On the magnetic topology of partially and fully convective stars

¹ Universität Göttingen, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: Ansgar.Reiners@phys.uni-goettingen.de
² Astronomy Department, University of California, Berkeley, CA, 94720, USA e-mail: basri@berkeley.edu

Received: 1 December 2008
Accepted: 2 January 2009

Abstract

We compare the amount of magnetic flux measured in Stokes V and Stokes I in a sample of early- and mid-M stars around the boundary to full convection (~M 3.5). Early-M stars possess a radiative core, mid-M stars are fully convective. While Stokes V is sensitive to the net polarity of magnetic flux arising mainly from large-scale configurations, Stokes I measurements can see the total mean flux. We find that in early-M dwarfs, only ~6% of the total magnetic flux is detected in Stokes V. This ratio is more than twice as large, ~14%, in fully convective mid-M dwarfs. The bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This is presumably because magnetic flux is mainly stored in small scale components. There is also more to learn about the effect of the weak-field approximation on the accuracy of strong field detections. In our limited sample, we see evidence for a change in magnetic topology at the boundary to full convection. Fully convective stars store a 2–3 times higher fraction of their flux in fields visible to Stokes V. We estimate the total magnetic energy detected in Stokes I and compare it to results from Stokes V. We find that in early-M dwarfs only ~0.5% of the total magnetic energy is detected in Stokes V while this fraction is ~2.5% in mid-M dwarfs.