Inner disk dynamics of classical T Tauri stars in NGC 2264

Abstract

The young open cluster NGC 2264 is a rich site of star formation, containing hundreds of young, accreting, low-mass stars known as classical T Tauri stars (CTTS). In December 2011, this cluster was observed simultaneously by the CoRoT and Spitzer satellites, as well as quasi-simultaneously by a number of ground-based telescopes, such as the CFHT and the VLT with the FLAMES multi-object spectrograph in a spectral range covering the H line (a strong indicator of accretion). NGC 2264 had also been observed previously by CoRoT in March 2008, and it became the target of a new FLAMES campaign in 2014 centered on the forbidden [OI] 6300Å line, a well-known tracer of jets and winds in young stars. The goal of this thesis is to explore the richness of data available for NGC 2264 in order to investigate the dynamics of inner circumstellar disks of CTTSs and, with this, to better understand the processes of mass accretion and mass ejection that occur therein. In the rst part of this work, we investigate a group of CTTSs that present photometric variability indicative of extinction from inner disk material. These systems are viewed at high inclinations and represent an opportunity to study the interaction between the stellar magnetosphere and the inner accretion disk through indirect measurements. We analyze their CoRoT light curves alongside their simultaneous Spitzer light curves in order to better understand the dust distribution in the inner disk region, and in this way nd indications of the presence of grains larger than typical interstellar grains. We investigate veiling variability in their FLAMES spectra and u¿¿r color variations, and in many cases nd evidence of hot spots that are spatially associated with occultations of the stellar photosphere by circumstellar material, indicating that the occulting structures may be located at the base of accretion columns. We nd that an occultation model of an inner disk warp, initially proposed to explain the photometric behavior of the CTTS AA Tau, is successful at reproducing the general aspects of most of the quasi-periodic light curves in our sample, nding warps of similar maximum height and azimuthal extension as had been found for AA Tau. We ascribe this AA Tau-like variability to a stable accretion regime and similar but non-periodic variability to an unstable accretion regime. We see indications that a transition, and even certain coexistence, between the two regimes may be common. In the second part of this work we investigate ejection processes through the forbidden [OI] 6300Å emission line. We separate the [OI] line proles into three distinct features: a high-velocity component (HVC), a broad low-velocity component (BLVC), and a narrow low-velocity component (NLVC). The HVC is believed to originate in bipolar, colimated jets, while the origin of the low-velocity components (LVC) is still under debate. Our BLVCs show blueshifts consistent with an origin in inner disk winds. The NLVC has been suggested to arise in X-ray or EUV driven photoevaporative disk winds, which is consistent with some of our ndings, but we have insucient evidence to conrm this. We nd that the luminosities of all components correlate positively with the stellar and accretion luminosity. We also nd that CTTSs accreting in the unstable regime tend to show broader LVCs and stronger HVCs than stars accreting in the stable regime. We only detect the HVC among systems with optically thick inner disks, while the BLVC is found in a few systems with an anemic disk and the NLVC is common among systems with thin inner disks. The ratio of detected NLVCs to BLVCs seems to reect the age of the star forming region, indicating that as a system evolves, the NLVC tends to dominate the [OI] emission. We also show that stellar properties such as eective temperature, mass, and stellar rotation, do not seem to inuence in a signicant way the propagation velocity or luminosity of protostellar jets, and it appears that it is the disk, rather than the star, that governs their launching mechanism.