Star-disk interaction in classical T Tauri stars revealed using wavelet analysis

Abstract

Context. The extension of the corona of classical T Tauri stars (CTTS) is is being widely discussed. The standard model of magnetic configuration of CTTS predicts that coronal magnetic flux tubes connect the stellar atmosphere to the inner region of the disk. However, differential rotation may disrupt these long loops. The results from hydrodynamic modeling of X-ray flares observed in CTTS that confirm the star-disk connection hypothesis are still controversial. Some authors suggest the presence of the accretion disk prevents the stellar corona extending beyond the co-rotation radius, while others are simply not confident with the methods used to derive loop lengths. Aims. We use independent procedures to determine the length of flaring loops in stars of the Orion Nebula Cluster, which has previously been analyzed using hydrodynamic models. Our aim is to disentangle the two scenarios that have been proposed. Methods. We present a different approach for determining the length of flaring loops that is based on the oscillatory nature of the loops after strong flares. We use wavelet tools to reveal oscillations during several flares. The subsequent analysis of these oscillations is based on the physics of coronal seismology. Results. Our results likely confirm the large extension of the corona of CTTS and the hypothesis of star-disk magnetic interaction in at least three CTTS of the Orion Nebula Cluster. Conclusions. Analyzing oscillations in flaring events is a powerful tool to determine the physical characteristics of magnetic loops in coronae in stars other than the Sun. The results presented in this work confirm the star-disk magnetic connection in CTTS.Key words. magnetohydrodynamics (MHD) -X-rays: stars -stars: magnetic field -stars: variables: T Tauri, Herbig Ae/Beprotoplanetary disks -stars: flare IntroductionClassical T Tauri stars (CTTS) and other young stellar objects are characterized by accreting gas from a surrounding gaseous disk. Accretion takes place through a funnel that connects the star to the inner region of the disk (e.g., Hartman 1989). Koenigl (1991) was the first to propose the scenario of magnetic driven accretion in CTTS. In this scenario, the star accretes material from the disk following stellar magnetic field lines connected to the disk (see Fig. 12 in Camenzind 1990). A similar scenario was previously proposed for accreting neutron stars by Ghosh & Lamb (1978).Some mechanisms of this star-disk interaction are not yet fully understood. Owing to differential rotation between the star and the disk, magnetic connection may be disrupted. Magnetohydrodynamic (MHD) simulations predict that the poloidal field connecting the disk to the star is wrapped up when their angular velocities differ substantially (Lovelace et al. 1995). As a consequence, magnetic loops inflate rapidly (Goodson et al. 1997). This expansion of poloidal field yields a magnetic configuration with three components: a closed stellar magnetosphere, a region of open field connected to the pole of the star, and a region...

X-Ray Flare Oscillations Track Plasma Sloshing along Star-disk Magnetic Tubes in the Orion Star-forming Region

“…∼ 10 − 20%, and the period is very long, i.e., ∼ 3 hours. These features are detected also in other flares on Orion PMS stars (López-Santiago et al 2016), but in these two flares they are particularly significant.…”

“…The feature at ∼ 30 ks is of short duration, i.e., less than 3 periods, and therefore not relevant in this work. The wave reconstruction (Torrence & Compo 1998;López-Santiago et al 2016) in Fig. 5b shows data and model oscillations with the same period and amplitude of the same order (within ∼ 30%).…”

“…This background noise is derived from the univariate lag-1 autoregressive, or Markov process. Details on the methodology are given in Torrence & Compo (1998) and summarized in López-Santiago et al (2016). In our work, we assume red noise instead of the white noise background typically used in the literature.…”

Plasma Sloshing in Pulse-Heated Solar and Stellar Coronal Loops

“…This does not seem to be so frequent in solar flares, probably because the length of the loops that brighten initially is often quite small. In spite of the smaller signal to noise, it is instead more frequent in stellar flares which can occur in giant magnetic channels (López-Santiago et al 2016).…”

“…The density fronts determine visible periodic fluctuations in the light curves, which may be detected. This kind of fluctuations has been detected in the solar non-flaring (Harrison 1987;Wang et al 2003) and flaring (Nakariakov & Melnikov 2009) corona and in stellar coronal flares (Mitra-Kraev et al 2005;Welsh et al 2006;Pandey & Srivastava 2009;López-Santiago et al 2016), and generally interpreted in terms of MHD harmonic modes.…”