Modelling the magnetic activity & filtering radial velocity curves of young Suns: the weak-line T Tauri star LkCa 4

TL;DR

This study uses spectropolarimetric and photometric data to map the magnetic and brightness features of the young star LkCa4, demonstrating effective filtering of stellar activity in radial velocity measurements to aid planet detection.

Contribution

It provides the first detailed magnetic and brightness maps of LkCa4, revealing a strong, mainly axisymmetric magnetic field and small differential rotation, and shows activity filtering can improve RV precision for young stars.

Findings

Magnetic field is strong and mainly axisymmetric with poloidal and toroidal components.

Differential rotation is weak, close to solid-body rotation.

Activity filtering reduces RV jitter to 0.055 km/s, enabling planet searches.

Abstract

We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star LkCa4 within the MaTYSSE programme, involving ESPaDOnS at the Canada-France-Hawaii Telescope. Despite an age of only 2Myr and a similarity with prototypical classical T Tauri stars, LkCa4 shows no evidence for accretion and probes an interesting transition stage for star and planet formation. Large profile distortions and Zeeman signatures are detected in the unpolarized and circularly-polarized lines of LkCa4 using Least-Squares Deconvolution (LSD), indicating the presence of brightness inhomogeneities and magnetic fields at the surface of LkCa4. Using tomographic imaging, we reconstruct brightness and magnetic maps of LkCa4 from sets of unpolarized and circularly-polarized LSD profiles. The large-scale field is strong and mainly axisymmetric, featuring a ~2kG poloidal component and a ~1kG toroidal component encircling the star at equatorial latitudes - the latter making LkCa4 markedly different from classical TTauri stars of similar mass and age. The brightness map includes a dark spot overlapping the magnetic pole and a bright region at mid latitudes - providing a good match to the contemporaneous photometry. We also find that differential rotation at the surface of LkCa4 is small, typically ~5.5x weaker than that of the Sun, and compatible with solid-body rotation. Using our tomographic modelling, we are able to filter out the activity jitter in the RV curve of LkCa4 (of full amplitude 4.3km/s) down to a rms precision of 0.055km/s. Looking for hot Jupiters around young Sun-like stars thus appears feasible, even though we find no evidence for such planets around LkCa4.