Current measurements suggest that the transiting exoplanet HD 149026b is a "super-Neptune," with an enormous heavy-element core. The existence of such a planet is a major challenge to planet formation theories. We propose to place the radius measurement on much firmer footing, by obtaining a NICMOS light curve with 0.4 mmag precision and 13 sec cadence. We will improve the radius measurement by a factor of 2.3, and more importantly, the result will be more robust because we will determine the stellar radius directly from the data. Numerous attempts to do this from the ground have failed.

HD 149026 will be observed in a special time series mode using the G141 grism as was tested in CAL/NIC 9642 and used in GO 9832. The approach for setting defocus will be adopted from CAL/NIC 11335 -- see text in Additional Comments below. First orbits will include a specification of orbital phase for the observations, a direct image to fix wavelengths and some exposures at different count levels with the grism to internally verify linearity. Orbits 2-5 will be time series of 4.0 s exposures using STEP2, NSAMP=5. Peak counts per pixel for the grism spectra should reach 85,000 e-, with sum over the full first order spectrum yielding 3.7e7 e- per exposure. The phasing is critical. Following previous NICMOS experience we do not expect that Orbit 1 will be sufficiently stable to contribute to analyses. We need most of orbit 2 to be before the transit starts, and most of orbit 5 to be after transit ends in order to provide data during times of stable photometry to enable removal of systematics that correlate with HST pointing, instrument temperature changes and focus drift. We also need the light curve composed of all four visits to yield nearly full phase coverage including ingress and egress. IMPLEMENTATION NOTES: Program uses NIC3-FIXD and CAMER-FOCUS= DEFOCUS optional parameter to repeat the degree of defocus used in GO-9832. STAR FORMATION 3 false 0.0 0.0 false false false false false AVAILABLE STAR G V-IV Hipparcos position and proper motions. J2000 16 30 29.6192 +38 20 50.32 J2000 NIC3 Spectroscopic No No G141 Phase and BETWEENs have been set with narrow ranges to select the 2007:356:02 opportunity identified via test scheduling. Should this, or any of the other narrow constraints on Visits 1-4 prove impossible to satisfy, then these will need to be revised to allow alternatives. 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In focus would be at about 80% of saturation. The POS TARGs used throughout these observations are intended to place the first order spectrum over x = 86-197, y = 63, i.e. on a very clean position in terms of flat field deviations and bad pixels. This is similar to position used previously for HD 209458 observations with minor tweak. Expected exposure level is about half of full well depth in center of spectrum. Narrow phase and BETWEEN constraints have been added to select a window at 2007:358:22 identified through early test scheduling. If unavailable in reality these will need to be changed. 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