The current Call for Proposals is for HST Cycle 34. 0 Assigned ID: 6957 ----- Attempting Submission 1 (Sun Mar 29 08:01:40 GMT 2026) ----- This is a JWST Cycle 5 joint JWST - HST proposal. The phase 1 is requested to select Cycle 33. FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser 0 ----- Attempting Submission 1 (Tue Apr 28 12:16:46 GMT 2026) ----- The background flux at the planets' locations is expected to be high because the planets are embedded in the bright stellar halo. Therefore, we choose to ignore the warnings regarding the FLASH levels and do not apply additional FLASH. Proposal Characterizing the Accreting Protoplanet Embedded in the Protoplantary Disk The planetary growth rate, specifically the mass accretion rate, is a key parameter in planet formation, as it ultimately determines a planet's final mass. Traditionally, accretion rates have been estimated using hydrogen emission lines based on the accretion flow model, originally developed for stars. However, Aoyama et al. recently proposed the accretion shock model as an alternative mechanism for hydrogen line emission. For planetary-mass objects, the shock model predicts accretion rates up to an order of magnitude higher than the flow model, making it crucial to determine which mechanism dominates. We propose a joint JWST-HST observing program of the protoplanet WISPIT 2 b to distinguish between the two models using hydrogen line ratios. For the first time, this study will enable more accurate estimates of accretion rates in accreting planets embedded within protoplanetary disks. Our pilot observations pave the way for applying these models to characterize future discoveries with JWST and the ELTs. We request contemporaneous JWST and HST observations as described in Science Justification. While Pa-alpha and Pa-beta will be observed with JWST, H-beta will be observed with HST/WFC3. Below, we provide the observing description for the HST observations. We propose a two-orbit campaign to observe the accreting planet WISPIT 2 b using the F487N filter. The C512C-SUB array of the WFC3/UVIS2 detector will be employed to minimize readout overheads. Since we will apply the Reference Star Differential Imaging (RDI) technique for PSF subtraction to mitigate self-subtraction, one orbit will be allocated to the target and the other to the reference star. We request that Visit-01 for the target (WISPIT 2 b) be fully scheduled within the JWST observing window. In contrast, Visit-02 for the reference star does not need to overlap with JWST observations. Each orbit will consist of four 224 s exposures to improve spatial sampling (see below), resulting in a total integration time of 896 s per orbit in F487N. Note that the exposure time has been reduced from 720 s in the Phase I proposal to 224 s to avoid saturation. The expected SNR is 7, which is sufficient to achieve our science goals. We adopt a four-point box dithering pattern to improve the spatial sampling of the images. The native pixel scale of 40 mas in the UVIS channel is undersampling for the spatial resolution of 70 mas in the F487N filter. The dithering distance will be half a pixel (20 mas) in either direction. At each dithering position, eight 28-second exposures will be taken to mitigate cosmic rays. The total integration times per orbit will be 896 s. The same procedure is applied to both the target and the reference star. The background flux at the planets' locations is expected to be high because the planets are embedded in the bright stellar halo. Therefore, we choose to ignore the warnings regarding the FLASH levels and do not apply additional FLASH. PI Jun Hashimoto is an expert of observing accreting exoplanets and brown dwarfs (e.g., Hashimoto et al. 2020, AJ, 159, 222; Hashimoto et al. 2025, AJ, 169, 93), and protoplanetary disks (e.g., disk observations; Hashimoto et al. 2025, AJ, 170, 156) at optical to radio wavelengths. In addition, he is PI of the ongoing intensive program with Subaru/SCExAO (S25B-065I: Planet-Forming Disks around M-type stars in Taurus). CoI Yuuhiko Aoyama, Gabriel Marleau, and Yasuhiro Hasegawa are experts on numerical simulations of accreting planets (e.g., Aoyama et al. 2018, ApJ, 86, 84; Aoyama, Marleau, et al. 2021, ApJ, 917, 30; Marleau 2025, ApJ, arXiv251000138; Hasegawa et al. 2024, AJ, 167, 105). CoI Taichi Uyama has sufficient experiences in JWST observations (e.g., Uyama et al. 2025 AJ, 169, 287). CoI Nagayoshi Ohashi was PI of the eDISK project that is an ALMA large program (Ohashi et al. 2023, ApJ, 951, 8). CoI Michihiro Takami has intensively studied planets and protoplanetary disks (e.g., Takami et al. 2025, PASP, 137c4504T). Exoplanets and Exoplanet Formation 6 false false false false false false false true SECONDARY JWST 11164 false SUPPORTED STAR EXTRA-SOLAR PLANETARY SYSTEM NO This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 STAR K III-I NO This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. J2000 J2000 WFC3/UVIS Imaging No No F487N WFC3/UVIS Imaging No No F487N POS-TARG DITHER 0.02 LINE 0.02 false false 90 4 0 WFC3-UVIS-DITHER-BOX 1 false /tmp/OPVisit199361368702812259.xml Timing link suitability includes constraints ([02 After 01 by at most 0.1 days]) The scheduling data was generated by CASM 34.0. CASM was run in Phase II mode. 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(Tic-Visit :proposal-id 18245 :visit-id "02" :proposal-version -1 :trans-version "20250929" :date "28-Apr-2026 18:49:43.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 146448 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18245 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18245 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 40 :internal-dur-at-end 197 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2977 :pcs-gap 0 ) true 1 0