0 Assigned ID: 467 ----- Attempting Submission 1 (Fri Apr 07 10:13:00 GMT 2017) ----- HST Phase I Proposal 467 (WASP117b.apt) successfully submitted. Receipt: # 467-1 0 WASP-117b is an unique exoplanet as it's elliptical orbit causes the temperature of the atmosphere to cross the 970 K temperature boundary, which separates the disequilibrium and equilibrium chemistry regime. Due to the long orbital period (10.022 days) compared to chemistry and dynamical time scales and due to the orbital orientation, we can expect the planet to be well in the disequilibrium chemistry temperature regime (Teff <970 K) during transit, which is identifiable by the under-abundance of methane compared to predictions with equilibrium chemistry. To characterize the atmosphere of WASP-117b, we thus propose to observe its primary transit with HST/WFC3. It is the only instrument currently available with sufficient stability to cover the 6 hour long, relatively deep (0.75%) transit over a quiet, bright (mv=10.14) host star and to obtain accurate transit spectroscopy to constrain methane-content in the atmosphere of WASP-117b. Two important science objectives can thus be fulfilled with only one HST measurement: a) Determining if the planet's atmosphere is in chemical equilibrium or not and b) Constraining the effective temperature, albedo and cloud coverage. Since the planet is expected to spend several days each in the disequilibrium and equilibrium chemistry regime over the course of one orbit, HST observations will also provide a vital first input for planning future phase resolved JWST observations. The latter will allow to investigate atmosphere and cloud composition changes as WASP-117b switches in between chemical regimes. We ask for a single HST/WFC3 transit observation in spatial scan and round trip mode over the course of 11 or 9 orbits (see below): We choose scan rate of 0.12 arcsec/sec and scan length of 10 arcsec with FGS control. Pos y for the scan was adjusted to -4.84. Each orbit begins with a direct image through the F126N filter for wavelength calibration. This scan rate was selected, using the exposure time calculator (ETC, pandexo) to keep the electron count [e-] between 20 000 and 40 000 per pixel over the observed 1.1 - 1.6 micron wavelegth range. This selection minimises non-linerarity in the ramp effect as the dominant source for systematics in WFC3, making it easier to correct for it. We further select the G141 grism to resolve the 1.1 -1.7 near infrared wavelength range with subarray GRISM 256, using sample mode SPARS10 and 15 samples. By resampling the G141 spectra into 15 bins, the resulting transmission spectra will have 50 ppm precision We like to use one visit with 11 or (alternatively) 9 consecutive orbits, pending success of ground based broad band photometry observations, which become possible after end of July. Thus we ask for observation to start after October 2017 to ensure success of observation. We are aware that such long visit with more than 8 orbits will have one orbit during the South Atlantic Anomaly without fine guidance. We accept this, but ask (1) that the SAA crossing orbit should not include orbits 4-9 for the 11 orbit case (without transit ephemeris update from ground based observations) or (2) that the SAA crossing orbit is put as the first orbit for the 9 orbit case (with transit ephemeris update from ground based observations) Planets and Planet Formation 6 false false false false false false false SUPPORTED STAR EXTRA-SOLAR PLANETARY SYSTEM F3-F9 This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/IR Spectroscopic No No G141 1567203462379 The scheduling data was generated by CASM 27.2. 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