The Surface Magnetic Activity of the Weak-Line T Tauri Stars TWA 7 and TWA 25

TL;DR

This study uses spectropolarimetric observations and Zeeman Doppler imaging to map the magnetic fields of TWA 7 and TWA 25, reducing activity-induced radial velocity noise and ruling out close-in giant planets.

Contribution

It provides the first large-scale magnetic field maps for these stars, revealing complex magnetic topologies and demonstrating the effectiveness of magnetic measurements in exoplanet detection.

Findings

Revealed predominantly toroidal, non-axisymmetric magnetic fields.

Reduced radial velocity jitter significantly, excluding close-in giant planets.

Showed activity-induced signals can mimic planetary signatures.

Abstract

We present an analysis of spectropolarimetric observations of the low-mass weak-line T Tauri stars TWA 25 and TWA 7. The large-scale surface magnetic fields have been reconstructed for both stars using the technique of Zeeman Doppler imaging. Our surface maps reveal predominantly toroidal and non-axisymmetric fields for both stars. These maps reinforce the wide range of surface magnetic fields that have been recovered, particularly in pre-main sequence stars that have stopped accreting from the (now depleted) central regions of their discs. We reconstruct the large scale surface brightness distributions for both stars, and use these reconstructions to filter out the activity-induced radial velocity jitter, reducing the RMS of the radial velocity variations from 495 m/s to 32 m/s for TWA 25, and from 127 m/s to 36 m/s for TWA 7, ruling out the presence of close-in giant planets for both stars. The TWA 7 radial velocities provide an example of a case where the activity-induced radial velocity variations mimic a Keplerian signal that is uncorrelated with the spectral activity indices. This shows the usefulness of longitudinal magnetic field measurements in identifying activity-induced radial velocity variations.