Understanding the atmospheric structure of T Tauri stars - I. Improved atomic physics applied to IUE data of BP Tauri

Recent advances in modelling the radiating character of dynamic laboratory and astrophysical plasmas are applied here in a new examination of the prop erties of the atmosphere of the classical T Tauri star BP Tau. We analyse a rchived International Ultraviolet Explorer (IUE) UV spectra of BP Tau. We a dopt a collisional-radiative model and utilize emission measure (EM) and di fferential emission measure (DEM) techniques to try to constrain the distri bution of emitting material in temperature in the atmosphere of this star. We use spectroscopic diagnostic techniques to probe atmospheric parameters such as electron density, and to set constraints on the volume of emission regions. This work is important for understanding the fundamental propertie s of BP Tau and other T Tauri stars, and for providing a more complete basi s for models of their atmospheres.