quasi-periodic
oscillations of the shocked structure at the base of these
columns. There is, however, no observational evidence of such
feature. These simulations rely on the assumption that accreted gas can
be described with an optically thin line cooling function. The main
goal of my work has been to go beyond this assumption. I started with
the improvement of the description of important microscopic processes
included in the 1D ALE RHD code AstroLabE. I worked then on the
building of adapted opacity tables, to take into account the coupling
between radiation and matter. The results show that even by taking into
account the absorption of a small fraction of radiation, the dynamics
of the shocked gas structure is significantly affected, and the
predicted oscillatory behavior may be suppressed.

I have concentrated on the coherent modeling of the stellar
chromosphere above which the accretion takes place. For this purpose, I
used a model based on acoustic waves heating. Although the
chromospheric shock waves perturb the dynamics of accretion (which
remains periodic), the computed luminosity presents modulations of
relative small amplitude.

The work highlights the importance of the radiative transfer in the
accretion process on young stars and the necessity of an adequate,
physically based, description of the radiative transfer. The methods I
have developed in this work will foster developments of
multi-dimensional simulations.

  • PHD thesis of Simon Chefdeville (ISM, Bordeaux) defended in 2014

Title : Dynamique de collisions moléculaires à très basse énergie. Mise en évidence
expérimentale de résonances quantiques

Theoretical calculations predict that the dynamics of rotational excitation
of CO or O2 molecules, induced by collisions with H2, are dominated by
quantum scattering resonances at very low energies. However, experimental
observation of these effects is challenging : very low collision energies
and high energy resolution are both required. Experiments performed with a
crossed molecular beam apparatus with variable intersection angle allow us
to observe the thresholds of the CO (j = 0 -> 1) transition at 3.85 cm-1
and the O2 (Nj = 10 -> 11) transition at 3.96 cm-1, which correspond to the
average kinetic energy of a gas below 4 K. The peaks in the integral cross
section’s collision energy dependence constitute the first experimental
observation of resonances in an inelastic process. The good agreement
between theory and experiment reinforces the confidence in the interaction
potentials used to deduce rate coefficients for modeling the interstellar
medium in the 1-20 K range. Our experimental results highlight the quantum
nature of molecular interactions at very low energies.

  • PHD thesis of Chantal Sleiman (Rennes 1) defended in 2014

Title : Réactivité chimique en phase gazeuse de molécules organiques d’intéret
atmosphérique et astrophysique

This thesis reports the experimental kinetic study of the gas phase
reactions of atmospheric and astrophysical interests. The knowledge of
the reactions rate constants is useful to understand the mechanisms of
formation and destruction of molecules in the Earth’s atmosphere and in
the interstellar medium.

On the atmospheric side, we have studied the reactions of a series of
hydroxyketones (4-hydroxy-2-butanone, 3-hydroxy-3-methyl-2-butanone and
4-hydroxy-4-methyl-2-pentanone), a large category of hydroxycarbonyls
with OH radicals and chlorine atoms Cl in order to determine their
atmospheric fate. We have measured the absolute rate constants of the
reactions of hydroxyketones with OH radicals by using the cryogenic cell
coupled to PLP-LIF technique (Pulsed Laser Photolysis - Laser Induced
Fluorescence) at room temperature and as function of pressure. The
relative rate constants of the reactions of the compounds investigated
with Cl atoms were measured at room temperature and atmospheric pressure
by using the atmospheric simulation chamber coupled to the analytical
detection techniques : FTIR and GC-MS. In addition, a mechanistic study
was also conducted in order to identify and quantify the products formed
from these reactions. The whole results are discussed in order to assess
the atmospheric implications of these hydroxyketones (lifetimes and
environmental impact).

On the astrophysical side, the kinetic of the reactions involving CN
radical and a series of nitrogen-containing molecules (methylamine,
dimethylamine, trimethylamine and acetonitrile) was studied over a wide
range of temperature (23 K - 354 K) using CRESU technique (a French
acronym standing for Cinétique de Réaction en Ecoulement Supersonique
Uniforme) and the cryogenic cell coupled to the detection technique
PLP-LIF. These experimental studies were accompanied by theoretical
studies to better understand the reaction mechanisms. Possible
astrophysical implications of the whole results have been discussed in
this study.

  • PhD thesis of P.Modica (IAS, Paris) defended in 2014

Title : From astrophysics to astrobiology : significance of laboratory
organic residues from photo-irradiation of cosmic ice analogs

Laboratory experiments have shown that ultraviolet photo-irradiation of
astrophysical ice analogs and their following warm-up until room
temperature lead to the formation of refractory organic residues. These
water-soluble residues consist of rich mixtures of organic compounds
including amino acids which have a potential importance for prebiotic
chemistry. These residues are considered as analogs of the organic
refractory materials that are thought to be synthesized on dust grains
in molecular clouds and/or in protoplanetary disks, as a product of ices
evolution, and that could be later accreted into comets and asteroids
and eventually be delivered to the early Earth. Hence, the study of
these analogs, produced under astrophysically relevant conditions,
represents a valid tool to investigate the processes at work for the
origin of complex organic molecules in the Solar System and in
particular the possible introduction of enantiomeric excesses in chiral

This PhD work is devoted to the study of these laboratory organic
residues, their characterization and the astrophysical applications of
the results. We used different analytical techniques such as gas
chromatography mass spectrometry (GC MS, classical and
multidimensional), Fourier transform ion cyclotron resonance mass
spectrometry (FT ICR MS), and infrared spectroscopy. We measured the
enantiomeric excesses induced in five chiral amino acids by UV
circularly polarized light (UV CPL) irradiation of our analogs and
insert our result in a coherent astrophysical scenario for the origin of
the enantiomeric excesses observed in meteoritic amino acids. We studied
the amino acid content of the Paris meteorite and evidence some
similarities with the distribution of the amino acids in our organic
residues. We also produced more realistic analogs of interstellar
grains, including a silicate surface, to test the potential effect of
such a surface on the formation and nature of organic residues. Finally,
we discuss the significance of these results in the astrophysical
context and the possible relationship between astrochemistry and
prebiotic chemistry.