Large-scale magnetic topologies of early M dwarfs^★

Abstract

We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0–M3), that is above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarized profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large‐scale magnetic topologies of six early M dwarfs. We find that early‐M stars preferentially host large‐scale fields with dominantly toroidal and non‐axisymmetric poloidal configurations, along with significant differential rotation (and long‐term variability); only the lowest‐mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large‐scale field resembling those found in mid‐M dwarfs. This abrupt change in the large‐scale magnetic topologies of M dwarfs (occurring at spectral type M3) has no related signature on X‐ray luminosities (measuring the total amount of magnetic flux); it thus suggests that underlying dynamo processes become more efficient at producing large‐scale fields (despite producing the same flux) at spectral types later than M3. We suspect that this change relates to the rapid decrease in the radiative cores of low‐mass stars and to the simultaneous sharp increase of the convective turnover times (with decreasing stellar mass) that models predict to occur at M3; it may also be (at least partly) responsible for the reduced magnetic braking reported for fully convective stars.

Effects of M dwarf magnetic fields on potentially habitable planets

“…Using ZDI, one can reconstruct the large-scale magnetic field (intensity and orientation) at the surface of the star from a series of circular polarisation spectra. In this work, we concentrated on dM stars and used the maps that were published in Donati et al (2008) and Morin et al (2008bMorin et al ( , 2010. Our sample consists of 15 stars with masses ranging from 0.1 to 0.75 M , spanning spectral types M0−M6.…”

“…Table 1 shows some properties of the entire sample of dM stars considered. Data from the first eight columns are taken from Donati et al (2008) and Morin et al (2008bMorin et al ( , 2010. Effective temperatures and bolometric luminosities are derived from NextGen models (Baraffe et al 1998).…”

“…Part of this limitation is justified by the fact that it was only recently that the large-scale magnetic field of dM stars was reconstructed for the first time . Since then, new observations showed that dM stars can harbour magnetic fields that are quite different from the solar one both in intensity and topology (Donati et al 2008;Morin et al 2008aMorin et al ,b, 2010Phan-Bao et al 2009). In particular, because of the close location of their HZ, planets orbiting in this region interact with significantly more intense interplanetary magnetic field than does the Earth.…”

Exploring the magnetic field complexity in M dwarfs at the boundary to full convection

“…However, these techniques are very challenging to apply for such faint objects. Nevertheless, it is still possible to observe the brightest objects and use a least square deconvolution (LSD) method (see Donati et al 1997) to construct an average line profile based on selected individual atomic lines for which Landé factors are accurately known, as was done in a series of papers by Donati et al (2006Donati et al ( , 2008 and Morin et al (2008). The authors found the presence of regions with strong magnetic fields that occupy large surface areas in selected M dwarfs, but the field geometry of the majority of them are dominated by the poloidal component, which indicates rather homogeneous magnetic fields.…”

Probing the magnetosphere of the M8.5 dwarf TVLM 513−46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

“…This is the case of the ECM emission that originates in a laminar source region. The existence of a large-scale well-ordered magnetic field in the fast-rotator fully-convective M-type stars (Donati et al 2006(Donati et al , 2008Reiners & Basri 2007Morin et al 2008aMorin et al ,b, 2010, could explain the observed similarity of the auroral radio emission in these extreme classes of stars, like the case of hot magnetic stars.…”