0 Assigned ID: 1571 ----- Attempting Submission 2 (Fri Mar 01 17:34:54 GMT 2013) ----- We propose to analyze archival HST-COS/STIS data with new spectral modeling techniques to determine the UV radiation field and molecular gas properties of the inner regions of low-mass protoplanetary disks (PPDs). HST-COS/STIS observations show in unprecedented detail a population of Lyman Alpha (LyA) photo-excited molecular gas in the inner disk (r < 10 AU), where the UV radiation field plays a strong role in gas abundances and planetary formation. While the LyA emission is scattered by interstellar and circumstellar material along the line of sight to most targets, the fluoresced CO and H2 emission lines are prevalent. We will simultaneously reconstruct the incident LyA radiation and determine the physical and dynamical properties of the fluorescing gas in a large sample (~40) of PPDs. Initial studies of some of these targets suggest a CO/H2 abundance ratio 3 to 4 orders of magnitude larger than expected from interstellar abundances, which would have direct bearing on the inferred gas mass available for planet-formation. However, this inference is based on different modeling techniques and different data sets from a very small number of objects. With the proposed large sample, a systematic approach will mitigate uncertainties related to time-variability and different model approaches, enabling accurate determination of relative abundances. We will also use the model LyA profiles and gas properties to determine extinctions towards each target. This AR proposal is submitted in parallel with a GO proposal to observe PPDs in the Orion Nebular Cluster, applying this same analysis to PPDs under external illumination from strong O and B star UV radiation fields. DEBRIS DISKS Default Medium false true false false false SUPPORTED