Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).

The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M-star) similar to 0.1 M-circle dot for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 +/- 0.1 and 2.3 +/- 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than similar to 0.45 L-circle dot and for stellar masses lower than similar to 0.3 M-circle dot is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M-star similar to 0.3-0.4 M-circle dot. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are similar to 10(-10) M-circle dot/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.

HD 141569 A is a pre-main sequence B9.5 Ve star surrounded by a prominent and complex circumstellar disk, likely still in a transition stage from protoplanetary to debris disk phase. Here, we present a new image of the third inner disk component of HD 141569 A made in the L' band (3.8 mu m) during the commissioning of the vector vortex coronagraph that has recently been installed in the near-infrared imager and spectrograph NIRC2 behind the W.M. Keck Observatory Keck II adaptive optics system. We used reference point-spread function subtraction, which reveals the innermost disk component from the inner working distance of similar or equal to 23 au and up to similar or equal to 70 au. The spatial scale of our detection roughly corresponds to the optical and near-infrared scattered light, thermal Q, N, and 8.6 mu m PAH emission reported earlier. We also see an outward progression in dust location from the L' band to the H band (Very Large Telescope/SPHERE image) to the visible (Hubble Space Telescope (HST)/STIS image), which is likely indicative of dust blowout. The warm disk component is nested deep inside the two outer belts imaged by HST-NICMOS in 1999 (at 406 and 245 au, respectively). We fit our new L'-band image and spectral energy distribution of HD 141569 A with the radiative transfer code MCFOST. Our best-fit models favor pure olivine grains and are consistent with the composition of the outer belts. While our image shows a putative very faint point-like clump or source embedded in the inner disk, we did not detect any true companion within the gap between the inner disk and the first outer ring, at a sensitivity of a few Jupiter masses.

Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. Aims. We explore the photometric and spectroscopic behavior of ASASSN-13db during the 2014-2017 outburst. Methods. We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the light curve of ASASSN-13db and the dynamical and physical properties of the accretion flow. Results. The 2014-2017 outburst lasted for nearly 800 days. A 4.15 d period in the light curve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blue-shifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of similar to 5800-6000 K in the accretion flow. Conclusions. Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate (<(M)over dot> = 1-3 x 10(-7) M-circle dot/yr), about two orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very-low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settle after the beginning of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power of time-resolved photometry and spectroscopy to explore the properties of variable and outbursting stars.

This thesis focuses on the evolution of zonal flows in self-gravitating accretion discs and their resulting effect on disc stability; it also studies the process of disc gravito-turbulence, with particular emphasis given to the way the turbulent state is able to extract energy from the background flow and sustain itself by means of a feedback. Chapters 1 and 2 provide an overview of systems involving accretion discs and a theoretical introduction to the theory of accretion discs, along with potential methods of angular momentum transport to explain the observed accretion rates. To address the issue of the gravito-turbulence self-sustenance, a compressible non-linear spectral code (dubbed CASPER) was developed from scratch in C; its equations and specifications are laid out in Chapter 3. In Chapter 4 an ideal (no viscosities or cooling) linear stability analysis to non-axisymmetric perturbations is carried out when a zonal flow is present in the flow. This yields two instabilities: a Kelvin-Helmholtz instability (active only if the zonal flow wavelength is sufficiently small) and one driven by self-gravity. A stability analysis of the zonal flow itself is carried out in Chapter 5 by means of an axisymmetric linear analysis, using non-ideal conditions. This considers instability due to both density wave modes (which give rise to overstability) and slow modes (which result in thermal or viscous instability) and, thanks a different perturbation wavelength regime, represents an extension to the classical theory of thermal and viscous instabilities. The slow mode instability is found to be aided by high Prandtl numbers and adiabatic index γ values, while quenched by fast cooling. The overstability is likewise stabilised by fast cooling, and occurs in a non-self-gravitational regime only if γ ≲ 1.305. Lastly, Chapter 6 illustrates the results of the non-linear simulations carried out using the CASPER code. Here the system settles into a state of gravito-turbulence, which appears to be linked to a spontaneously-developing zonal flow. Results show that this zonal flow is driven by the slow mode instability discussed in Chapter 5, and that the presence of zonal flows triggers a non-axisymmetric instability, as seen in Chapter 4. The role of the latter is to constrain the zonal flow amplitude, with the resulting zonal flow disruption providing a generation of shearing waves which permits the self-sustenance of the turbulent state.

Stars form from the collapse of molecular clouds and evolve in an environment rich in gas and dust before becoming Main Sequence stars. During this phase, characterised by the presence of a protoplanetary disc, stars manifest changes in the structure and luminosity. This thesis performs a multi-wavelength analysis, from optical to mm range, on a sample of young stars (YSOs), mainly Classical T Tauri (CTTS). The purpose is to study optical and infrared variability and its relation with the protoplanetary disc. Longer wavelength, in the mm range, are used instead to investigate the evolution of the disc, in terms of dust growth. In optical, an F-test on a sample of 39 CTTS reveals that 67\% of the stars are variable. The variability, quantified through pooled sigma, is visible both in magnitude amplitudes and changes over time. Time series analysis applied on the more variable stars finds the presence of quasi periodicity, with periods longer than two weeks, interpreted either as eclipsing material in the disc happening on a non-regular basis, or as a consequence of star-disc interaction via magnetic field lines. The variability of YSOs is confirmed also in infrared, even if with lower amplitude. No strong correlations are found between optical and infrared variability, which implies a different cause or a time shift in the two events. By using a toy model to explore their origin, I find that infrared variations are likely to stem from emissions in the inner disc. The evolution of discs in terms of dust growth is confirmed in most discs by the analysis of the slope of the spectral energy distribution (SED), after correcting for wind emission and optical depth effects. However, the comparison with a radiative transfer model highlights that a number of disc parameters, in particular disc masses and temperature, dust size distribution and composition, can also affect the slope of the SED.