----- Attempting Submission 1 (Thu May 10 20:07:00 EDT 2007) ----- Assigned ID: 470 ----- Attempting Submission 1 (Fri Jan 26 17:09:58 EST 2007) ----- A Hard Look at Stellar Disks at the Epoch of Planet Formation We propose to use HST/ACS/SBC and Chandra/ACIS-S3 to observe the high energy fluxes of 4 stars surrounded by disks in the newly discovered aggregate 25 Ori, the most populous 10 Myr group known within 500 pc. Our observations will cover the 1-25A and 1250-2000A bandpasses, and will complement our optical and Spitzer data for these objects, to provide essential input to physically-consistent models of disk structure and chemistry in the age range around 10 Myr, thought to be a critical period in the planet-forming process. We will be able to determine the Ne/O ratio and determine if the anomalous metal abundances observed in X-ray spectra of young stars are an evolutionary or an environmental effect. Our proposed observations will double the number of 10 Myr old accreting stars with known high energy radiation fields, and will be the first FUV observations of low mass accreting stars in an OB association.
ACS prism observations are being made of 4 young stellar objects in 25 Ori to study the FUV continuum of these objects. The spectral element is the PR130L being used with the ACS/SBC. A single observation of an orbit in length is scheduled for OB1A-776 and OB1A-1192. For OB1A-1200 and OB1A-1630, the observation will be 2 orbits in length. A pair of short observations is also scheduled, as these are needed for wavelength calibration. The pair of short images is being taken with the F140LP filter on the SBC/ACS. One of our targets (OB1A-1192) was identified by the BOT tool as posing a hazard to the ACS/SBC based on assuming an incorrect spectral type of 05. The correct spectral type for this object is K7. Within the visit for this object, BOT also identifies the following target as a hazard: N91A012945 at 05 25 47, +01 45 38.34. However, plotting this object on a 2MASS color-color diagram shows that it cannot be higher than an F0 star and it therefore falls within the limits of Table 7.4 in the ACS handbook. 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