18415( 2) - 14-Aug-2026 16:30:36 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18415 Version: 2 Check-in Time: 14-Aug-2026 16:30:36 STScI Edit Number: 0 Title Hydrodynamic escape in a young sub-Neptune progenitor ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Leonardo Dos Santos Space Telescope Science Institute Y CoI: Prof. Aline Vidotto Universiteit Leiden N CoI: Prof. Kevin France University of Colorado at Boulder N CoI: Madelyn Broome University of California - Santa Cruz N CoI: Mr. Andres Presa Universiteit Leiden N CoI: Prof. David Ehrenreich University of Geneva, Department of N Astronomy CoI: Dr. Vincent Bourrier University of Geneva, Department of N Astronomy CoI: Dr. Joshua D. Lothringer Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract During their early lives (< 100 Myr), planets experience hydrodynamic escape, a process in which their H+He envelopes evaporate vigorously to space and entrain metallic species (such as C, N, O and Mg) in their outflows. This is thought to be the their main evolutionary channel of young (< 100 Myr) sub-Jovian worlds and is used as a convincing explanation for demographic features in the exoplanet population. Although predicted by theory, including for our own early Solar System rocky planets, to this day we lack decisive observational evidence for hydrodynamic escape occurring in a sub-Jovian exoplanet younger than 100 Myr. Thus, this key hypothesis that is foundational for our understanding of planetary evolution remains to be tested against observations. In this program, we propose to observe escaping metals in the young (45 Myr) Neptune-sized exoplanet DS Tuc Ab, which is predicted to be a progenitor to a sub-Neptune, the most common type of exoplanet in the solar neighborhood. We used a leading-edge modeling framework to simulate the transmission spectrum of DS Tuc Ab and a well-tested experimental design to predict observable escape signals; we estimate that, within two transits, we can securely detect escaping carbon and tentatively other species in the upper atmosphere of the planet. The results of this observational campaign will allow us to accurately measure the mass-loss rate of DS Tuc Ab, which has challenging to obtain with previous observations of this planet using escaping H and He. This measurement will enable us to perform the first empirical test to hypotheses involving the role of hydrodynamic escape in exoplanet evolution. ------------------------------------------------------------------------------------ Observations Description ------------------------ We shall observe two transits of the young exoplanet DS Tuc Ab using COS/FUV. The objective is to detect metals escaping from the planet, and producing in-transit absorption in emission lines between 1100 and 1400 angstrom. DS Tuc is a binary system where the A component is brighter, so it is automatically acquired instead of the companion DS Tuc B. The angular separation between the 18415( 2) - 14-Aug-2026 16:30:36 - [ 2] components is 5 arcsec, so DS Tuc B will be outside of the COS aperture. This program is time sensitive because it can only be executed when the planet transits. Thus, we setup this 14-orbit program in the following structure: - Visit 1, pre-transit 1 baseline: 2 consecutive orbits happening ~12 hours before the transit 1 midpoint to establish the out-of-transit spectrum temporally far from the transit center. - Visit 2, transit 1: 5 consecutive orbits starting a few hours before transit center - Visit 3, pre-transit 2 baseline: 2 consecutive orbits happening ~11.25 hours before the transit 2 midpoint (the offset from 12 h is necessary to cover the Earth occultation gaps from transit 1) - Visit 4, transit 2: 5 consecutive orbits starting a few hours before transit center The program is broken into 4 visits instead of 2 because UV visits are limited to a maximum of 5 consecutive orbits. So, we establish pre-transit exposures with enough offset from the transits visits to avoid the SAA passages. To achieve this observing setup, we set phase constraints on the pre-transit visits and set an "After Visit" constraint for the transit visits with enough flexibility in the after range to facilitate scheduling. We will use the G130M grating centered at 1291 angstrom with the PSA aperture and both SEGMENTs on aiming for the best wavelength coverage, including z=0 Lyman-alpha. We will use the TIME-TAG mode to allow us to break down each exposure into sub-exposures, which in turn is necessary to obtain a precise time series of fluxes. As described in the Phase 2 instructions, we shall use LP7 and FP-POS=3, 4. ------------------------------------------------------------------------------------ 18415( 2) - 14-Aug-2026 16:30:36 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 DS-TUC-A EXT-STAR, EXTRA-SOLAR RA=23H39M39.4808S +/- J2000 V = V = 8.226 B=9.161, R=9.28, J=7.122 PLANETARY SYSTEM, G 0.0000127", DEC=-69D11'44.71" 7.785 V-IV +/- 0.0000122" Reference Frame: ICRS Extended: NO Comments: 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. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.014927611885390706 -0.06755100000646053 0.022636700000000003 ------------------------------------------------------------------------------------------------------------------------------------ 18415( 2) - 14-Aug-2026 16:30:36 - [ 4] Visit: 01 (Baseline 1) Visit Requirements: Period 8.138268 D AND ZERO-PHASE HJD2458332.30997 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DS-TUC-A COS/NUV ACQ/IMA BOA MIRRORA 1 8 S (8 S) PHASE 0.9133394640399325 GE TO 0.9184593085408345 ------------------------------------------------------------------------------------------------------------------------------------ 2 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2265 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ DS-TUC-A 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE DS-TUC-A 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2265.0 S G DS-TUC-A 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2652.0 S G 18415( 2) - 14-Aug-2026 16:30:36 - [ 5] Visit: 02 (Transit 1) Visit Requirements: AFTER 01 BY 11.5 H TO 12.5 H On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DS-TUC-A COS/NUV ACQ/IMA BOA MIRRORA 1 8 S (8 S) GE ------------------------------------------------------------------------------------------------------------------------------------ 2 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2265 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ DS-TUC-A 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE DS-TUC-A 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2265.0 S G DS-TUC-A 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2652.0 S G DS-TUC-A 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 3 2652.0 S G DS-TUC-A 5 COS/FUV TIME-TA PSA G130M 1291 none none none none 4 2652.0 S G DS-TUC-A 6 COS/FUV TIME-TA PSA G130M 1291 none none none none 5 2652.0 S G 18415( 2) - 14-Aug-2026 16:30:36 - [ 6] Visit: 03 (Baseline 2) Visit Requirements: Period 8.138268 D AND ZERO-PHASE HJD2458332.30997 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DS-TUC-A COS/NUV ACQ/IMA BOA MIRRORA 1 8 S (8 S) PHASE 0.917179347415609 GE TO 0.922299191916511 ------------------------------------------------------------------------------------------------------------------------------------ 2 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2265 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ DS-TUC-A 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE DS-TUC-A 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2265.0 S G DS-TUC-A 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2652.0 S G 18415( 2) - 14-Aug-2026 16:30:36 - [ 7] Visit: 04 (Transit 2) Visit Requirements: AFTER 03 BY 11.5 H TO 12.5 H On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DS-TUC-A COS/NUV ACQ/IMA BOA MIRRORA 1 8 S (8 S) GE ------------------------------------------------------------------------------------------------------------------------------------ 2 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2265 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=3, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 DS-TUC-A COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=4121, FP-POS=4, 1 2000 S G LIFETIME-POS=LP7 see below (2652 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ DS-TUC-A 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE DS-TUC-A 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 2265.0 S G DS-TUC-A 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 2652.0 S G DS-TUC-A 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 3 2652.0 S G DS-TUC-A 5 COS/FUV TIME-TA PSA G130M 1291 none none none none 4 2652.0 S G DS-TUC-A 6 COS/FUV TIME-TA PSA G130M 1291 none none none none 5 2652.0 S G 18415( 2) - 14-Aug-2026 16:30:36 - [ 8] Summary Form for Proposal 18415 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures BOA, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=4121, FP-POS=3, FP-POS=4, LIFETIME-POS=LP7 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 11.5 H TO 12.5 H, AFTER 03 BY 11.5 H TO 12.5 H, PHASE 0.9133394640399325 TO 0.9184593085408345, PHASE 0.917179347415609 TO 0.922299191916511, Period 8.138268 D AND ZERO-PHASE HJD2458332.30997 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names DS-TUC-A ------------------------------------------------------------------------------------------------------------------------------------