Dynamo processes in the T Tauri star V410 Tau

Abstract

We present new brightness and magnetic images of the weak‐line T Tauri star V410 Tau, made by using data from the Narval spectropolarimeter at Télescope Bernard Lyot (TBL). The brightness image shows a large polar spot and significant spot coverage at lower latitudes. The magnetic maps show a field that is predominantly dipolar and non‐axisymmetric with a strong azimuthal component. The field is 50 per cent poloidal and 50 per cent toroidal, and very little differential rotation is apparent from the magnetic images. A photometric monitoring campaign on this star has previously revealed V‐band variability of up to 0.6 mag, but in 2009 the light curve is much flatter. The Doppler image presented here is consistent with this low variability. Calculating the flux predicted by the mapped spot distribution gives a peak‐to‐peak variability of 0.04 mag. The reduction in the amplitude of the light curve, compared with previous observations, appears to be related to a change in the distribution of the spots rather than the number or area. This paper is the first from a Zeeman–Doppler imaging campaign being carried out on V410 Tau between 2009 and 2012 at the TBL. During this time, it is expected that the light curve will return to a high‐amplitude state, allowing us to ascertain whether the photometric changes are accompanied by a change in the magnetic field topology.

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

“…The probable absence of a radiative core and the non-detection of differential rotation in this work and other investigations Skelly et al 2010) leaves not much room for speculation about the nature of the dynamo operating in the interior of V410 Tau -at least in the framework of classical mean field dynamos. As has been shown by Küker & Rüdiger (1999) a likely explanation for the generation of the magnetic field in WTTS can be given in terms of a α 2 -dynamo.…”

“…In a recent magnetic investigation of V410 Tau Skelly et al (2010) found that 50% of the total field has a toroidal component and the other 50% are poloidal and that the majority of the field shows strongly inclined large-scale azimuthal fields which are distributed over one entire hemisphere, whereas our reconstructed topology has only 27% toroidal fields and 73% poloidal fields with a strong radial and bipolar component around the polar region. Moreover, in contrast to our reconstructed maps where the strong magnetic fields are well correlated with the temperature, see Fig.…”

“…Without having the possibility for doing such a detailed comparison of the data as well as of the inversion codes we can only speculate about the reasons for the disparity of the two reconstructions. But we want to emphasize that in contrast to Skelly et al (2010) our approach makes no assumptions on the surface topology in terms of a spherical harmonic decomposition or potential field structure. It is furthermore fully based on polarized radiative transfer and line profile modeling and it pursues a strategy which simultaneously invert the temperature and magnetic field.…”

“…At the end we also want to comment on the apparent differences between the reconstructed surface fields of Skelly et al (2010) and our magnetic and temperature maps. In a recent magnetic investigation of V410 Tau Skelly et al (2010) found that 50% of the total field has a toroidal component and the other 50% are poloidal and that the majority of the field shows strongly inclined large-scale azimuthal fields which are distributed over one entire hemisphere, whereas our reconstructed topology has only 27% toroidal fields and 73% poloidal fields with a strong radial and bipolar component around the polar region.…”

“…More direct evidence of photospheric magnetic activity could be obtained by Doppler images, in particular for V410 Tau (Joncour et al 1994;Strassmeier et al 1994;Hatzes 1995;Rice & Strassmeier 1996;Rice et al 2011). Skelly et al (2010) presented the first Zeeman-Doppler map of V410 Tau. Surprisingly, the reconstructed large-scale field exhibit a rather complex structure dispersed over different latitudes with strongly inclined fields that are only weakly correlated with the spots shown in their brightness maps.…”