Through the study of accretion onto the young, low-mass stars known as T Tauri Stars (TTS), we can better understand the formation of our solar system. Gas is funneled along stellar magnetic field lines into magnetospheric accretion columns where it reaches free-fall velocities and shocks at the stellar surface, generating emission that carries information about the inner regions of the protoplanetary disk. Accretion is a variable process, with characteristic timescales ranging from minutes to years. In this dissertation, I use simulations, models, and observations to provide insight into the driving forces of mass accretion rate variability on timescales of minutes to weeks and the structure of the inner disk. Using hydrodynamic simulations, I find that steady-state, transonic accretion occurs naturally in the absence of any other source of variability. If the density in the inner disk varies smoothly in time with roughly day-long time-scales (e.g., due to turbulence), traveling shocks develop within the accretion column, which lead to rapid increases in the accretion luminosity followed by slower declines. I present the largest Hubble Space Telescope (HST) spectral variability study of TTS to date. I infer mass accretion rates and accretion column surface coverage using newly updated accretion shock models. I find typical changes in the mass accretion rate of order 10% and moderate changes in the surface coverage for most objects in the sample on week timescales. Individual peculiar epochs are further discussed. I find that the inner disk is inhomogeneous and that dust may survive near the magnetic truncation radius. Next, I link 2-minute cadence light curves from the Transiting Exoplanet Survey Satellite (TESS) to accretion using ground-based U-band photometry. Additional HST observations for one target enable more detailed connections between TESS light curves and accretion. I also use the TESS light curves to identify rotation periods and patterns of quasi-periodicity. Finally, I connect hydrodynamic simulations, accretion shock models, and stellar rotation to predict signatures of a turbulent inner disk. I generate light curves from these models to make comparisons to previous month-long photometric monitoring surveys of TTS using metrics of light curve symmetry and periodicity.

I develop the first magnetospheric accretion model to take account of the observed complexity of T Tauri magnetic fields, and the influence of stellar coronae. It is now accepted that accretion onto classical T Tauri stars is controlled by the stellar magnetosphere, yet to date the majority of accretion models have assumed that the stellar magnetic field is dipolar. By considering a simple steady state accretion model with both dipolar and complex magnetic fields I find a correlation between mass accretion rate and stellar mass of the form M[dot above] proportional to M[asterisk subscript, alpha superscript], with my results consistent within observed scatter. For any particular stellar mass there can be several orders of magnitude difference in the mass accretion rate, with accretion filling factors of a few percent. I demonstrate that the field geometry has a significant effect in controlling the location and distribution of hot spots, formed on the stellar surface from the high velocity impact of accreting material. I find that hot spots are often at mid to low latitudes, in contrast to what is expected for accretion to dipolar fields, and that particularly for higher mass stars, accreting material is predominantly carried by open field lines. Material accreting onto stars with fields that have a realistic degree of complexity does so with a distribution of in-fall speeds. I have also modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating from surface magnetograms I find that T Tauri coronae are compact and clumpy, such that rotational modulation arises from X-ray emitting regions being eclipsed as the star rotates. Emitting regions are close to the stellar surface and inhomogeneously distributed about the star. However some regions of the stellar surface, which contain wind bearing open field lines, are dark in X-rays. From simulated X-ray light curves, obtained using stellar parameters from the Chandra Orion Ultradeep Project, I calculate X-ray periods and make comparisons with optically determined rotation periods. I find that X-ray periods are typically equal to, or are half of, the optical periods. Further, I find that X-ray periods are dependent upon the stellar inclination, but that the ratio of X-ray to optical period is independent of stellar mass and radius. I also present some results that show that the largest flares detected on T Tauri stars may occur inside extended magnetic structures arising from the reconnection of open field lines within the disc. I am currently working to establish whether such large field line loops can remain closed for a long enough time to fill with plasma before being torn open by the differential rotation between the star and the disc. Finally I discuss the current limitations of the model and suggest future developments and new avenues of research.

Le processus d'accrétion joue un rôle crucial dans le scénario de formation stellaire. Il régit l'interaction des étoiles jeunes avec leurs disques, en régulant l'échange de masse et de moment cinétique; ainsi, il a un impact durable sur leur évolution. De plus, l'accrétion est un ingrédient essentiel de la physique des systèmes étoile-disque à l'époque de formation planétaire. Selon le modèle d'accrétion magnétosphérique, une cavité de quelques rayons stellaires s'étend de la surface de l'étoile au bord interne du disque. L'interaction se produit donc par le champ magnétique stellaire, qui pénètre le disque interne et l'attèle à l'objet central. Des colonnes d'accrétion se développent du disque interne suivant les lignes de champ, et atteignent l'étoile à des vitesses presque de chute libre. L'impact à la surface crée des chocs localisés, qui sont responsables de l'excès de luminosité UV distinctif des systèmes accrétants par rapport aux objets non-accrétants. L'évolution temporelle intrinsèque et l'effet d'alternance du côté visible des objets au cours de leur rotation se mélangent dans la variabilité photométrique typique des étoiles jeunes, révélée par les campagnes de suivi.Durant ma thèse, j'ai mené une étude statistique du processus d'accrétion et de sa variabilité dans la région NGC 2264 (3 Myr). Cet amas contient plus de 700 membres, repartis entre étoiles avec disque (45%) et sans disque. J'ai qualifié l'accrétion par la diagnostique de l'excès UV; les étoiles de l'amas privées de disque définissent le niveau d'émission de référence au-dessus duquel l'excès UV provenant du choc d'accrétion est décelé et mesuré. Mon étude se base sur un jeu de données photométriques obtenues au télescope Canada-France-Hawaii (CFHT), comprenant un relevé profond en 4 filtres (u,g,r,i) et un suivi simultané de variabilité optique (bande r) et UV (bande u) d'une durée de 2 semaines et avec échantillonnage de l'ordre des heures. Dans une première étape de cette étude, je convertis les excès UV en taux d'accrétion pour obtenir une image globale du processus à travers l'amas et examiner sa dépendance envers les paramètres stellaires. Le taux d'accrétion moyen corrèle avec la masse de l'étoile, bien qu'une dispersion significative autour de cette tendance moyenne soit observée à chaque masse. Je montre que cet étalement ne peut pas être justifié par la variabilité des objets; une diversité de mécanismes d'accrétion et de stades évolutifs dans l'amas pourrait contribuer à la vaste gamme de régimes d'accrétion décelés. Ensuite, j'explore les signatures dans l'UV propres à des types distincts d'étoiles jeunes variables. Je montre que les étoiles accrétantes présentent en général une variabilité plus prononcée que les objets sans disque, et que les respectives variations de couleur sont cohérentes avec une origine différente de la variabilité associée aux deux groupes. Pour le premier groupe, ce sont les chocs d'accrétion à dominer, alors que le deuxième est dominé par des taches froides à la surface, dérivant de l'activité magnétique stellaire. Je compare les variations photométriques mesurées sur bases de quelques heures, quelques jours et quelques années, afin de déterminer quelles soient les composantes de variabilité les plus importantes. L'échelle de temps de quelques jours prévaut sur les autres délais investigués dans la variabilité enregistrée pour ces étoiles jeunes, avec une contribution majeure provenant de l'effet de modulation rotationnelle. Enfin, j'analyse les propriétés de rotation des étoiles de l'amas à partir d'un jeu de courbes de lumière optiques, d'une durée de 38 jours, obtenues avec le satellite CoRoT près de la campagne d'observation au CFHT. Je reconstruis la distribution de périodes de l'amas et montre que les objets sans disque tournent statistiquement plus vite que les objets accrétants. Cette connexion entre les propriétés d'accrétion et celles de rotation peut être interprétée dans le scénario de disk-locking. / Disk accretion plays a most important role in the star formation scenario. It governs the interaction of young stars with their disks, with a long-lasting impact on stellar evolution, by providing both mass and angular momentum regulation. Accretion is also a central ingredient in the physics of star-disk systems at the epoch when planets start to form. In the picture of magnetospheric accretion, a cavity of a few stellar radii extends from the star surface to the inner disk rim. The star-disk interaction is then mediated by the stellar magnetic field, whose lines thread the inner disk and couple it to the central object. Material from the inner disk is channeled along the field lines in accretion columns that reach the star at near free-fall velocities. The impact produces localized hot shocks at the stellar surface, which determine the distinctive UV excess emission of accreting objects relative to non-accreting sources. Intrinsic time evolution, and varying visibility of surface features during stellar rotation, combine in the characteristic photometric variability of young stars, revealed by monitoring surveys.In this thesis, I investigate the statistical properties of disk accretion and of its variability in the young open cluster NGC 2264 (3 Myr). This comprises a population of over 700 objects, about similarly distributed between disk-bearing (45%) and disk-free sources. I characterize accretion from the UV excess diagnostics; disk-free cluster members define the reference emission level over which the UV excess linked to accretion is detected and measured. The study is based on a homogeneous photometric dataset obtained at the Canada-France-Hawaii Telescope (CFHT), composed of a deep mapping of the region in four different bands (u,g,r,i) and of simultaneous optical (r-band) and UV (u-band) monitoring on timescales from hours to days for a period of 2 weeks. In the first part of the study, UV excesses are converted to accretion luminosities and mass accretion rates to derive a global picture of the accretion process across the cluster, and to investigate the dependence of the typical accretion properties on stellar parameters such as mass and age. A robust correlation is detected between the average accretion rate and stellar mass, but a significant dispersion in accretion rates is observed around this average trend at any given mass. I show that the extent of this spread cannot be accounted for by typical variability on week timescales; I discuss several aspects, including a diversity in accretion mechanisms and a non-negligible evolutionary spread among cluster members, which may contribute to the broad range of accretion regimes detected. In the second part of the study, I explore the variability signatures in the UV that pertain to different types of variable young stars. I show that accreting objects typically exhibit stronger variability than non-accreting objects, and that the color properties associated with the two groups are consistent with a statistically distinct origin of the variability features in the two cases. These are dominated, in the first case, by hot accretion spots, and in the second, by cold spots linked to magnetic activity. I compare the amounts of variability on timescales of hours, days and years, to assess the dominant components. The mid term (days) appears to be the leading timescale for variability in young stars up to years, with a major contribution from rotational modulation. In the third part of the study, I use a set of 38 day-long optical light curves obtained with the CoRoT satellite, close to the epoch of the CFHT survey, to investigate periodicity and rotation properties in NGC 2264. I derive the period distribution for the cluster and show that accreting and non-accreting objects exhibit statistically distinct properties: the second rotate on average faster than the first. I then illustrate the connection between accretion and rotation properties in the disk-locking scenario.

We present Gemini Planet Imager polarized intensity imagery of HD 100453 in Y, J, and K1 bands that reveals an inner gap (9-18 au), an outer disk (18-39 au) with two prominent spiral arms, and two azimuthally localized dark features that are also present in Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) total intensity images. Spectral energy distribution fitting further suggests that the radial gap extends to 1 au. The narrow, wedge-like shape of the dark features appears similar to predictions of shadows cast by an inner disk that is misaligned with respect to the outer disk. Using the Monte Carlo radiative transfer code HOCHUNCK3D, we construct a model of the disk that allows us to determine its physical properties in more detail. From the angular separation of the features, we measure the difference in inclination between the disks (45 degrees) and their major axes, PA = 140 degrees east of north for the outer disk, and 100 degrees for the inner disk. We find an outer-disk inclination of 25 degrees +/- 10 degrees from face-on, in broad agreement with the Wagner et al. measurement of 34 degrees. SPHERE data in J and H bands indicate a reddish disk, which indicates that HD 100453 is evolving into a young debris disk.

<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>

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.

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.

Dans ce travail de thèse, nous analysons pour la première fois à l'aide de la technique d'imagerie doppler, les raies photosphériques de deux étoiles T Ttauri à faibles raies d'émission, pour cartographier leur brillance de surface et localiser ainsi les inhomogenéités en température, qui signent la présence de forts champs magnétiques. Pour ce faire, nous avons développé une méthode numerique fondée sur la paramétrisation de taches, dont les caractéristiques sont déterminées lors de la minimisation, de type moindres carrés, des différences entre les modèles de raie calculés et l'ensemble des raies (issues de divers éléments chimiques) observées à plusieurs phases rotationnelles. Nous étudions le type et l'amplitude de déformations engendrées par la présence de taches froides, ce qui nous permet d'en déduire les conditions optimales d'application de l'imagerie doppler. Nous etudions par ailleurs les limitations intrinséques de cette technique déterminées par la qualité des données (résolution instrumentale et rapport signal sur bruit). L'application à des étoiles T Tauri, jeunes étoiles en fin de formation, similaires au soleil dans sa jeunesse, nous permet de mettre en évidence la présence de larges taches froides, préférentiellement localisées aux latitudes élevées. La position de ces taches suggère une géometrie de type dipolaire pour le champ magnétique. Seule une étude exhaustive pourra déterminer si cette géometrie est commune à toute cette classe d'étoiles, ou si elle est liée à des caractéristiques particulières (fort taux de rotation 70km/s ; 80 km/s). cette réponse est cruciale pour déterminer l'origine du champ magnétique : fossile, engendré par un mecanisme particulier de type dynamo, peut-etre les deux à la fois. L'application future de cette technique aux etoiles T Tauri classiques, supposées etre entourées d'un disque d'accrétion, sera determinante pour comprendre l'interaction entre l'environnement circumstellaire et l'objet central.

<p>This thesis is a detailed observational study of the accretion processes in T Tauri stars (TTS). The interaction between the central star, the circumstellar disk and the magnetic field gives rise to a wide range of features in the spectra of TTS. The current picture of TTS is based on rather simple models assuming that accretion is a homogeneous and axisymmetric process. Although these models have been successful in explaining some observational signatures of TTS such as the shape of emission lines, the static nature of these models makes them unsuitable for describing the strong variability of the veiling spectrum and emission lines of TTS. An improved understanding of this variability is of key importance to study the dynamic processes related to the accretion flow and the winds.</p><p>This study is based on a set of high-quality spectroscopic observations with the UVES spectrograph at the 8-m VLT in 2000 and 2002. These spectra, with exposure times as short as 10-15 minutes, have high spectral resolution and high signal-to-noise ratios and cover a large part of the optical wavelength range. From this dataset we determine the basic physical parameters of several TTS and model their photospheres. These models then serve as a basis for a detailed investigation of variations of the veiling continuum and line emission. We confirm that the level of veiling correlates with some of the strongest emission lines and that coherent changes in accretion occur on a timescale of a few hours, comparable to the free-fall time from the disk to the star. From the properties of the emission lines formed close to the central star and in the stellar wind we derive restrictions on the geometry of the observed systems.</p><p>Because the intrinsic axial symmetry of a single star makes it almost impossible to disentangle rotational modulation from inhomogeneity and axial asymmetry of the accretion flow, we study a series of spectra of a close spectroscopic binary at different orbital phases and derive the 3D structure of flows between the disk and the star. Finally, we calculate the profiles of hydrogen emission lines by iteratively solving 3D NLTE radiative transfer in a state-of-the-art magnetospheric model.</p>