The Ultraviolet Environment of Protoplanetary Systems

Young stars undergoing the conversion of pre-main-sequence circurnstellar disks into protoplanetary systems are strong sources of high energy (FUV/EUV/X-ray) radiation that controls the physical and chemical processes in their circumstellar environment out to hundreds of AU from the star. The high energy emission, both as radiation and particles, resulting from magnetic activity on the central star controls the thermal structure of disks, the formation process of planetesimals, and the photoexcitation and photoionization of protoplanets and young planetary atmospheres. Modeling of the dust and gas evolution requires an accurate understanding of the local radiation field throughout the UV and X-ray spectral regions, even those parts of the spectrum that are impossible to observe from Earth. Most young stars are well over 100 pc from the Sun and are consequently hard to observe in the UV at even moderate spectral resolution. Fortunately, members of the "Local Association" [offshots from the ongoing Sco-Cen star-formation complex], whose ages range from 7 Myr to a few hundred Myr, surround the Sun at distances of 50 pc or less and permit the detailed study of the early evolution of stellar activity. I review the current understanding of UV and Xray stellar activity during pre-main-sequence evolution and within the various moving groups of the Local Association, and discuss how modeling observations of a few well-studied examples could improve our understanding of the role of the stellar radiation in shaping the protoplanetary environment.