ULTRAVIOLET SPECTROSCOPY OF THE HOTSPOT IN THE CLASSICAL T TAURI STAR DI CEP : OBSERVATIONAL INDICATIONS OF MAGNETICALLY CHANNELLED ACCRETION

T Tauri stars (TTS) are low-mass pre-main-sequence stars that are accreting mass from the surrounding disc. The hotspots detected in some of them are probably heated by the release of gravitational energy in the accretion of the disc material on to the star. In this work we study the UV spectrum of the hotspot detected in DI Cep to constrain the physical mechanisms heating the spot and to study the possible role of the magnetic field in channelling the accretion flow. DI Cep is a classical TTS, classified as G8 IV, with a hotspot (T similar to 8500 K) covering 1-3 per cent of the visible hemisphere. We have carried out a monitoring campaign with the Short Wavelength spectrograph (1200-2000 Angstrom) and the optical FES Camera of the International Ultraviolet Explorer (IUE) from 1992 July 12 to 26. The UV spectrum of DI Cep shows excess emission in the continuum from 1700 Angstrom towards longer wavelengths with respect to a G8 IV star. The far-UV spectrum is dominated by strong emission lines of OI, CIV, Si IV, Si II and Si III], with typical surface fluxes of similar to 10(6) erg cm(-2) s(-1). The UV fluxes (lines and continuum) vary in phase and reach the maximum when the optical flux (FES) does. The light curves are similar in all the lines: the emission from the hotspot is detected above a baseline flwe probably produced by the stellar atmosphere. There is a broad range of temperatures in the hotspot (from 10(4) to 10.5 K) that is similar to that observed in the plages of magnetically active cool stars (e.g. II Peg). However, in DI Cep the light cuwes of the UV lines and continuum are correlated with the optical continuum (T/-bnnd) light curve. DI Cep as a whole deviates only slightly from active stars in the CIV-Si II and CIV-CII flux-flux relations (there is a factor of 2 excess of Si II with respect to CIV when compared with the regression line fitted to active stars). This suggests that the chromosphere and transition region of DI Cep are heated by a mechanism similar to that of the active main-sequence stars. However, the spot is significantly shifted from these relations in the flux-flux diagrams, displaying an excess of Si II (or a defect of CIV) with respect to the surface fluxes emitted by magnetically active stars. The spot alone radiates as much energy as the rest of the atmosphere, and the spot surface fluxes are similar to 10(8) erg cm(-2) s(-1) (typically 2 orders of magnitude larger than those corresponding to the atmosphere). Our observations support the theories in which the accreting material is magnetically channelled on to the stellar surface. Variations in the temperature of the spot between observations taken 1 year apart suggest that the infalling material is more likely channelled by a transient loop structure attached to the star than by a strong stellar dipolar field. The total energy radiated in the far UV lines plus the UV continuum excess is greater than or equal to 0.07 L. This can be accounted for by the accretion of greater than or equal to 6x10(-9) M. yr(-1) from the corotation radius (8.3 R*).