The Balmer wavelength range of BP Tauri

We have analyzed all the observations of BP Tauri taken by the International Ultraviolet Explorer (IUE) in the low-resolution (Delta lambda similar to 6 Angstrom), long-wavelength (lambda = 1850-3350 Angstrom) range. This data set contains 61 spectra. We observe variability in the ultraviolet continuum of Delta m(cont) similar to 1 mag and variability in the Mg II line flux of Delta m(Mg II) similar to 0.8 mag. Moreover, these spectra do not show any correlation between the continuum flux and the Mg II line flux, thus resolving a standing controversy in the literature concerning the origin of the Mg II line flux. There is no correlation between the color temperature of the UV continuum and the average value of its flux. Using models of the accretion process recently developed by other authors, we obtain energy fluxes, accretion spot sizes, and accretion rates from the IUE observations of BP Tauri. We find average energy fluxes of 5.0 x 10(11) ergs cm(-2) s(-1) average spot sizes of 4.4 x 10(-3) times the stellar surface, and average accretion rates of 1.6 x 10(-8) M. yr(-1) Our analysis shows that the particle energy flux and the UV flux in the stellar surface are proportional to each other. Most strikingly, we observe a correlation between accretion rate and spot size, with the spot size increasing as the square of the accretion rate. Based on the results of a simulation, we conclude that geometrical effects (i.e., the varying inclination of the spot with respect to the observer) are not enough to account for this effect. Current models of the accretion process fail to reproduce such an effect, suggesting the need of using more realistic descriptions of the stellar field when treating magnetospheric accretion. There may also be an unmodeled efficiency factor that determines how matter is loaded into the field lines. Nondipole fields, geometry, oblique shocks, and the possibility of "limb brightening" should be taken into account when creating models and explaining the results of observations of T Tauri stars.