I found the explanation for the orientation to be unclear. Personally I feel that the defaults for orientation should always have X = east and Y = north, and the observer should not have to include an orientation angle to obtain this result. I was also surprised to find that the pointings are not centered on the position given in the targets. This should also be the default, and the offsets should already be included. Both of these issue caused a lot of wasted time while I tried to figure out why my spiral patterns didn't look as I expected when overlaid on an image. ----- Attempting Submission 1 (Wed May 09 16:55:18 GMT 2007) ----- Assigned ID: 1177 ----- Attempting Submission 1 (Thu Feb 08 16:06:54 EST 2007) ----- Phase I Proposal 1177 (cover-hst16-rcw38) successfully submitted. A NICMOS Survey for Proplyds in the RCW 38 Massive Embedded Cluster We propose a search for line emission from photoevaporating protoplanetary disks in the Massive Embedded Cluster RCW 38. These disks would be analogous to the "proplyds" discovered in the Orion Nebula: disks around young low mass stars which are being photoionized by a nearby O star. We will search for these disks in RCW 38 using narrowband imaging in the lines of Paschen alpha and molecular hydrogen (1-0) S(1) with NICMOS. The RCW 38 region is an excellent target for determining whether proplyds are observable in large numbers outside of Orion. It is a young embedded cluster hosting a few hundred low mass young stars with a large percentage showing infrared excess indicating the presence of disks. About 100 of these stars are found within 0.1 pc of the central O5 star, and the cluster is located within a cleared cavity 0.2 pc in size, embedded within a molecular cloud, exposing the cluster members directly to the UV radiation from the O star. Unlike Orion, but like many other young clusters, RCW 38 is not seen in visible light, and infrared imaging is needed. The best line in the infrared for revealing proplyds is the Paschen alpha line, which is not detectable from the ground. Only HST is able to perform these observations. From these observations we will estimate the lifetime of the evaporating disks, and ascertain whether these disks will survive long enough to form planets.
The region around the central O5 star (RCW 38 IRS 2) containing most of the cluster members will be imaged with NIC2 with a 3 x 2 mosaic in the Pa-alpha line (F187N) and its narrowband continuum (F190N), and with a 2 x 2 mosaic in the H_2 (1-0) S(1) line (F212N) and its narrowband continuum (F215N). With offsets of 17.5" between frames the field-of-view of the final images will be 51.6" x 35.4" (F187N and F190N) and 35.4" (F212N and F215N). We estimate the sensitivity required for the Pa-alpha observations using the published fluxes of the Orion disks (Churchwell et al. 1987) and estimating the flux of equivalent sources at the distance (1.7 kpc) and extintion (A_V ~ 10) of RCW38. The resulting flux for a typical proplyd is ~3.8e-4 Jy in Pa-alpha. The 3 x 2 mosaic will be imaged with one mosaic per orbit (55 minutes/orbit, and two filters), with an integration time of 300 seconds per mosaic position per filter, resulting in a sensitivity of S/N ~50 for compact sources. We will integrate in both filters before moving to the next position to minimize drift between the Pa-alpha frames and the continuum frames. This will minimize artifacts due to interpixel gain variations in the NICMOS detectors when subtracting the continuum from the line data. Two orbits are required for the observations. In a similar manner we estimate the flux of a typical proplyd to be ~2e-5 Jy in H_2 (Chen et al. 1998). Because it is much fainter than Pa-alpha a longer integration time is required. Hence we restrict ourselves to imaging the inner region with a 2 x 2 mosaic. In order to achive a S/N ~16 a total integration time per mosaic position of 35 minutes is required, or 4 orbits per filter (F212N and its continuum filter F215N). All observations will use the MULTIACCUM readout mode. The 3x2 mosaic has orientation restrictions, but not the 2x2 mosaic. The change in relative orientation between orbits using the same filters need to be small, so that the mosaics can be built. 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