0 Assigned ID: 2404 ----- Attempting Submission 1 (Fri Apr 09 22:57:37 GMT 2021) ----- HST Phase I Proposal 2404 successfully submitted. Receipt: # 2404-1 LONG SU LIKELY TO INTERSECT THE SAA We encountered an issue with timing requirements. To maximize the number of secondary eclipses observed in our visit, we require the visit to begin at a planetary orbital phase between 0.39 and 0.54. However, placing this Phase constraint on the first exposure of the visit (after supplying the Period and Zero Phase HJD for the Visit) results in an error: "PHASE CRITICAL VISIT HAS PHASE WINDOW LARGER THAN PERIOD." To ensure successful submission of the Phase II, we have removed this timing constraint, but would still like to apply it to the visit to ensure the best result from our observations. Thank you! -482963072 ----- Attempting Submission 1 (Thu Jul 29 20:53:52 GMT 2021) ----- Our visit is a continuous observation of ~3 planetary orbital phases for this ultra short period planet. If executed continuously, the ratio of HST to planet orbit will result in good phase coverage. If broken into multiple visits, the planetary orbital phase at start of observations would need to be scheduled carefully (with, e.g., the timing of subsequent visits dependent upon the timing of previous visits), and data separated by many days or weeks may also be affected by changing levels of stellar activity. Scheduling and data analysis will therefore likely be simpler for a single 11-orbit visit, and photon-limited measurement precision has been achieved for past phase-curve observations totalling more than 11 orbits (e.g. Stevenson et al. 2014, Kreidberg et al. 2018). We do not expect an SAA crossing (if it must occur) to significantly affect our data quality. Existing on the extreme edges of the exoplanet population, ultra short period (USP) rocky planets offer new opportunities to study the physicochemical properties of planetary interiors, surfaces, and atmospheres. The immense irradiation and tidal forces experienced by these planets open observational avenues that remain closed for their long-period counterparts: their primordial atmospheres photoevaporate, opening their surfaces to observation; their tidally locked daysides melt and vaporize, enabling observation of surface material in new phase states; and unless heat is redistributed efficiently, their rock vapor atmospheres may condense on their cold nightsides. We propose for 11 orbits of WFC3 data to measure the phase curve of the lava world TOI-2431b, a rocky planet with a 5.4-hour orbital period, the shortest among planets with a known density. It is the best rocky planet for thermal emission studies using HST/WFC3, and with the majority of the sky now surveyed by TESS, it is likely to remain so. The planet is expected to have lost its primordial atmosphere, but it may well possess a rock vapor atmosphere sustained by its surface magma ocean. These observations will allow us to: 1) determine the Bond albedo of the planet, which can constrain the properties of the surface magma ocean; 2) to detect a rock vapor atmosphere or a thicker volatile atmosphere from its spectrum or the presence of a hotspot offset indicative of heat redistribution; and 3) constrain the interior structure by refining the planetary radius and iron mass fraction of the planet, which will help inform USP planet formation models and the design of future observations. Our observations will consist of time-series WFC3/G141 spectroscopy covering 3 orbital phases of TOI-2431 b. Because of the short orbital period, this is only 11 HST orbits. We require 3 orbital phases to acquire full phase coverage given the visibility of this equatorial target (52 minutes per orbit). If executed continuously, the ratio of HST to planet orbit will result in good phase coverage; if broken into multiple visits, the planetary orbital phase at start of observations would need to be scheduled carefully. Scheduling will therefore likely be simpler for a single 11-orbit visit, and photon-limited measurement precision has been achieved for past phase-curve observations totalling more than 11 orbits (e.g. Stevenson et al. 2014, Kreidberg et al. 2018). We therefore request a single 11-orbit visit, understanding that the SAA might not be avoided. To determine the optimal scan rate and readout pattern, we used the open-source PandExo package available on GitHub, which includes both HST and JWST observation planning utilities. We will use the 512x512 subarray to improve the duty cycle and ensure that there is a sufficient region on the detector to estimate the background. Each orbit will begin with a direct image of the target star for wavelength calibration. The remainder of each orbit will consist of G141 spectroscopy. We will observe in spatial scan mode to keep the maximum per pixel counts below 40000 electrons. We will use the SPARS25/NSAMP=7 readout mode and a scan rate of 0.2226 arcsec/sec. To maximize the duty cycle of the observations, we will alternate between forward and reverse scanning along the detector. With 3 continuous orbits of the planet corresponding to 11 HST orbits, we will sample the full phase of the planet with the precision required to determine the dayside temperature, bond albedo, and redistribution with high precision. With our observing design, we will be able to distinguish between a thin gas melt equilibrium atmosphere and a range of atmospheric redistributions for the planet. For redistributions greater than 33% the difference will be >3 sigma. The PI is an expert in the identification and characterization of transiting planets with a focus on observational constraints on planet formation and migration. The Co-PI is an expert in the analysis of exoplanet atmospheres and phase curves, particularly for lava worlds. The Co-Is include experts in the identification of transiting planets, the measurement of their masses, the characterization of their atmospheres and surfaces, and the testing and development of atmospheric models, including for USP rocky planets. Collectively, we have obtained and analyzed hundreds of orbits of HST data, Spitzer data sets, and ground-based observations to study exoplanet atmospheres and phase curves, leading to numerous papers, including on the subject of USP rocky planets. We are well positioned to perform the WFC3 data analysis, generate models, and interpret and publish results. Exoplanets and Exoplanet Formation 6 false false false false false false false SUPPORTED STAR EXTRA-SOLAR PLANETARY SYSTEM K V-IV HIP11707 TIC258804746 J2000 J2000 J = 8.356; H = 7.729; K = 7.554 WFC3/IR Spectroscopic No No G141 1642004748863 /private/var/folders/z3/xy_g1v_s5fz9yrhzb8thz4cr0001qc/T/OPVisit16669485445415029602.xml The scheduling data was generated by CASM 29.2. CASM was run in Phase II mode. 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