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In this project we employ tools such as dynamic time warping (DTW) and variography for detailed time series study of TTS light curves. The main goal is to be able to classify the light curves, identify recurring patterns that might unveil potential planetary signatures that are hidden behind obvious signals in the data (e.g periodic modulations due to spots or dips, bursting events, etc..).

The accretion of material from the protoplanetary disk onto the star is essential in the formation of Sun-like stars and in setting the properties of the disk at the time when planet formation happens. Although often described by simple and static models, the accretion process is inherently variable. For this reason, variability of young stars is perhaps the most important source of information on the accretion process and on the behavior of the innermost region of the systems. We designed a program to study six young stars in the Chamaeleon I star-forming region with the aim of characterizing the accretion process in time, to study the accretion diagnostic parameters and to examine the innermost part of the circumstellar disk. We combined the high-precision optical photometric observations of the TESS space telescope with contemporaneous ground-based IJHK-band photometry, and multi-epoch high-resolution optical spectroscopic observations obtained by the VLT/ESPRESSO and the 2.2m /FEROS spectrographs. Besides uncovering the periodic light variations that can be attributed to the stellar rotation, the uninterrupted TESS observations allowed us to examine the shorter timescale fluctuations due to accretion variability and also revealed flare-like events in the light curves. Our high-resolution spectroscopy allowed us to examine the amplitude, timescale and pattern of variability in accretion tracers which carry information on the distribution and the kinematics of the accreting material, the density structure of the inner disk, stellar activity, and the presence of outflows or jets. Where we have contemporaneous photometry and spectroscopy we linked the photometric variability to spectroscopic variations., {"references":["Hussain et al ., 2009, MNRAS, Vol 398, Issue 1, p 189 200","Carpenter et al ., 2001, AJ, Vol 121, Issue 6, p 3160 3190","Costigan et al ., 2014, MNRAS, Vol . 440, Issue 4, p 3444 3461"]}

The rotational evolution of accreting pre-main-sequence stars is influenced by its magnetic interaction with its surrounding circumstellar disk. Using the PLUTO code, we perform 2.5D magnetohydrodynamic, axisymmetric, time-dependent simulations of star-disk interaction---with an initial dipolar magnetic field structure, and a viscous and resistive accretion disk---in order to model the three mechanisms that contribute to the net stellar torque: accretion flow, stellar wind, and magnetospheric ejections (periodic inflation and reconnection events). We investigate how changes in the stellar magnetic field strength, rotation rate, and mass accretion rate (changing the initial disk density) affect the net stellar torque. All simulations are in a net spin-up regime. We fit semi-analytic functions for the three stellar torque contributions, allowing for the prediction of the net stellar torque for our parameter regime, and the possibility of investigating spin-evolution using 1D stellar evolution codes. The presence of an accretion disk appears to increase the efficiency of stellar torques compared to isolated stars, for cases with outflow rates much smaller than accretion rates, because the star-disk interaction opens more of the stellar magnetic flux compared to that from isolated stars. In our parameter regime, a stellar wind with a mass loss rate of $\approx 1 \%$ of the mass accretion rate is capable of extracting $\lesssim 50 \%$ of the accreting angular momentum. These simulations suggest that achieving spin-equilibrium in a representative T Tauri case within our parameter regime, e.g., BP Tau, would require a wind mass loss rate of $\approx 25\%$ of the mass accretion rate., 26 pages, 14 figures

Photometric observations of accreting, low-mass, pre-main-sequence stars (i.e., Classical T Tauri stars; CTTS) have revealed different categories of variability. Several of these classifications have been linked to changes in $\dot{M}$. To test how accretion variability conditions lead to different light-curve morphologies, we used 1D hydrodynamic simulations of accretion along a magnetic field line coupled with radiative transfer models and a simple treatment of rotation to generate synthetic light curves. We adopted previously developed metrics in order to classify observations to facilitate comparisons between observations and our models. We found that stellar mass, magnetic field geometry, corotation radius, inclination, and turbulence all play roles in producing the observed light curves and that no single parameter is entirely dominant in controlling the observed variability. While the periodic behavior of the light curve is most strongly affected by the inclination, it is also a function of the magnetic field geometry and inner disk turbulence. Objects with either pure dipole fields, strong aligned octupole components, or high turbulence in the inner disk all tend to display accretion bursts. Objects with anti-aligned octupole components or aligned, weaker octupole components tend to show light curves with slightly fewer bursts. We did not find clear monotonic trends between the stellar mass and empirical classification. This work establishes the groundwork for more detailed characterization of well-studied targets as more light curves of CTTS become available through missions such as the Transiting Exoplanet Survey Satellite (TESS)., Comment: 18 pages, 11 figures

During the last decades, observations and numerical simulations are helping us to characterize young star-disk systems. For such matter, the challenging goal of this project is to reinforce the synergy between observational and theoretical perspectives. On one hand, observations help us to constrain numerical simulations. On the other hand, the latest are useful the better understand the evolution along time of the physical mechanisms responsible for some spectroscopic features we can detect in YSOs (Young Stellar Objects) spectra., Au cours des dernières décennies, les observations et les simulations numériques nous aident à caractériser les jeunes systèmes étoile-disque. Pour cette question, le but ambitieux de ce projet est pour renforcer la synergie entre les perspectives d'observation et théoriques. D'une part, les observations nous aident à contraindre les simulations numériques. D'autre part, l'observations sont utiles pour mieux comprendre l'évolution au cours du temps des mécanismes physiques responsables pour certaines caractéristiques spectroscopiques, qui nous pouvons détecter dans les spectres des YSO (Young Stellar Objects).

Classical T Tauri Stars (CTTSs) are young stars accreting mass from their circumstellar disk. According to the largely accepted magnetospheric accretion scenario, the disk extends up to the truncation radius. In this region, the magnetic field is strong enough to disrupt the inner part of the disk and to channel the material towards the star, thereby forming accretion columns. The material falls onto the star at free fall velocity and hits the stellar surface; this produces shocks that heat the plasma up to a few million degrees. In the last twenty years, the X-ray and UV observations of these systems have raised several questions. In particular, the value predicted by theoretical models is systematically above the observed X-ray luminosity, and, also, the UV lines arising from these regions show complex profiles, which cannot be easily interpreted with current accretion models based only on magnetohydrodynamical effects. To tackle these problems, we modelled the structure and the dynamics of the plasma in the impact region, using radiation hydrodynamics simulations that include, for the first time, the effects of radiative transport in the Non Local Thermodynamic Equilibrium (non-LTE) regime. We found that the radiation arising from the shocked plasma is partially absorbed by the unshocked accretion column. This might explain the excess of X-ray flux predicted by MHD models in which only radiative losses are considered. Moreover, due to the absorption of radiation, the pre-shock down-falling accreted material is gradually heated up to a few 10^5 K due to irradiation of X-rays arising from the shocked plasma at the impact region. We discuss the implication of this pre-shock heating for the UV and X-ray emission arising from the impact region.

The main purpose of the dissertation is on the basis of long-term light curves, to be classified the variability of 28 PMS stars and to be draw conclusions about the physical mechanisms initiating observed changes in their brightness. 22 of the investigated stars are located in the dense molecular cloud L935, known as 'Gulf of Mexico' (NGC 7000/IC 5070), and 6 stars are located in the vicinity of the reflection nebula NGC 7129. The multicolour photometric observations that we present were performed from 1993 to 2015 with the 2-m RCC, the 50/70-cm Schmidt and the 60-cm Cassegrain telescopes of the Rozhen NAO (Bulgaria) and the 1.3-m RC telescope of the Skinakas Observatory (Greece). All frames were taken through a standard Johnson--Cousins set of filters. The studied stars were classified as follows: V752 Cyg, V1539 Cyg, V1716 Cyg, FHO 26, FHO 29, LkHα 186, LkHα 187, LkHα 191, [KW97] 53-17, [KW97] 53-22, [KW97] 53-23, V391 Cep, NGC 7129 S V2 and 2MASS J21403576+6635000 show characteristics for classical T Tauri stars; V1538 Cyg, V1929 Cyg, [KW97] 53-20 and NGC 7129 S V1 are probably weak-line T Tauri stars; LkHα 189 and [KW97] 53-11 show characteristics for both type T Tauri stars and spectral observations are needed for their exact classification; V350 Cep shows indications for EXor and/or FUor-type variability; V521 Cyg, FHO 27, FHO 28 and NGC 7129 S V3 show characteristics for UXor-type variability; V1957 Cyg, V2051 Cyg and [KW97] 53-36 likely are evolved PMS stars or post-T Tauri stars. Periodicity was discovered for 3 of the investigated stars. V1716 Cyg indicates 4.15-day period, V1929 Cyg indicates 0.43-day period and LkHα 189 indicates 2.45-day period.