Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AATau

TL;DR

This study uses spectropolarimetric observations to map the magnetic field and accretion processes of the classical T Tauri star AATau, revealing a tilted magnetic dipole, strong variability in accretion rates, and evidence of star-disc magnetic coupling leading to stellar spin-down.

Contribution

It provides detailed magnetic field maps and accretion analysis of AATau, demonstrating active star-disc magnetic interactions and the star's likely propeller regime, advancing understanding of stellar angular momentum loss.

Findings

AATau hosts a 2-3kG tilted magnetic dipole.

Accretion rate varies from -9.6 to -8.5 in log Msun/yr.

AATau is likely in the propeller regime, expelling most accreting material.

Abstract

From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at CFHT and TBL, we report the detection of Zeeman signatures on the prototypical classical TTauri star AATau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AATau. We find that AATau hosts a 2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AATau host large cool spots at photospheric level and accretion regions at chromospheric level. The logarithmic accretion rate at the surface of AATau at the time of our observations is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in Msun/yr); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the stellar magnetic field) matches the corotation radius (where the Keplerian period equals the stellar rotation period) - a necessary condition for accretion to occur. It suggests that AATau is largely in the propeller regime, with most of the accreting material in the inner disc regions being expelled outwards and only a small fraction accreted towards the surface of the star. The strong variability in the observed surface mass-accretion rate and the systematic time-lag of optical occultations (by the warped accretion disc) with respect to magnetic and accretion-powered emission maxima also support this conclusion. Our results imply that AATau is being actively spun-down by the star-disc magnetic coupling and appears as an ideal laboratory for studying angular momentum losses of forming Suns in the propeller regime.