Line and continuum radiative transfer modelling of AA Tau

Claire F Esau,Jerome Bouvier,Tim J Harries

doi:10.1093/mnras/stu1211

Abstract

We present photometric and spectroscopic models of the Classical T Tauri star AA Tau. Photometric and spectroscopic variability present in observations of AA Tau is attributed to a magnetically induced warp in the accretion disc, periodically occulting the photosphere on an 8.2--day timescale. Emission line profiles show signatures of both infall, attributed to magnetospherically accreting material, and outflow. Using the radiative transfer code TORUS, we have investigated the geometry and kinematics of AA Tau's circumstellar disc and outflow, which is modelled here as a disc wind. Photometric models have been used to constrain the aspect ratio of the disc, the offset angle of the magnetosphere dipole with respect to the stellar rotation axis, and the inner radius of the circumstellar disc. Spectroscopic models have been used to constrain the wind and magnetosphere temperatures, wind acceleration parameter, and mass loss rate. We find observations are best fitted by models with a mass accretion rate of $5\times10^{-9}$ M$_\odot$ yr$^{-1}$, a dipole offset of between $10^\circ$ and $20^\circ$, a magnetosphere that truncates the disc from 5.2 to 8.8 R$_\star$, a mass-loss-rate to accretion-rate ratio of ~ 0.1, a magnetosphere temperature of 8500 -- 9000 K, and a disc wind temperature of 8000 K.

Highlights

Classical T Tauri stars (CTTs) are low-mass pre-main-sequence stars
We have focused on fitting Hβ which displays the classical inverse P Cygni (IPC) absorption which is associated with the magnetospheric accretion paradigm
We found that models with a mass accretion rate of ∼5 × 10−9 M yr−1 yielded an accretion luminosity of up to 6.1 × 10−2 L, consistent with the value of 6.5 × 10−2 L calculated by B99

Summary

Introduction

Classical T Tauri stars (CTTs) are low-mass pre-main-sequence stars. Spectroscopic studies of CTTs show high-velocity redshifted absorption components in their recombination lines, providing evidence of accretion from the circumstellar disc (e.g. Edwards et al 1994; Muzerolle, Calvet & Hartmann 1998). Spectra show the presence of excess ultraviolet (UV) emission These can be explained by a magnetospheric accretion model (Bertout, Basri & Bouvier 1988; Konigl 1991; Hartmann, Hewett & Calvet 1994, and references therein), in which the magnetosphere of a CTT truncates the circumstellar disc, at which point material attaches to the field lines and falls on to the photosphere. The accreting material travels ballistically and its kinetic energy is liberated as thermal radiation on impact with the photosphere, producing hotspots near the magnetosphere poles. These are the sources of the excess UV emission. Typical accretion rates of CTTs range between 10−7 and 10−9 M yr−1 (e.g. Basri & Bertout 1989)

Results

Discussion

Conclusion