Neste trabalho investigamos os efeitos de choques (induzidos por supernovas) e de turbulência magneto-hidrodinâmica no processo de formação estelar. Primeiramente, considerando o impacto de um remanescente de supernova (RSN) com uma nuvem neutra magnetizada derivamos analiticamente um conjunto de condições através das quais estas interações podem levar à formação de estruturas densas capazes de tornarem-se gravitacionalmente instáveis e formar estrelas. Usando estas condições, construímos diagramas do raio do RSN, $R_$, versus a densidade inicial da nuvem, $n_c$, os quais delimitam um domínio no espaço paramétrico onde a formação estelar é permitida. Estes diagramas foram testados através de simulações numéricas magneto-hidrodinâmicas tridimensionais (3D MHD) onde seguimos a evolução espaço-temporal da interação de um RSN com uma nuvem auto-gravitante. Verificamos que a análise numérica está de acordo com os resultados previstos pelos diagramas. Observamos ainda que a presença de um campo magnético fraco, $\\sim 1 \\; \\mu$G, inicialmente homogêneo e perpendicular à velocidade de impacto do RSN, resulta em uma pequena diminuição da região permitida para formação estelar nos diagramas quando comparado a diagramas para nuvens não magnetizadas. Já um campo magnético mais intenso ($\\sim 10\\;\\mu$G) causa um encolhimento significativo nestas, como esperado. Embora derivados de considerações analíticas simples estes diagramas fornecem uma ferramenta útil para identificar locais onde a formação estelar pode ter sido induzida pelo impacto de uma onda de choque de SN. Aplicações a algumas regiões de nossa Galáxia (como a Grande Concha de CO na direção de Escorpião e a Nuvem Periférica 2 na direção da constelação de Cassiopeia) mostram que a formação estelar nestes locais pode ter sido induzida por uma onda de choque de um RSN em passado recente, quando se consideram valores específicos para as condições iniciais das nuvens impactadas.%, para valores específicos de raio do RSN e uma faixa de densidades iniciais possíveis para estas nuvens. Avaliamos também a eficiência de formação estelar efetiva para estas interações e encontramos que esta é geralmente menor do que os valores observados para a nossa Galáxia (sfe $\\sim$ 0.01$-$0.3). Este resultado é consistente com outros trabalhos da literatura e também sugere que este mecanismo, embora poderoso para induzir a formação de estruturas, turbulência supersônica e eventualmente formação estelar local, não parece ser suficiente para induzir a formação estelar global em galáxia normais, nem mesmo quando o campo magnético é desprezado. Além do estudo acima, exploramos ainda a formação estelar considerando a injeção prévia de turbulência (por um mecanismo físico arbitrário) em nuvens magnetizadas. Para uma nuvem ou glóbulo de nuvem molecular formar estrelas deve haver transporte de fluxo magnético das regiões internas mais densas para as regiões externas menos densas da nuvem, de outra forma o colapso poderá ser impedido pela força magnética. Consideramos aqui um novo mecanismo. Reconexão magnética rápida, a qual ocorre em presença de turbulência, pode induzir um processo de difusão eficiente dos campos magnéticos. Neste trabalho investigamos esse processo por meio de simulações numéricas 3D MHD e suas implicações para a formação estelar, estendendo um estudo prévio realizado para nuvens de simetria cilíndrica e sem auto-gravidade (Santos-Lima et al. 2010). Aqui consideramos nuvens mais realistas com potenciais gravitacionais esféricos (devido a estrelas embebidas) e também levando em conta os efeitos da auto-gravidade do gás. Determinamos, pela primeira vez, quais as condições em que o transporte do campo magnético devido à difusão por reconexão turbulenta leva uma nuvem inicialmente subcrítica a tornar-se super-crítica e capaz de colapsar para formar estrelas. Nossos resultados indicam que a formação de um núcleo supercrítico é resultado de uma complexa interação entre gravidade, auto-gravidade, intensidade do campo magnético e turbulência aproximadamente trans-sônica e trans-Alfvénica. Em particular, a auto-gravidade favorece a difusão do campo magnético por reconexão turbulenta e, como resultado, seu desacoplamento do gás colapsante torna-se mais eficiente do que quando apenas um campo gravitacional externo está presente. Demonstramos que a difusão por reconexão turbulenta é capaz de remover fluxo magnético da maior parte das nuvens investigadas, porém somente uma minoria desenvolve núcleos aproximadamente críticos ou super-críticos, o que é consistente com as observações. A formação destes é restrita ao seguinte intervalo de condições iniciais para as nuvens: razão pressão térmica-pressão magnética, $\\beta \\sim 1$ a $3$, razões entre a energia turbulenta e a energia magnética $E_/E_\\sim 1.62$ a $2.96$, e densidades $50 < n < 140$ cm$^$, quando consideramos massas estelares M$_{\\star}\\sim 25$M$_{\\odot}$, implicando uma massa total da nuvem (gás + estrelas) M$_\\lesssim 120$M$_{\\odot}$. / In this work, we have investigated the effects of shocks (induced by supernovae) and magnetohydrodynamical turbulence in the process of star formation. Considering first, the impact of a supernova remnant (SNR) with a neutral magnetized cloud we derived analytically a set of conditions through which these interactions can lead to the formation of dense structures able to become gravitationally unstable and form stars. Using these conditions, we have built diagrams of the SNR radius, $R_{SNR}$, versus the initial cloud density, $n_c$, that constrain a domain in the parameter space where star formation is allowed. These diagrams have been also tested by means of three-dimensional magneto-hydrodynamical (3D MHD) numerical simulations where the space-time evolution of a SNR interacting with a self-gravitating cloud is followed. We find that the numerical analysis is in agreement with the results predicted by the diagrams. We have also found that the effects of a weak homogeneous magnetic field ($\\sim 1 \\; \\mu$G) approximately perpendicular to the impact velocity of the SNR results only a small decrease of the allowed zone for star formation in the diagrams when compared with the diagrams with non-magnetized clouds. A larger magnetic field ($\\sim 10\\;\\mu$G) on the other hand, causes a significant shrinking of the star formation zone, as one should expect. Although derived from simple analytical considerations, these diagrams provide a useful tool for identifying sites where star formation could be triggered by the impact of a SN blast wave. Applications of them to a few regions of our own Galaxy (e.g., the large CO shell in the direction of Scorpious, and the Edge Cloud 2 in the direction of the Cassiopeia constellation) have revealed that star formation in those sites could have been triggered by shock waves from SNRs in a recent past, when considering specific values of the SNR radius and the initial conditions in the neutral clouds. We have also evaluated the effective star formation efficiency for this sort of interaction and found that it is generally smaller than the observed values in our Galaxy (sfe $\\sim$ 0.01$-$0.3). This result is consistent with previous work in the literature and also suggests that the mechanism presently investigated, though very powerful to drive structure formation, supersonic turbulence and eventually, local star formation, does not seem to be sufficient to drive $global$ star formation in normal star forming galaxies, not even when the magnetic field is neglected. Besides the study above, we have also explored star formation considering a priori injection of turbulence (by an arbitrary physical mechanism) in magnetized clouds. For a molecular cloud clump to form stars some transport of magnetic flux may be required from the denser, inner regions to the outer regions of the cloud, otherwise this can prevent the gravitational collapse. We have considered here a new mechanism. Fast magnetic reconnection which takes place in the presence of turbulence can induce a process of reconnection diffusion of the magnetic field. In this work, we have investigated this process by means of 3D MHD numerical simulations considering its implications on star formation. We have extended a previous study which considered clouds with cylindrical geometry and no self-gravity (Santos-Lima et al. 2010). Here, we considered more realistic clouds with spherical gravitational potentials (from embedded stars) and also accounted for the effects of the gas self-gravity. We demonstrated that reconnection diffusion takes place. We have also, for the first time, determined the conditions under which reconnection diffusion is efficient enough to make an initially subcritical cloud clump to become supercritical and collapse. Our results indicate that the formation of a supercritical core is regulated by a complex interplay between gravity, self-gravity, magnetic field strength and nearly transonic and trans-Alfvénic turbulence. In particular, self-gravity helps reconnection diffusion and, as a result, the magnetic field decoupling from the collapsing gas becomes more efficient than in the case when only an external gravitational field is present. We have demonstrated that reconnection diffusion is able to remove magnetic flux from most of the collapsing clumps analysed, but only a few of them develop nearly critical or supercritical cores, which is consistent with the observations. Their formation is restricted to a range of initial conditions for the clouds as follows: thermal to magnetic pressure ratios $\\beta \\sim$ 1 to 3, turbulent to magnetic energy ratios $E_{turb}/E_{mag}\\sim 1.62$ to $2.96$, and densities $50 < n < 140$ cm$^{-3}$, when considering stellar masses M$_{\\star}\\sim 25$M$_{\\odot}$, implying total (gas+stellar) masses M$_{tot} \\lesssim 120$M$_{\\odot}$.

Superconductor-Insulator-Superconductor (SIS) mixers offer the best noise properties of any heterodyne mixing technique at millimetre wavelengths. In astronomy, they are used for sensitive spectroscopy, which is vital for understanding the properties of the cold interstellar medium, including regions of star formation activity. Modern SIS receivers have noise properties that are &Tilde;3 times the quantum limit, and it is now becoming increasingly difficult to lower the noise properties any further. In this thesis, I investigate two techniques that extend the capability of SIS receivers. The first technique is extending the instantaneous bandwidth of the receivers, i.e., the intermediate frequency bandwidth (IFBW). For spectral line sources, wide IFBW expands the survey depth to allow multiple emission lines to be observed simultaneously. Here, I present a new SIS mixer device at 230 GHz. The planar circuit was minimised to reduce any parasitic capacitances that may limit the IFBW. Experimentally, the device provides excellent noise temperatures down to 36 K and an IFBW extending from approximately 0-11 GHz. Simulation software was developed to better understand the performance of this device, and it suggests that the IFBW can be extended to higher frequencies if the IF measurement chain is upgraded. The second technique that I investigate is increasing the number of receivers in the focal plane of the receiver, i.e., adding more pixels. There are many challenges involved in this task including how to fit multiple receivers into a small space, how to properly cool the receiver, and how to deliver the local-oscillator signal. Here, I present a new 1 × 4 focal plane array. This array is acting as a demonstrator for a new array architecture that can be expanded into many more pixels in the future. It uses cascaded waveguide power splitters to divide the local-oscillator signal, and then waveguide directional couplers to combine the LO with the astronomical signals. Finally, I present CO(J=1&rarr;0) measurements from 34 galaxies in the 5MUSES survey. These measurements trace the amount of cold molecular gas present in these galaxies. By comparing these measurements to other metrics that trace star formation activity (e.g., infrared luminosity), I was able to form empirical relationships between the observed quantities. I also combined these results with other star formation studies from nearby and high redshift galaxies to form scaling relationships spanning a large fraction of cosmic time.

La naissance des étoiles massives est aujourd’hui encore mal comprise. En particulier, les conditions initiales de leur formation restent largement inconnues. Pour éclairer cette question, nous avons réalisé un relevé spectral complet non biaisé avec le télescope 30 m de l’IRAM vers la protoétoile massive CygX-N63 (M ~ 58 M◦ et L~ 340 L◦). Nous avons mis en évidence une complexité moléculaire significative avec plus de 40 espèces. L’ion CF+ est observé pour la première fois dans une protoétoile. Une possible première détection de l’espèce prébiotique CH2NH dans une protoétoile est aussi proposée, ainsi qu’une première détection de DOCO+. Cette étude spectroscopique, accompagnée d’observations interférométriques avec le Plateau de Bure, permet de séparer la contribution des différentes régions : enveloppe froide, région tiède, région de type hot core et flot bipolaire. L’enveloppe est constituée d’une grande quantité de gaz froid peu évolué, offrant un potentiel important pour la compréhension des phases précoces de la formation stellaire massive et compatible avec un scénario d’effondrement monolithique. La modélisation chimique montre que la chimie de ce gaz est encore hors équilibre, malgré sa haute densité, et confirme la jeunesse de la protoétoile avec un âge chimique de seulement ~ 1000 ans. N63 est un précurseur de hot core plutôt qu’un hot corino massif. Il serait donc possible de distinguer, grâce à des diagnostics chimiques évolutifs, les précurseurs d’étoiles massives des protoétoiles de masse faible ou ntermédiaire. / High-mass star formation is still poorly understood. In particular the initial conditions of their formation are unknown. To explore this question, a complete unbiased spectral survey was conducted with the IRAM 30 m telescope toward the massive protostar CygX-N63 (M~58 M◦ and L~ 340 L◦). A significant molecular complexity is found, with more than 40 species. The ion CF+ is observed for the first time in a protostar. A possible first detection of the prebiotic species CH2NH in a protostar and a first detection of DOCO+ are proposed. This spectroscopic study, along with Plateau de Bure interferometric observations, allows us to separate the contribution of different regions : cold envelope, lukewarm region, hot corelike region and outflow. The envelope contains large amounts of cold and young gas, which gives us the opportunity to better understand the early phases of massive star formation. The chemical modeling shows that the chemistry is still out of equilibrium, despite its high density, and confirms the youth of the protostar with a chemical age of ~ 1000 years. N63 is a hot core precursor rather than a massive hot corino. The use of chemical diagnostics of the evolution would then allow to distinguish massive star precursors from low-mass or intermediate-mass protostars.

With the Hubble Space Telescope and its near-infrared capabilities, it is now possible to probe deep into the epoch of reionization, improving our understanding of galaxy evolution through cosmic history. Whether it is via colour-selection or fitting the spectral energy distribution, it has now become routine to amass large samples of galaxies as distant as redshift z = 8, with the current frontier of observations at z = 9 - 10. The new Hubble Frontier Fields (HFF) programme provides the potential to study the most distant, intrinsically faint background galaxies through the gravitational lensing provided by a foreground galaxy cluster. This thesis presents a study of the galaxy population at z = 9 - 10 that exploits this phenomenon. In an initial search of the first two HFF cluster+parallel pointings, Abell 2744 and MACS J0416.1-240, we unveil twelve candidate high-redshift galaxies at 8:4 < z < 9:5, and are thus able to place constraints on the galaxy UV luminosity function at z = 9. For this study, we employ the "blank-field" method, whereby we confine attention to only the homogeneously deep, relatively low-magnification regions of the imaging. We are able to demonstrate evidence for a smooth decline in UV luminosity density between z ≃ 8 and z ≃ 9, in contrast to reports in the recent literature of a steep drop-off at these redshifts. We extend this study to include the new MACS J0717.5+3745 and MACS J1149+2223 cluster+parallel pointings, and supplement the search for z ≃ 9-10 galaxies with twenty CLASH cluster pointings. From a search over an area ≃ 130 sq. arcmin, we are able to present 33 galaxy candidates with photometric redshift solutions in the range 8:4 < zphot < 11:2. Our new results reinforce the argument for a smoothly-evolving LF between z ≃ 8 and z ≃ 9, which can be equally well modelled by a factor ≃ 2 drop in Φ* or a dimming of ≃ 0:5 mag in M*. We also find evidence that this smooth decline in the UV luminosity function, and hence UV luminosity density, continues to z ≃ 10. As well as considering the galaxy population at z = 9 - 10, this thesis presents a study of the stellar populations of galaxies at z ≥ 5. We are able to extend the luminosity baseline and measure the colour-magnitude relation at z = 5 - 8, through a combination of probing intrinsically faint galaxies behind cluster fields, in conjunction with both ultra-deep, pencil beam imaging such as the Hubble Ultra Deep Field (HUDF) and wider, shallower imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This allows us to make inferences about the underlying stellar populations of galaxies at these epochs, and provides a unique insight into the colours of intrinsically faint, lensed galaxies as faint as M1500 ~ -14. We find that the data is consistent with an essentially unchanged average UV slope (β) for a given luminosity across the redshift range z = 5-8. We also find that the data favours a mild flattening of the colour-magnitude relation with redshift between z = 5 and z = 8.

Previous studies have shown that thermal feedback from protostars plays a key role in the process of low-mass star formation. In this thesis, we explore the effects of protostellar heating on the formation of stellar clusters. We describe new methods for modelling protostellar accretion luminosities and protostellar evolution in calculations of star formation. We then present results of a series of numerical simulations of stellar cluster formation which include these effects, and examine their impact. We begin by investigating the dependence of stellar properties on the initial density of molecular clouds. We find that the dependence of the median stellar mass on the initial density of the cloud is weaker than the dependence of the thermal Jeans mass when radiative effects are included. We suggest that including protostellar accretion luminosities and protostellar evolution may weaken this dependence further, and may account for the observed invariance of the median stellar mass in Galactic star-forming regions. Next, we investigate the effects of including accretion feedback from sink particles on the formation of small stellar groups. We find that including accretion feedback in calculations suppresses fragmentation even further than calculations that only include radiative transfer within the gas. Including feedback also produces a higher median stellar mass, which is insensitive to the sink particle accretion radius used. Finally, we compare calculations of small stellar clusters which model the evolution of protostars using a live stellar model with those which use a fixed stellar structure. We find that the dynamics of the clusters are primarily determined by the accretion luminosities of protostars, but that the relative effects of protostellar evolution depend on the accretion rate and advection of energy into the protostar. We also demonstrate how such calculations may be used to study the properties of young stellar populations.

The work in this thesis concerns the host galaxies of a class of luminous, yet heavily-obscured, quasars at z$\sim$2 - a peak epoch of both star formation and black hole accretion. Here, we seek to characterise the star-forming properties of these obscured quasars to improve our understanding of galaxy-quasar coevolution. A key issue facing host galaxy studies among populations of the most luminous quasars is being able to disentangle the galaxy emission from that of the quasar. With combined observations from the Dark Energy Survey (DES), the VISTA Hemisphere Survey (VHS) and the UKIDSS Large Area Survey (ULAS), we exploit the quasar dust extinction in our sample to demonstrate that the quasar and galaxy emission can be separated via SED-fitting in these systems. By isolating the galaxy emission in this way, we estimate instantaneous SFRs for the galaxies in our sample, based on the restframe UV emission. In general, we find obscured quasars to reside in prodigiously star forming hosts with 25 $\lesssim$ SFR$_{\rm{UV}}$ $\lesssim$ 365 M$_{\rm{\odot}}$yr$^{-1}$. Furthermore, we show that the most luminous quasars reside in the most actively star-forming galaxies, potentially indicating the same gas supply is fuelling both star formation and accretion on to the black hole. Having isolated the galaxy emission via SED-fitting, we test our ability to model the restframe-UV emission of obscured z$\sim$2 quasar hosts in 2D. Until now, morphological studies of luminous quasar hosts have typically been limited to low redshifts or relied on space-based imaging. By making use of a multi-band modelling code however, we demonstrate that it is possible to accurately infer several galaxy properties (i.e. the position of the galaxy in the image, (X,Y), its radius, R$_{\rm{eff}}$, axis ratio, q$_{\rm{GAL}}$, angle of orientation, $\theta$, and S\'rsic index, $n$), based on current ground-based imaging, accross the full range of galaxy and quasar luminosities considered in our sample. This potentially opens the door to future ground-based morphological studies of obscured quasars at high redshift. At sub-mm wavelengths, thermal emission from cold dust peaks, meaning these wavelengths can be used to probe the dust heating by star formation, effectively giving a measure of the obscured star formation in the galaxy. Using targeted observations from SCUBA-2, we trace the 850$\mu$m emission in a sample of obscured quasars, finding evidence for prodigious star formation $ > $ 2400 M$_$yr$^$ in three of the 19 quasars in our sample. The detection rate of our obscured quasar sample is found to be consistent with that of both more heavily-obscured Hot-DOGs and UV-luminous quasars, once the samples have been matched in luminosity and redshift. Furthermore, we find evidence that several of the obscured quasars lie in overdense regions of the sky ($\sim$ 3 times denser than sub-mm blank fields).

This thesis presents my work characterising star formation in Gould Belt molecular clouds using submillimetre observations from SCUBA-2 on the James Clerk Maxwell Telescope (JCMT). I use these observations alongside data from previously published surveys using instruments including the Spitzer Space Telescope. I investigate the effect of including submillimetre data on the numbers, classifications and lifetimes of Young Stellar Objects (YSOs) in Gould Belt molecular clouds, particularly protostars. Following a literature review, I use SCUBA-2 450 and 850 μm observations to characterise star formation in the Lupus I molecular cloud. A total of eleven previously identified YSOs are detected with SCUBA-2, as well as eleven starless cores. Two cores have masses greater than the Jeans mass, and one has a virial parameter of 1.1 0.4, meaning these cores could be unstable against collapse. I use submillimetre emission to calculate disk masses, and find that one YSO has a disk mass greater than the minimum mass solar nebula. I find that Lupus I has a high percentage of both protostars and Very Low Luminosity Objects (VeLLOs). I also fit YSO Spectral Energy Distributions (SEDs) with models, allowing protostellar envelope masses and temperatures to be calculated, and interstellar extinction to be constrained for some YSOs. The signs of recent and future star formation support the hypothesis that a shock has triggered a star forming event in Lupus I. I also use SCUBA-2 data in conjunction with archival Spitzer and Herschel data to produce SEDs for five new candidate First Hydrostatic Cores (FHSCs) in Serpens South. These observations were then fit with models by the first author of this work, Alison Young. This work was able to identify two of the FHSC candidates as probable FHSCs, and constrain the rotation rate and inclination of one of them. I use JCMT Gould Belt Survey (GBS) observations of ten molecular clouds to produce an updated catalogue of protostars in these clouds. I use the FellWalker algorithm to find individual sources in the SCUBA-2 maps, and match them to the Spitzer YSO catalogue of Dunham et al. (2015). I use bolometric temperature to classify 362 out of 592 candidates as Class 0 or Class I protostars - a factor of two increase compared to the Spitzer catalogue due to improved submillimetre coverage. I find that protostellar lifetimes of 0.59 – 0.89 Myr - approximately 25 % longer than previously estimated. I also calculate protostellar luminosities, envelope masses, and envelope temperatures, and examine the distributions. Finally, I newly identify 19 protostars as VeLLOs, and increase the number of known VeLLOs in these clouds by a factor of two.

Les étoiles de masse intermédiaire (2M⊙ ≤ M ≤ 10M⊙) sont parmi les sources dominantes du champ interstellaire FUV dans la Galaxie. Elles régulent les phases du milieu interstellaire et l’ensemble des processus de formation stellaire galactique. Alors que les protoétoiles de type solaire et massives ont été et continuent à faire l’objet de nombreuses études, la formation des étoiles intermédiaires a été relativement peu étudiée. Leur structure physique, composition chimique et leur richesse moléculaire sont un domaine à explorer.L’objectif de ma thèse est d’obtenir un recensement détaillé et aussi complète que possible des propriétés physico-chimiques d’une protoétoile isolée de masse intermédiaire. Notre choix s’est porté sur Cep E-mm (100 L⊙).J’ai pour cela complété un relevé spectral de l’émission moléculaire dans les bandes (sub)millimétriques entre 72 et 350 GHz avec le télescope de 30m de l’IRAM. La sensibilité des observations a permis d’identifier la présence de nombreuses molécules complexes organiques (COMs) dans l’enveloppe de la protoétoile, mais aussi, plusieurs espèces moléculaires inhabituelles dans le jet généré par la protoétoile. Des observations complémentaires avec le télescope de 30m ont permis de cartographier l’émission moléculaire à grande échelle (20’’ à 11’’ ; 15000 à 8000 UA). En parallèle, des cartes interférométriques de l’émission moléculaire entre 86 – 90 GHz et 216 – 220 GHz ont été obtenues avec l’interféromètre de l’IRAM (NOEMA) à 1.4’’ (1000 UA) de résolution angulaire. Ces observations m’ont permis d’obtenir une première description de la distribution de l’émission moléculaire au sein de l’enveloppe, des grandes échelles, dans les parties extérieures de l’enveloppe étendue, aux petites échelles dans la région d’un hot corino. Les études présentées ici ont suivi un travail méticuleux de réduction et d’analyse des données, single-dish et interférométriques. Plus précisément, j’ai identifié et séparé les contributions à l’émission détectée dans le lobe du télescope de 30m de l’IRAM des différentes régions physiques du cœur protostellaire. De ce fait, j’ai identifié et caractérisé quatre composantes physiques qui diffèrent par leurs propriétés spectroscopiques et leurs conditions d’excitation : l’enveloppe étendue, le hot corino, le flot bipolaire basse vitesse et le jet à haute vitesse. Enfin, l’anisotropie de la distribution de brillance du flot et du jet bipolaire ne peut pas être modélisée par l’approche ‘’classique’’ d’une source gaussienne. J’ai développé des outils spécifiques semi-analytiques pour calculer de manière approchée, et plus raisonnable, le couplage entre le lobe du télescope et la source. / Intermediate-mass stars (2 M⊙ ≤ M ≤ 10 M⊙) are among the dominant sources of FUV interstellar field in the Galaxy. They regulate the phases of interstellar medium and the whole process of galactic star formation. While solar-type and massive protostars have been and continue to be the subject of many studies, the formation of intermediate stars has been relatively little studied. Their physical structure, chemical composition and molecular richness are still a subject to explore.The aim of my thesis is to obtain a detailed census, as complete as possible ,of the physical and chemical structure of an isolated intermediate-mass protostar: Cep E-mm (100 L⊙).I have completed a spectral survey of the molecular emission in the (sub)millimetre bands between 72 and 350 GHz with IRAM 30m telescope. The sensitivity of the observations made it possible to identify the presence of numerous complex organic molecules (COMs) in the protostar envelope, but also several unusual molecular species in the protostellar jet. Additionally, further observations with the IRAM 30m telescope made it possible to map the molecular emission at large scale (20’’ to 11’’; 15000 to 8000 AU). In parallel, interferometric maps of the molecular emission between 86 – 90 GHz and 216 – 220 GHz were obtained with NOEMA, the IRAM interferometer, at 1.4’’ (1000 AU) of angular resolution. These observations allowed me to obtain the distribution of molecular emission within the source, from large scales in the outer parts of the extended envelope, to the small scales in the hot corino region. The single-dish and interferometric observations were reduced and analysed in a meticulous manner. More precisely, I identified and separated the molecular emission contribution from the different physical regions as observed with the IRAM 30m telescope. I have identified and characterized fours physical components that differ in their spectroscopic properties and excitation conditions: the extended envelope, the hot corino, the bipolar outflow and the high-velocity jet. Finally, the anisotropy of the brightness distribution from the outflow system cannot be modelled by the “classical” Gaussian approach. I have developed specific tools to estimate, in a semi-analytical manner, the coupling between the telescope lobe and the source.

La formation des étoiles massives reste, à ce jour, encore mal connue à cause de l'extrême quantité d'énergie que ces étoiles dégagent, limitant en conséquence leurs masses théoriques et contredisant les observations de ce type d'étoile. Les observatoires du futur (en particulier l'observatoire spatial Herschel) vont tenter de répondre à cette problématique grâce notamment aux émissions moléculaires de l'eau. L'analyse précise et correcte de ces données, dans l'avenir, nécessite donc dès aujourd'hui un travail associant des observations et des modélisations des objets concernés. C'est dans ce but que cette thèse a consisté en l'élaboration d'une méthode de modélisation dite « globale » d'objets proto-stellaires massifs (proto-amas ou cœurs denses massifs). Celle-ci a permis une description physique et une étude chimique des multiples cœurs denses massifs étudiées, et a ouvert de nombreuses voies vers des aspects évolutifs. Elle a également donné des indices pour affiner le programme d'observation en temps garanti WISH des raies moléculaires de l'eau et confirmé le rôle clef de cette molécule pour la compréhension de la formation des étoiles massives.

<p>Optical lucky imaging polarimetry of HL Tau and XZ Tau in the Taurus-Auriga molecular cloud was carried out with the instrument PolCor at the Nordic Optical Telescope (NOT). The results show that in both the V- and R-band HL Tau show centrosymmetric structures of the polarization angle in its northeastern outflow lobe (degree of polarization ~30%). A C-shaped structure is detected which is also present at near-IR wavelengths (Murakawa, 2008), and higher resolution optical images (Stapelfeldt, 1995). The position angle of the outflow is 47.5+-7.5 degrees, which coincides with previous measurements and the core polarization is observed to decrease with wavelength and a few scenarios are reviewed. Measuring the outflow witdh versus distance and wavelength shows that the longer wavelengths scatter deeper within the cavity wall of the outflow. In XZ Tau the binary is partially resolved, it is indicated by an elongated intensity distribution. The polarization of the parental cloud is detected in XZ Tau through the dichroic extinction of starlight. Lucky imaging at the NOT is a great way of increasing the resolution, shifting increases the sharpness by 0.1 asec and selection the sharpest frames can increase the seeing with 0.4 asec, perhaps more during better conditions.</p>