Modeling the RV jitter of early M dwarfs using tomographic imaging

TL;DR

This study demonstrates how Zeeman Doppler Imaging can effectively model and reduce stellar activity-induced radial velocity jitter in early M dwarfs, improving exoplanet detection prospects.

Contribution

The paper introduces a modified ZDI technique applied to spectropolarimetric data, enabling detailed magnetic and surface activity mapping of early M dwarfs, and quantifies its effectiveness in jitter reduction.

Findings

ZDI can decrease activity jitter amplitude by factors of 2-3, up to 6 in optimal cases.

Dark spots cover less than 2% of stellar surfaces in the sample.

Magnetic field topologies suggest bi-stability of dynamo processes in these stars.

Abstract

In this paper we show how tomographic imaging (Zeeman Doppler Imaging, ZDI) can be used to characterize stellar activity and magnetic field topologies, ultimately allowing to filter out the radial velocity (RV) activity jitter of M-dwarf moderate rotators. This work is based on spectropolarimetric observations of a sample of five weakly-active early M-dwarfs (GJ 205, GJ 358, GJ 410, GJ479, GJ 846) with HARPS-Pol and NARVAL. These stars have v sin i and RV jitters in the range 1-2 km/s and 2.7-10.0 m/s rms respectively. Using a modified version of ZDI applied to sets of phase-resolved Least-Squares- Decon- volved (LSD) profiles of unpolarized spectral lines, we are able to characterize the distribution of active regions at the stellar surfaces. We find that darks spots cover less than 2% of the total surface of the stars of our sample. Our technique is e cient at modeling the rotationally mod- ulated component of the activity jitter, and succeeds at decreasing the amplitude of this com- ponent by typical factors of 2-3 and up to 6 in optimal cases. From the rotationally modulated time-series of circularly polarized spectra and with ZDI, we also reconstruct the large-scale magnetic field topology. These fields suggest that bi-stability of dynamo processes observed in active M dwarfs may also be at work for moderately active M dwarfs. Comparing spot distributions with field topologies suggest that dark spots causing activity jitter concentrate at the magnetic pole and/or equator, to be confirmed with future data on a larger sample.