Magnetic fields of intermediate-mass T Tauri stars

I. Magnetic detections and fundamental stellar parameters^⋆

¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: florent.villebrun@univ-grenoble-alpes.fr
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ IRAP, Universite de Toulouse, CNRS, CNES, UPS, 31400 Toulouse, France
⁴ LESIA, Observatoire de Paris, PSL Research University, CNRS UMR 8109, 5 Place Jules Janssen, 92195 Meudon, France
⁵ Institut de Physique de Rennes, Universite de Rennes 1, CNRS UMR 6251, 35042 Rennes, France
⁶ Department of Astronomy, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
⁷ Department of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005, USA
⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁹ University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350, Australia
¹⁰ University of Exeter, Department of Physics and Astronomy, Stoker Road, Devon, Exeter EX4 4QL, UK
¹¹ LUPM, Université de Montpellier & CNRS, Montpellier Cedex 05, France

Received: 1 June 2018
Accepted: 24 October 2018

Abstract

Context. The origin of the fossil magnetic fields detected in 5 to 10% of intermediate-mass main sequence stars is still highly debated.

Aims. We want to bring observational constraints to a large population of intermediate-mass pre-main sequence (PMS) stars in order to test the theory that convective-dynamo fields generated during the PMS phases of stellar evolution can occasionally relax into fossil fields on the main sequence.

Methods. Using distance estimations, photometric measurements, and spectropolarimetric data from HARPSpol and ESPaDOnS of 38 intermediate-mass PMS stars, we determined fundamental stellar parameters (T_eff, L and v sin i) and measured surface magnetic field characteristics (including detection limits for non-detections, and longitudinal fields and basic topologies for positive detections). Using PMS evolutionary models, we determined the mass, radius, and internal structure of these stars. We compared different PMS models to check that our determinations were not model-dependant. We then compared the magnetic characteristics of our sample accounting for their stellar parameters and internal structures.

Results. We detect magnetic fields in about half of our sample. About 90% of the magnetic stars have outer convective envelopes larger than ∼25% of the stellar radii, and heavier than ∼2% of the stellar mass. Going to higher mass, we find that the magnetic incidence in intermediate-mass stars drops very quickly, within a timescale on the order of few times 0.1 Myr. Finally, we propose that intermediate-mass T Tauri stars with large convective envelopes, close to the fully convective limit, have complex fields and that their dipole component strengths may decrease as the sizes of their convective envelopes decrease, similar to lower-mass T Tauri stars.