Herbig Ae/Be (HAeBe) stars are intermediate mass (2-10 solar mass) pre-main sequence stars with circumstellar disks. Observing planets within these young disks would greatly aid in understanding planet formation processes and timescales particularly around massive stars. So far, only one planet, HD 100546 b, has been confirmed to orbit a HAeBe star. With over 250 HAeBe stars known, and several observed to have disks with structures thought to be related to planet formation, it seems likely that there are as yet undiscovered planetary companions within the circumstellar disks of some of these young stars. Direct detection of a low-luminosity companion near a star requires high contrast imaging, often with the use of a coronagraph, and the subtraction of the central star's point spread function (PSF). Several processing algorithms have been developed in recent years to improve PSF subtraction and enhance the signal-to-noise of sources close to the star. However, many HAeBe stars were observed via direct imaging before these algorithms came out. We used the PSF subtraction program PynPoint to reprocess archival images of HAeBe stars from the Advanced Camera for Surveys on the Hubble Space Telescope to increase the likelihood of detecting a planet in their disks. We believe we have recovered the known planet around HD 100546 and possibly its candidate second companion. We also detect new candidate sources in the vicinities of HD 141569 and HD 163296. Further observations are needed to confirm the reality of these detections and also establish their association with the host stars.

La performance accrue des instruments dans une vaste gamme de longueur d'onde a montré qu'une grande partie des étoiles jeunes étaient entourées de matière sans doute répartie dans un disque. La détection de tels systèmes par imagerie directe est difficile du fait de la faible taille angulaire de ces objets, ainsi que de leur faible luminosité en comparaison avec l'étoile. Il semble aujourd'hui que près de 50 \% d'étoiles pré-séquence principale de faible masse, dites étoiles T Tauri, soient entourées d'un disque. Cette proportion est beaucoup moins évidente pour les étoiles plus massives, dites Herbig Ae, du fait de leur faible nombre et du faible nombre de détections directes.<br />Jusqu'à présent, seule l'interprétation de la distribution spectrale d'énergie (SED) de ces objets permettait d'inférer la structure géométrique des disques autour des étoiles Herbig Ae. Les modèles sont fortement dégénerés et il est primordial d'obtenir des images, seules preuves directes de la présence ou non de disque. <br />Cette thèse a permis de mettre en évidence l'apport de l'imagerie en infrarouge moyen pour lever une partie de la dégénérescence liée à l'interprétation de la SED et mieux contraindre, pour plusieurs objets, les paramètres des disques, comme le rayon externe minimal ou l'inclinaison. De plus, nous présentons un nouveau mode d'observation avec VISIR, l'instrument pour les observations en infrarouge moyen au VLT (ESO, Chili): le mode BURST. Ce mode permet d'atteindre la limite de résolution spatiale déterminée par la diffraction. Grâce à l'imagerie en infrarouge faite avec cet instrument, cette thèse a permis de déterminer, pour la première fois, la géométrie (structure évasée) d'un disque autour d'une étoile massive inférée au préalable par l'interprétation des SEDs.

Herbig Ae/Be (HAeBe) stars are the pre-main sequence progenitors of main sequence A and B stars, and are characterized observationally by strong emission in spectral lines and significant infra-red excess that results from their presence in dust-obscured regions. These stars are usually surrounded by a complex environment composed of gas and dust and often a significant stellar wind and a circumstellar disc. This complex circumstellar environment can have a significant affect on their spectral energy distributions, leading to large systematic uncertainties in determinations of their effective temperatures from photometric methods. In an attempt to improve temperature determinations for HAeBe stars, we have conducted an experiment to evaluate the potential of high-resolution spectra to constrain their atmospheric parameters. To this end, high-resolution (R~68 000) and low-resolution (R~1500) spectra obtained using the ESPaDOnS spectropolarimeter (at the Canada-France-Hawaii telescope) and the FORS1 spectropolarimeter (at the Very Large Telescope) have been used with an automatic spectrum fitting procedure. This procedure compares spectroscopic data to a grid of synthetic LTE, solar abundance spectra, spanning a range in effective temperature, surface gravity, and micro-turbulence. This analysis was applied to the spectra of a sample of twelve previously well-studied HAeBe stars. Our temperatures were found to be consistent with previously published values, while providing much lower uncertainties - in some cases about 5 times smaller. Numerous methods were investigated to obtain these quantitative uncertainties (chi-squared statistics, Bayesian analysis, Monte Carlo bootstrap method, individual temperature sensitive line region analysis). We conclude that our method can be used to efficiently and effectively obtain temperatures of HAeBe stars in addition to providing us with a characterization of the degree of departure of the spectrum from solar abundance, LTE photospheric models. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-02-13 10:23:04.9

The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought to be due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares, or traveling shock waves, and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T. Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. The isotropic (Kolmogorov power spectrum) turbulent component is synthesized along the individual particle trajectories. We have investigated the energy range [0.1-10] GeV, consistent with expectations from Chandra X-ray observations of large flares on T. Tauri stars and recent indications by the Herschel Space Observatory of a significant contribution of energetic particles to the disk ionization of young stars. In contrast with a previous theoretical study finding a dominance of energetic particles over X-rays in the ionization throughout the disk, we find that the disk ionization is likely dominated by X-rays over much of its area, except within narrow regions where particles are channeled onto the disk by the strongly tangled and turbulent magnetic field. The radial thickness of such regions is 5 stellar radii close to the star and broadens with increasing radial distance. This likely continues out to large distances from the star (10 au or greater), where particles can be copiously advected and diffused by the turbulent wind.

Aims. The aim of this work is to constrain properties of the disk around the archetype FU Orionis object, FU Ori, with as good as similar to 25 au resolution. Methods. We resolved FU Ori at 29-37 GHz using the Karl G. Jansky Very Large Array (JVLA) in the A-array configuration, which provided the highest possible angular resolution to date at this frequency band (similar to 0 ''.07). We also performed complementary JVLA 8-10 GHz observations, Submillimeter Array (SMA) 224 GHz and 272 GHz observations, and compared these with archival Atacama Large Millimeter Array (ALMA) 346 GHz observations to obtain the spectral energy distributions (SEDs). Results. Our 8-10 GHz observations do not find evidence for the presence of thermal radio jets, and constrain the radio jet/wind flux to at least 90 times lower than the expected value from the previously reported bolometric luminosity-radio luminosity correlation. The emission at frequencies higher than 29 GHz may be dominated by the two spatially unresolved sources, which are located immediately around FU Ori and its companion FU Ori S, respectively. Their deconvolved radii at 33 GHz are only a few au, which is two orders of magnitude smaller in linear scale than the gaseous disk revealed by the previous Subaru-HiCIAO 1.6 mu m coronagraphic polarization imaging observations. We are struck by the fact that these two spatially compact sources contribute to over 50% of the observed fluxes at 224 GHz, 272 GHz, and 346 GHz. The 8-346 GHz SEDs of FU Ori and FU Ori S cannot be fit by constant spectral indices (over frequency), although we cannot rule out that it is due to the time variability of their (sub)millimeter fluxes. Conclusions. The more sophisticated models for SEDs considering the details of the observed spectral indices in the millimeter bands suggest that the >29 GHz emission is contributed by a combination of free-free emission from ionized gas and thermal emission from optically thick and optically thin dust components. We hypothesize that dust in the innermost parts of the disks (less than or similar to 0.1 au) has been sublimated, and thus the disks are no longer well shielded against the ionizing photons. The estimated overall gas and dust mass based on SED modeling, can be as high as a fraction of a solar mass, which is adequate for developing disk gravitational instability. Our present explanation for the observational data is that the massive inflow of gas and dust due to disk gravitational instability or interaction with a companion/intruder, was piled up at the few-au scale due to the development of a deadzone with negligible ionization. The piled up material subsequently triggered the thermal instability and the magnetorotational instability when the ionization fraction in the inner sub-au scale region exceeded a threshold value, leading to the high protostellar accretion rate.

We present proper motion measurements of 37 jets and HH objects in the Carina Nebula measured in two epochs of H alpha images obtained similar to 10 yr apart with Hubble Space Telescope/Advanced Camera for Surveys (ACS). Transverse velocities in all but one jet are faster than greater than or similar to 25 km s(-1), confirming that the jet-like H alpha features identified in the first epoch images trace outflowing gas. Proper motions constrain the location of the jet-driving source and provide kinematic confirmation of the intermediate-mass protostars that we identify for 20/37 jets. Jet velocities do not correlate with the estimated protostar mass and embedded driving sources do not have slower jets. Instead, transverse velocities (median similar to 75 km s(-1)) are similar to those in jets from low-mass stars. Assuming a constant velocity since launch, we compute jet dynamical ages (median similar to 10(4) yr). If continuous emission from inner jets traces the duration of the most recent accretion bursts, then these episodes are sustained longer (median similar to 700 yr) than the typical decay time of an FU Orionis outburst. These jets can carry appreciable momentum that may be injected into the surrounding environment. The resulting outflow force, dp/dt, lies between that measured in low- and high-mass sources, despite the very different observational tracers used. Smooth scaling of the outflow force argues for a common physical process underlying outflows from protostars of all masses. This latest kinematic result adds to a growing body of evidence that intermediate-mass star formation proceeds like a scaled-up version of the formation of low-mass stars.

L'étude de l'hydrogène moléculaire est fondamentale pour une meilleure compréhension des mécanismes de formation stellaire et planétaire. En effet, le H2 est la molécule la plus abondante dans l'environnement circumstellaire des étoiles jeunes, et permet<br />donc d'estimer les quantités totales de gaz disponibles à chaque étape de l'évolution de la protoétoile vers la séquence principale. Dans ce contexte, j'ai mené une étude sur la composante gazeuse, et en particulier sur l'hydrogène moléculaire, dans l'environnement circumstellaire d'un échantillon d'étoiles pré-séquence principale, dites étoiles Ae/Be de Herbig, à différents stades de leur évolution vers la séquence principale.<br /><br />A partir des spectres observés par le satellite FUSE dans<br />l'ultraviolet lointain, j'ai mis en évidence plusieurs mécanismes d'excitation de l'hydrogène moléculaire qui sont clairement corrélés à la structure de l'environnement circumstellaire. En particulier, pour les étoiles de type Be, qui sont les plus jeunes de l'échantillon, les diagrammes d'excitation de l'hydrogène moléculaire circumstellaire peuvent être assez bien reproduits par un modèle de région de photodissociation. Mon analyse montre que ces étoiles sont entourées d'une enveloppe circumstellaire, reste<br />du nuage dans lequel elles se sont formées. Les étoiles de type Ae/B9 de l'échantillon, connues pour être entourées de disques, forment un groupe plus hétérogène. Pour la plupart de ces étoiles, du fait des angles d'inclinaison, le gaz présent dans les disques est très rarement observé car la ligne de visée ne traverse pas les disques. Lorsque du H2 d'origine circumstellaire<br />est observé, j'ai mis en évidence la présence d'un milieu chaud très proche de l'étoile, excité par collisions. En utilisant un modèle de disque ouvert et en supposant que la poussière et le gaz sont couplés, j'ai montré que le gaz chaud que l'on observe ne se situe pas dans le disque, mais peut avoir plusieurs origines. Le gaz chaud peut provenir d'une région chaude de type chromosphère étendue ou de la photoévaporation du disque.<br /><br />Ces différences de structure dans l'environnement circumstellaire des étoiles de Herbig Ae et Be reflètent la différence d'évolution de ces deux groupes d'étoiles. En effet, cette structuration différente du milieu circumstellaire peut être expliquée par une évolution plus rapide des étoiles de Herbig de type Be qui sont associées à de plus forts champs de rayonnement. <br /><br />Ces résultats représentent des contraintes fortes sur les<br />conditions physiques dans lesquelles se trouve le gaz circumstellaire, qui, une fois complétées par de nouvelles observations, permettront d'avoir une compréhension globale de la structure et de l'évolution de l'environnement circumstellaire des étoiles jeunes.

L'évolution du moment cinétique des étoiles de masse intermédiaire (1.5-15 Msun) et l'importance du champ magnétique dans cette évolution, surtout dans la phase pré-séquence principale (PMS), sont des aspects encore très mal connus de l'évolution stellaire. Mes travaux se situent dans ce cadre, et leur objectif est d'une part de fournir des contraintes observationnelles, et d'autre part d'effectuer les premiers pas vers une modélisation complète de l'évolution pré-séquence principale à ces masses-là, tenant compte de la rotation et du champ magnétique.<br /><br />Dans un premier temps, j'ai utilisé le système binaire à éclipses PMS RS Cha pour tester la physique contenue dans les modèles d'évolution des étoiles PMS. Pour cela j'ai tout d'abord complété notre connaissance des paramètres de ce système en redéterminant la masse des 2 composantes et en mesurant précisément leur métallicité, à l'aide de données spectroscopiques. La comparaison des paramètres fondamentaux des composantes PMS de RS Cha à ceux calculés m'a permis de valider les modèles actuels d'évolution PMS, à condition d'utiliser la nouvelle composition chimique solaire d'Asplund et al. (2004). Ces travaux apportent par ailleurs une confirmation indépendante des résultats d'Asplund et al., au moment où un débat est engagé sur la validité de ces nouvelles abondances.<br /><br />La deuxième partie de ma thèse fut consacrée à l'étude du magnétisme d'un échantillon de 50 étoiles Ae/Be de Herbig, étoiles PMS de masse intermédiaire, à l'aide du spectropolarimètre ESPaDOnS, nouvellement installé au CFHT. Nous avons détecté 4 étoiles magnétiques dans cet échantillon, c'est-à-dire que la proportion d'étoiles magnétiques parmi les étoiles Ae/Be de Herbig est proche de celle des étoiles Ap/Bp magnétiques parmi les étoiles A et B de la séquence principale. A l'aide d'un modèle simple de rotateur oblique, j'ai montré que la structure de ces champs magnétiques est proche d'une structure dipolaire à grande échelle comme le champ magnétique des étoiles Ap/Bp. Ces résultats constituent un argument fort en faveur de l'hypothèse d'un champ fossile pour expliquer l'origine du champ magnétique des étoiles Ap/Bp.

We have imaged GM Aurigae with the Hubble Space Telescope, detected its disk in scattered light at 1400 and 1650 angstrom, and compared these with observations at 3300 angstrom, 5550 angstrom, 1.1 mu m, and 1.6 mu m. The scattered light increases at shorter wavelengths. The radial surface brightness profile at 3300 angstrom shows no evidence of the 24 au radius cavity that has been previously observed in submillimeter observations. Comparison with dust grain opacity models indicates that. the surface of the entire disk is populated with submicron grains. We have compiled a. spectral energy distribution from 0.1 mu m to 1 mm. and used it to constrain a model of the star + disk system that includes the submillimeter cavity using the Monte Carlo radiative transfer code by Barbara Whitney. The best-fit model image indicates that the cavity should be detectable in the F330W bandpass if the cavity has been cleared of both large and small dust grains, but we do not detect it. The lack of an observed cavity can be explained by the presence of submicron grains interior to the submillimeter cavity wall. We suggest one explanation for this that. could be due to a planet of mass <9 M-J interior to 24 au. A unique cylindrical structure is detected in the far-UV data from the Advanced Camera for Surveys/ Solar Blind Channel. It is aligned along the system semiminor axis, but does not resemble an accretion-driven jet. The structure is limb. brightened and extends 190 +/- 35 au above the disk midplane. The inner radius of the limb. brightening is 40 +/- 10 au, just beyond the submillimeter cavity wall.

The notion that low- to intermediate-mass young stellar objects (YSOs) gain mass at a constant rate during the early stages of their evolution appears to be challenged by observations of YSOs suffering sudden increases of the rate at which they gain mass from their circumstellar discs. Also, this idea that stars spend most of their lifetime with a low accretion rate and gain most of their final mass during short-lived episodes of high accretion bursts, helps to solve some long-standing problems in stellar evolution. The original classification of eruptive variables divides them in two separate subclasses known as FU Orionis stars (FUors) and EX Lupi stars (EXors). In this classical view FUors are at an early evolutionary stage and are still gaining mass from their parent envelopes, whilst EXors are thought to be older objects only surrounded by an accretion disc. The problem with this classical view is that it excludes younger protostars which have higher accretion rates but are too deeply embedded in circumstellar matter to be observed at optical wavelengths. Optically invisible protostars have been observed to display large variability in the near-infrared. These and some recent discoveries of new eruptive variables, show characteristics that can be attributed to both of the optically-defined subclasses of eruptive variables. The new objects have been proposed to be part of a new class of eruptive variables. However, a more accepted scenario is that in fact the original classes only represent two extremes of the same phenomena. In this sense eruptive variability could be explained as arising from one physical mechanism, i.e. unsteady accretion, where a variation in the parameters of such mechanism can cause the different characteristics observed in the members of this class. With the aim of studying the incidence of episodic accretion among young stellar objects, and to characterize the nature of these eruptive variables we searched for high amplitude variability in two multi-epoch infrared surveys: the UKIDSS Galactic Plane Survey (GPS) and the Vista Variables in the Via Lactea (VVV). In order to further investigate the nature of the selected variable stars, we use photometric information arising from public surveys at near- to farinfrared wavelengths. In addition we have performed spectroscopic and photometric follow-up for a large subset of the samples arising from GPS and VVV. We analyse the widely separated two-epoch K-band photometry in the 5th, 7th and 8th data releases of the UKIDSS Galactic Plane Survey. We find 71 stars with _K > 1 mag, including 2 previously known OH/IR stars and a Nova. Even though the mid-plane is mostly excluded from the dataset, we find the majority (66%) of our sample to be within known star forming regions (SFRs), with two large concentrations in the Serpens OB2 association (11 stars) and the Cygnus-X complex (27 stars). The analysis of the multi-epoch K-band photometry of 2010-2012 data from VVV covering the Galactic disc at |b| < 1◦ yields 816 high amplitude variables, which include known variables of different classes such as high mass X-ray binaries, Novae and eclipsing binaries among others. Remarkably, 65% of the sample are found concentrated towards areas of star formation, similar to the results from GPS. In both surveys, sources in SFRs show spectral energy distributions (SEDs) that support classification as YSOs. This indicates that YSOs dominate the Galactic population of high amplitude infrared variable stars at low luminosities and therefore likely dominate the total high amplitude population. Spectroscopic follow-up allows us to confirm the pre-main sequence nature of several GPS and VVV Objects. Most objects in both samples show spectroscopic signatures that can be attributed to YSOs undergoing high states of accretion, such as veiling of photospheric features and CO emission, or show FUor-like spectra. We also find a large fraction of objects with 2.12 μm H2 emission that can be explained as arising from shock-excited emission caused by molecular outflows. Whether these molecular outflows are related to outbursts events cannot be confirmed from our data. Adding the GPS and VVV spectroscopic results, we find that between 6 and 14 objects are new additions to the FUor class from their close resemblance to the near-infrared spectra of FUors, and at least 23 more objects are new additions to the eruptive variable class. For most of these we are unable to classify them into any of the original definitions for this variable class. In any case, we are adding up to 37 new stars to the eruptive variable class which would double the current number of known objects. We note that most objects are found to be deeply embedded optically invisible stars, thus increasing the number of objects belonging to this subclass by a much larger factor. In general, objects in our samples which are found to be likely eruptive variable stars show a mixture of characteristics that can be attributed to both of the optically-defined classes. This agrees well with the recent discoveries in the literature. Finally, we are able to derive a first rough estimate on the incidence of episodic accretion among class I YSOs in the star-forming complex G305. We find that _ 9% of such objects are in a state of high accretion. This number is in agreement with previous theoretical and observational estimates among class I YSOs.