The physics of magnetospheric accretion in Classical T Tauri Stars from the observation of their hot spots

• Applicant: Professorin Dr. Beate Stelzer
• Subject Area: Astrophysics and Astronomy
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 559915914

Classical T Tauri stars (CTTSs) are young, low-mass stars that accrete material from a circumstellar disk. The strong stellar magnetic field truncates the disk at a distance of few stellar radii, and the matter free-falls onto the star along the magnetic field lines. Accretion ends at the stellar surface with the formation of a shock that produces a hot spot on the stellar surface. Three-dimensional magnetohydrodynamic simulations have shown that CTTSs may accrete in either a stable or an unstable regime relative to the Rayleigh-Taylor instability. In the stable regime, accretion produces a polar hot spot which rotates with the stellar rotation period. In the unstable regime, accretion produces several hot spots which rotate with the Keplerian velocity at the disc truncation radius. Therefore, unstable accretion sometimes produces quasi-periodic oscillations at periods shorter than the stellar rotation. Observationally, the hot spot can be studied both photometrically and spectroscopically. Photometric light curves are used to derive the rotational periods of the hot spot. The spectroscopic analysis is based on the study of the narrow components (NCs) of the emission lines in the optical spectrum. The radial velocity of the NCs suffers periodic modulations with an amplitude that, under the assumption of a point-like hot spot, depends on the latitude of the spot. Different species have different amplitudes, resulting in a latitudinal distribution on the stellar surface. Recent works have highlighted the existence of a dichotomy: in many systems, the radial velocity of the NCs is modulated with a stable period that coincides with the stellar rotation period, while the light curves often show no indication of that period. In this project, we will investigate the properties of the hot spots in CTTSs. We will use multi-epoch high resolution spectroscopy and high-cadence photometric data, and focus on five of the most studied CTTSs, that is, TW Hya, GM Aur, BP Tau, RU Lup, and EX Lup. The first four are the so-called 'monitoring targets' of the Hubble Space Telescope (HST) UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. In particular, we intend to implement new codes that take into account the extension of the hot spot for both photometric and radial velocity modulation. We will test this "extended hot spot model" on the observed radial velocity modulation of the emission line NCs, with the aim of showing that the latitudinal distribution of the emitting species is the signature of an onion-like structure of the hot spot. Next, we will study the vertical stratification of the shock structure by analyzing the line asymmetry of the NCs of different emission lines, using high-resolution spectra from ESPRESSO@VLT and CHIRON@CTIO. Finally, we will investigate the dichotomy of the spectro-photometric signatures of the hot spot, focusing on TW Hya and studying it with simultaneous CHIRON spectra and TESS light curves.

• DFG Programme: Research Grants