261 research outputs found
A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone
Methanol (CHOH) is found to be abundant and widespread towards the
Central Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin
is, however, not fully understood. It was proposed that the high cosmic ray
ionisation rate in this region could lead to a more efficient non-thermal
desorption of this species formed on grain surfaces, but it would also mean
that this species is destroyed in a relatively short timescale. In a first
step, we run chemical models with a high cosmic ray ionisation rate and find
that this scenario can only reproduce the lowest abundances of methanol derived
in this region (10-10). In a second step, we investigate
another scenario based on episodic explosions of grain mantles. We find a good
agreement between the predicted abundances of methanol and the observations. We
find that the dominant route for the formation of methanol is through
hydrogenation of CO on the grains followed by the desorption due to the grain
mantle explosion. The cyclic aspect of this model can explain the widespread
presence of methanol without requiring any additional mechanism. We also model
silicon monoxide (SiO), another species detected in several molecular clouds of
the Galactic Centre. An agreement is found with observations for a high
depletion of Si (Si/H 10) with respect to the solar abundance.Comment: Accepted in MNRA
High DO/HDO ratio in the inner regions of the low-mass protostar NGC1333 IRAS2A
Water plays a crucial role both in the interstellar medium and on Earth. To
constrain its formation mechanisms and its evolution through the star formation
process, the determination of the water deuterium fractionation ratios is
particularly suitable. Previous studies derived HDO/HO ratios in the warm
inner regions of low-mass protostars. We here report a detection of the DO
1-1 transition toward the low-mass protostar NGC1333 IRAS2A
with the Plateau de Bure interferometer: this represents the first
interferometric detection of DO - and only the second solar-type protostar
for which this isotopologue is detected. Using the observations of the HDO
5-6 transition simultaneously detected and three other HDO
lines previously observed, we show that the HDO line fluxes are well reproduced
with a single excitation temperature of 21821 K and a source size of
0.5 arcsec. The DO/HDO ratio is (1.20.5)
10, while the use of previous HO observations give an
HDO/HO ratio of (1.70.8) 10, i.e. a factor of 7
lower than the DO/HDO ratio. These results contradict the predictions of
current grain surface chemical models and indicate that either the surface
deuteration processes are poorly understood or that both sublimation of grain
mantles and water formation at high temperatures (230 K) take place in
the inner regions of this source. In the second scenario, the thermal
desorption of the grain mantles would explain the high DO/HDO ratio, while
water formation at high temperature would explain significant extra production
of HO leading to a decrease of the HDO/HO ratio.Comment: Accepted for publication in ApJ Letters; 12 pages, 2 figure
La deutération de l'eau dans les régions de formation stellaire : apport des données spectroscopiques Herschel/HIFI
L'eau (H2O) est une des molécules les plus abondantes du milieu interstellaire. En plus d'être un ingrédient nécessaire à l'apparition de la Vie, elle joue également un rôle important dans le processus de formation stellaire à travers le refroidissement du gaz chaud et contrôle aussi la chimie de nombreuses autres espèces, que ce soit en phase gazeuse ou à la surface des grains. Etudier sa forme deutérée HDO constitue un moyen unique, à travers l'estimation du rapport HDO/H2O, de contraindre les mécanismes de formation de l'eau ainsi que de mieux comprendre l'origine de l'eau des océans terrestres. Les résultats récents obtenus avec le satellite Herschel montrent en effet que le rapport HDO/H2O observé dans les comètes est similaire à celui mesuré dans les océans (~ 1.5 10-4), suggérant que l'eau pourrait avoir été apportée sur Terre par les comètes lors de grands bombardements (Hartogh et al. 2011). Dans cette thèse, je me suis intéressée à l'étude de l'eau deutérée durant les premières étapes de la formation stellaire, la phase de Classe 0, qui précèdent la formation du disque proto-planétaire menant à la naissance des planètes et des comètes. En modélisant avec un code 1D de transfert radiatif hors-Equilibre Thermodynamique Local les profils des nombreuses raies de HDO et H218O observées avec l'instrument HIFI (Heterodyne Instrument for Far-Infrared) de l'Observatoire Spatial Herschel et des télescopes terrestres (IRAM, JCMT), j'ai déterminé des rapports HDO/H2O de la proto-étoile de type solaire IRAS 16293-2422 de l'ordre de 2% dans le hot corino, la partie interne de l'enveloppe suffisamment chaude (T>100 K) pour que les molécules d'eau collées à la surface des grains désorbent en phase gazeuse, et de 0.5% dans l'enveloppe externe plus froide. Grâce à ce travail (Coutens et al. 2012), la présence en avant-plan d'une couche d'absorption riche en eau a été mise en évidence observationnellement pour la première fois. Elle pourrait être due à des processus de photo-désorption des molécules d'eau piégées dans les manteaux de glace des grains, en bordure de nuage moléculaire, par le champ interstellaire UV. Les estimations des rapports HDO/H2O ainsi que D2O/HDO dans cette source permettent de contraindre les conditions de formation de l'eau dans ce type d'objet et suggèrent notamment que l'eau se serait probablement formée avant l'effondrement gravitationnel du nuage. Cette étude a ensuite été étendue à d'autres proto-étoiles de type solaire NGC 333 IRAS4A et NGC1333 IRAS4B pour lesquelles j'ai estimé les abondances d'eau deutérée et constaté qu'une couche d'absorption étendue entourait également ces sources. Les rapports HDO/H2O élevés de la proto-étoile IRAS 16293-2422 suggèrent que des mécanismes nécessaires à la diminution de ces rapports isotopiques prennent place entre l'étape de Classe 0 et la formation des comètes. Il faudra néanmoins étudier un plus grand échantillon de proto-étoiles pour savoir si cette tendance est générale ou non. Les abondances de HDO obtenues dans les proto-étoiles NGC1333 IRAS4A et NGC1333 IRAS4B serviront donc à de prochaines estimations des rapports HDO/H2O. Enfin, je me suis également attachée à étudier l'eau deutérée dans des sources proto-stellaires beaucoup plus massives et plus lumineuses que les proto-étoiles de type solaire et présente ici le cas de la région HII ultra-compacte G34.26+0.15.Water (H2O) is one of the most abundant molecules in the interstellar medium. In addition to being a primordial ingredient in the emergence of life, this species plays an essential role in the process of star formation through the cooling of warm gas. It also controls the chemistry for many species, either in the gas phase or on the grain surfaces. Studying its deuterated form HDO is a unique opportunity, through the estimation of the HDO/H2O ratio, to constrain the mechanisms of water formation and to better understand the origin of water contained in terrestrial oceans. Indeed, recent results obtained with the Herschel satellite show that the HDO/H2O ratio observed in comets is similar to the value measured in oceans (~ 1.5 10-4), which suggests that comets could have brought a large fraction to Earth to form the oceans during heavy bombardments (Hartogh et al. 2011). In this thesis, I was interested in the study of deuterated water in the first stages of star formation, the Class 0 stage, which precede the formation of the protoplanetary disk leading to the birth of comets and planets. Through a 1D non-Local Thermodynamic Equilibrium radiative transfer modeling of the line profiles of the numerous HDO and H218O transitions detected with the HIFI (Heterodyne Instrument for Far-Infrared) instrument onboard the Herschel Space Observatory and ground-based telescopes (IRAM, JCMT), I determined that the HDO/H2O ratios of the solar-type protostar IRAS 16293-2422 was about 2% in the hot corino, the inner part of the protostellar envelope sufficiently warm (T > 100 K) to desorb in gas phase the water molecules trapped in the icy grain mantles, and about 0.5% in the colder part of the envelope. This study (Coutens et al. 2012) also allowed me to show that an absorbing layer rich in water surrounds the protostar. This layer could be produced by the photo-desorption through the UV field of the water molecules frozen on the grains, on the edges of the molecular cloud. The HDO/H2O ratios as well as the D2O/HDO ratios determined in IRAS 16293-2422 enable to constrain the conditions of water formation in this kind of objects and in particular suggest that water would be formed before the gravitational collapse of the cloud. This study was then extended to other solar-type protostars NGC1333 IRAS4A and NGC1333 IRAS4B, for which I estimated the abundances of deuterated water and noticed that an extended absorbing layer also surrounds these sources. The high HDO/H2O ratios determined in IRAS 16293-2422 suggest that mechanisms are required between the Class 0 stage and the comets formation to decrease these isotopic ratios. It is however necessary to study a larger sample of protostars to know if this trend is observed in most of the sources. The HDO abundances obtained in NGC1333 IRAS4A and NGC1333 IRAS4B will consequently be useful to estimate their HDO/H2O ratios. Finally, I also studied deuterated water in protostellar objects more massive and more luminous than solar-type protostars and show here the case of the ultra-compact HII region G34.26+0.15
On the Formation of Deuterated Methyl Formate in Hot Corinos
Methyl formate, HCOOCH, and many of its isotopologues have been detected
in astrophysical regions with considerable abundances. However, the recipe for
the formation of this molecule and its isotopologues is not yet known. In this
work, we attempt to investigate, theoretically, the successful recipe for the
formation of interstellar HCOOCH and its deuterated isotopologues. We used
the gas-grain chemical model, UCLCHEM, to examine the possible routes of
formation of methyl formate on grain surfaces and in the gas-phase in low-mass
star-forming regions. Our models show that radical-radical association on
grains are necessary to explain the observed abundance of DCOOCH in the
protostar IRAS~16293--2422. H-D substitution reactions on grains significantly
enhance the abundances of HCOOCHD, DCOOCHD, and HCOOCD. The
observed abundance of HCOOCHD in IRAS 16293--2422 can only be reproduced if
H-D substitution reactions are taken into account. However, HCOOCHD remain
underestimated in all of our models. The deuteration of methyl formate appears
to be more complex than initially thought. Additional studies, both
experimentally and theoretically, are needed for a better understanding of the
interstellar formation of these species.Comment: 13 pages , 3 figures, 5 tables , accepted for publications in MNRA
Stratégies non pharmacologiques et réponse aux antidépresseurs dans un modèle murin de dépression : effets comportementaux et mécanismes neurobiologiques
Les troubles majeurs dépressifs concernent plus de 300 millions de personnes à travers le monde. Si des composés pharmacologiques permettent de traiter la dépression, les substances commercialisées pour cette indication présentent certaines limites thérapeutiques. Notamment, on observe pour tous ces composés, un faible taux de réponse, un taux élevé de rechute et/ou un long délai d'action. En effet, la plupart des antidépresseurs nécessitent un traitement au long cours, avant que les premiers signes thérapeutiques ne soient observables, ce qui constitue un inconvénient clinique majeur. Au niveau mécanistique, ceci s'explique par le fait que l'administration chronique d'antidépresseur entraîne des modifications cérébrales qui nécessitent plusieurs semaines, voire plusieurs mois, pour se mettre en place. Dans ce contexte, il apparaît pertinent d'identifier de nouvelles solutions permettant une action plus rapide et plus durable sur les symptômes dépressifs. Pour ce faire, les approches non pharmacologiques suscitent un intérêt croissant puisqu'elles visent les causes des symptômes comportementaux, apparaissant ainsi comme des alternatives aux traitements pharmacologiques. En effet, on sait aujourd'hui que le style de vie est un facteur de déclenchement de la dépression majeure, et les effets protecteurs d'une alimentation saine, d'une vie sociale riche et de l'exercice physique sur la santé mentale ont été décrits. Au cours de ce travail de thèse, nous avons examiné si et comment ces éléments environnementaux contribuent à traiter le trouble dépressif lorsqu'ils sont proposés seuls ou combinés à un antidépresseur classique. A l'aide de souris modèle de dépression, nous avons montré que le séjour en environnement enrichi réduit le délai d'action de la venlafaxine. Ensuite nous avons déterminé que l'effet bénéfique de cette combinaison est associé à la désorganisation rapide de la plasticité des interneurones GABAergiques de l'hippocampe, impliquant la matrice extracellulaire qui entoure ces neurones, ainsi que des effets sur la neurogenèse hippocampique adulte. Nous avons également observé que l'arrêt des stimulations environnementales aggrave le phénotype pseudo-de´pressif chez les animaux, alors que l'exercice physique combiné avec un traitement antidépresseur induit des effets bénéfiques précoces mais partiels sur le comportement. Dans l'ensemble, nos travaux montrent un effet bénéfique des stratégies thérapeutiques non pharmacologiques et identifient les interneurones GABAergiques à parvalbumine comme une cible pertinente sur laquelle agir pour réduire le délai d'action des antidépresseurs actuellement disponibles sur le marché.Major depressive disorders affect more than 300 million people worldwide. Although pharmacological compounds are available to treat depression, those marketed for this indication have some therapeutic limitations. In particular, all these compounds present a low response rate, a high relapse rate and/or a long onset of action. Indeed, most antidepressants require a long-term treatment before the first therapeutic outcomes, which is a major clinical drawback. Mechanistically, this can be explained by the fact that chronic administration of antidepressants results in brain changes requiring several weeks, or even months, to occur. In this context, it seems relevant to identify new therapies allowing a faster and long-lasting action on depressive symptoms. To this end, there is growing interest in non-pharmacological strategies that target the causes of behavioral symptoms and thus appear to be alternatives to pharmacological treatments. Indeed, it is known that lifestyle is a triggering factor for major depression, and the protective effects of a healthy diet, rich social life, and physical exercise on mental health have been described.
This PhD work examined whether and how, these environmental elements contribute to treating depressive disorder when proposed alone or combined with a conventional antidepressant. Using a mouse model of depression, we showed that living in an enriched environment reduces the onset of action of venlafaxine. We then determined that the beneficial effect of this combination is associated with a rapid disruption of plasticity of GABAergic interneurons underpinned by the extracellular matrix surrounding these neurons and by regulatory effects on adult hippocampal neurogenesis. We observed in contrast, that the discontinuation of environmental stimuli aggravates the depressive-like phenotype of animals, whereas physical exercise in combination with an antidepressant treatment induces early, but partial, beneficial effects. Overall, our work shows a beneficial effect of non-pharmacological therapeutic strategies and identifies GABAergic parvalbumin interneurons as a relevant target to consider in order to reduce the delay of action of currently available antidepressants
On the accretion process in a high-mass star forming region - A multitransitional THz Herschel-HIFI study of ammonia toward G34.26+0.15
[Abridged] Our aim is to explore the gas dynamics and the accretion process
in the early phase of high-mass star formation. The inward motion of molecular
gas in the massive star forming region G34.26+0.15 is investigated by using
high-resolution profiles of seven transitions of ammonia at THz frequencies
observed with Herschel-HIFI. The shapes and intensities of these lines are
interpreted in terms of radiative transfer models of a spherical, collapsing
molecular envelope. An accelerated Lambda Iteration (ALI) method is used to
compute the models. The seven ammonia lines show mixed absorption and emission
with inverse P-Cygni-type profiles that suggest infall onto the central source.
A trend toward absorption at increasingly higher velocities for higher
excitation transitions is clearly seen in the line profiles. The lines show only very weak emission, so these absorption profiles
can be used directly to analyze the inward motion of the gas. This is the first
time a multitransitional study of spectrally resolved rotational ammonia lines
has been used for this purpose. Broad emission is, in addition, mixed with the
absorption in the ortho-NH line, possibly tracing a molecular
outflow from the star forming region. The best-fitting ALI model reproduces the
continuum fluxes and line profiles, but slightly underpredicts the emission and
absorption depth in the ground-state ortho line . The derived
ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core
similar to recent findings for translucent clouds in sight lines toward W31C
and W49N. We find evidence of two gas components moving inwards toward the
central region with constant velocities: 2.7 and 5.3 kms, relative
to the source systemic velocity. The inferred mass accretion rates derived are
sufficient to overcome the expected radiation pressure from G34.26+0.15.Comment: 20 pages, 18 figures, accepted by A&A 3 October 201
A high HDO/HO ratio in the Class I protostar L1551 IRS5
Water is a very abundant molecule in star-forming regions. Its deuterium
fractionation is an important tool for understanding its formation and
evolution during the star and planet formation processes. While the HDO/HO
ratio has been determined toward several Class 0 protostars and comets, the
number of studies toward Class I protostars is limited. We aim to study the
water deuteration toward the Class I binary protostar L1551 IRS5 and to
investigate the effect of evolutionary stage and environment on variations in
the water D/H ratio. Observations were made using the NOEMA interferometer. The
HDO 3-2 transition at 225.9 GHz and the HO
3-2 transition at 203.4 GHz were covered with a spatial
resolution of 0.5'' 0.8'', while the HDO 4-4
transition at 143.7 GHz was observed with a resolution of 2.0'' 2.5''.
We used both LTE and non-LTE models. The three transitions are detected. The
line profiles display two peaks, one at 6 km s and one at 9
km s. We derive an HDO/HO ratio of (2.1 0.8)
10 for the redshifted component and a lower limit of 0.3
10 for the blueshifted component due to the blending with the redshifted
CHOCH emission. The HDO/HO in L1551 IRS5 is similar to the ratios
in isolated Class 0 sources and to the Class I V883 Ori, while it is
significantly higher than in the clustered Class 0 sources and the comets. This
suggests that the chemistry of protostars in low source densities clouds share
more similarities with the isolated sources than the protostars of very dense
clusters. If Class 0 protostars with few sources around and isolated Class 0
objects are comparable in the HDO/HO ratio, it would mean that there is
little water reprocessing from the Class 0 to Class I protostellar stage.Comment: Accepted in A&A, 10 pages, 8 figure
The ALMA-PILS survey: First tentative detection of 3-hydroxypropenal (HOCHCHCHO) in the interstellar medium and chemical modeling of the CHO isomers
Characterizing the molecular composition of solar-type protostars is useful
for improving our understanding of the physico-chemical conditions under which
the Sun and its planets formed. In this work, we analyzed the Atacama Large
Millimeter/submillimeter Array (ALMA) data of the Protostellar Interferometric
Line Survey (PILS), an unbiased spectral survey of the solar-type protostar
IRAS~16293--2422, and we tentatively detected 3-hydroxypropenal (HOCHCHCHO) for
the first time in the interstellar medium towards source B. Based on the
observed line intensities and assuming local thermodynamic equilibrium, its
column density is constrained to be 10 cm, corresponding to
an abundance of 10 relative to methanol, CHOH. Additional
spectroscopic studies are needed to constrain the excitation temperature of
this molecule. We included HOCHCHCHO and five of its isomers in the chemical
network presented in Manigand et al. (2021) and we predicted their chemical
evolution with the Nautilus code. The model reproduces the abundance of
HOCHCHCHO within the uncertainties. This species is mainly formed through the
grain surface reaction CHCHO + HCO HCOCHCHO, followed by
the tautomerization of HCOCHCHO into HOCHCHCHO. Two isomers, CHCOCHO
and CHCOHCHO, are predicted to be even more abundant than HOCHCHCHO.
Spectroscopic studies of these molecules are essential in searching for them in
IRAS~16293--2422 and other astrophysical sources.Comment: Accepted in A&A Letter
Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B
Aims. The aim of this paper is to study deuterated water in the solar-type
protostars NGC1333 IRAS4A and IRAS4B, to compare their HDO abundance
distribution with other star-forming regions, and to constrain their HDO/H2O
ratios. Methods. Using the Herschel/HIFI instrument as well as ground-based
telescopes, we observed several HDO lines covering a large excitation range
(Eup/k=22-168 K) towards these protostars and an outflow position. Non-LTE
radiative transfer codes were then used to determine the HDO abundance profiles
in these sources. Results. The HDO fundamental line profiles show a very broad
component, tracing the molecular outflows, in addition to a narrower emission
component and a narrow absorbing component. In the protostellar envelope of
NGC1333 IRAS4A, the HDO inner (T>100 K) and outer (T<100 K) abundances with
respect to H2 are estimated at 7.5x10^{-9} and 1.2x10^{-11}, respectively,
whereas, in NGC1333 IRAS4B, they are 1.0x10^{-8} and 1.2x10^{-10},
respectively. Similarly to the low-mass protostar IRAS16293-2422, an absorbing
outer layer with an enhanced abundance of deuterated water is required to
reproduce the absorbing components seen in the fundamental lines at 465 and 894
GHz in both sources. This water-rich layer is probably extended enough to
encompass the two sources as well as parts of the outflows. In the outflows
emanating from NGC1333 IRAS4A, the HDO column density is estimated at about
(2-4)x10^{13} cm^{-2}, leading to an abundance of about (0.7-1.9)x10^{-9}. An
HDO/H2O ratio between 7x10^{-4} and 9x10^{-2} is derived in the outflows. In
the warm inner regions of these two sources, we estimate the HDO/H2O ratios at
about 1x10^{-4}-4x10^{-3}. This ratio seems higher (a few %) in the cold
envelope of IRAS4A, whose possible origin is discussed in relation to formation
processes of HDO and H2O.Comment: 16 pages, 13 figure
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