The Balmer Wavelength Range of BP Tauri

We have analyzed all the observations of BP Tauri taken by the InternationalUltraviolet Explorer in the low resolution (6 A), long wavelength (from 1850 to 3350 A) range. This dataset contains 61 spectra. These spectra do not showany correlation between the continuum flux and the MgII line flux, thusresolving a standing controversy in the literature concerning the origin of theMgII line flux. There is no correlation between the color temperature of the UVcontinuum and the average value of its flux. Using models of the accretionprocess developed by Calvet & Gullbring (1998), we obtain energy fluxes,accretion spot sizes, and accretion rates from the IUE observations of BPTauri. We find average energy fluxes of 5.0 10^11 ergs cm^{-2} s^{-1}, averagespot sizes of 4.4 10^{-3} times the stellar surface, and average accretionrates of 1.6 10^-8 msun/yr. Our analysis shows that the particle energy fluxand the UV flux in the stellar surface are proportional to each other. Moststrikingly, we observe a correlation between accretion rate and spot size, withthe spot size increasing as the square of the accretion rate. Based on theresults of a simulation, we conclude that geometrical effects are not enough toaccount for this effect. Current models of the accretion process fail toreproduce such an effect, suggesting the need of using more realisticdescriptions of the stellar field when treating magnetospheric accretion. Theremay also be an unmodelled efficiency factor that determines how matter isloaded into the field lines.Comment: 34 pages, 3 tables, 11 figures. Accepted in the Astrophysical Journa