Non-stationary dynamo and magnetospheric accretion processes of the classical T Tauri star V2129 Oph

We report here the first results of a multiwavelength campaign focussing on magnetospheric accretion processes of the classical T Tauri star V2129 Oph. In this paper, we present spectropolarimetric observations collected in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope and contemporaneous photometry secured with the SMARTS facility. Circularly polarized Zeeman signatures are clearly detected, both in photospheric absorption and accretion-powered emission lines, from time-series of which we reconstruct new maps of the magnetic field, photospheric brightness and accretion-powered emission at the surface of V2129 Oph using our newest tomographic imaging tool - to be compared with those derived from our old 2005 June data set, re-analysed in the exact same way. We find that in 2009 July, V2129 Oph hosts octupolar and dipolar field components of about 2.1 and 0.9 kG, respectively, both tilted by about 20 degrees with respect to the rotation axis; we conclude that the large-scale magnetic topology changed significantly since 2005 June (when the octupole and dipole components were about 1.5 and three times weaker, respectively), demonstrating that the field of V2129 Oph is generated by a non-stationary dynamo. We also show that V2129 Oph features a dark photospheric spot and a localized area of accretion-powered emission, both close to the main surface magnetic region (hosting fields of up to about 4 kG in 2009 July). We finally obtain that the surface shear of V2129 Oph is about half as strong as solar. From the fluxes of accretion-powered emission lines, we estimate that the observed average logarithmic accretion rate (in M-circle dot yr-1) at the surface of V2129 Oph is -9.2 +/- 0.3 at both epochs, peaking at -9.0 at magnetic maximum. It implies in particular that the radius at which the magnetic field of V2129 Oph truncates the inner accretion disc is 0.93 and 0.50 times the corotation radius (where the Keplerian period equals the stellar rotation period) in 2009 July and 2005 June, respectively.