18413( 2) - 17-Aug-2026 19:03:44 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18413 Version: 2 Check-in Time: 17-Aug-2026 19:03:44 STScI Edit Number: 0 Title Catching Carbon Outflow: A New FUV Probe of Escaping Exoplanet Atmospheres ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Ms. Lakeisha M. Ramos Rosado The Johns Hopkins University Y CoI: Prof. David K. Sing The Johns Hopkins University Y CoI: Dr. Leonardo Dos Santos Space Telescope Science Institute Y CoI: Dr. Joshua D. Lothringer Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract Atmospheric escape driven by stellar high energy irradiation is a primary driver of exoplanet evolution and composition. Direct measurements of escaping gas traditionally rely on Lyman alpha, the metastable He I triplet, and a handful of UV metal lines; each tracer faces observational or interpretive limits. Expanding the set of robust spectral diagnostics is therefore critical to break model degeneracies and to measure mass loss, composition, and ionization structure across a wider range of systems. Theoretical models identify C III 977.02 as a particularly strong FUV line that forms at large radii in hydrodynamic outflows; for canonical hot Jupiters, models predict an in transit C III depth of ~39%. Our program aims to validate C III as a practical tracer of hydrodynamic escape: a detection would open a new FUV window on carbon loss and atmospheric evolution, while a non detection will place meaningful constraints on mass loss, ionization, and model assumptions. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will observe HD 209458b in four visits using COS/FUV with the G140L grating centered at 1280 Angstroms. This setting simultaneously covers our diagnostic line, C III 977.02 Angstroms (segment B, 900-1165 Angstroms), and stellar continuum/context lines (segment A, 1280-2391 Angstroms) used to monitor activity. We use a single FP-POS setting within each visit to preserve a consistent solution across the exposures being co-added, which is required given the low S/N expected on the C III line. The FP-POS value differs between the Baseline and Transit visits in order to balance the cumulative FP-POS usage across the program, per COS IHB guidance (Sec. 5.8). Two of the four visits are 5-orbit Transit visits, scheduled to cover the transit of HD 209458b (P ~ 3.52 days), capturing ingress, mid-transit, egress, and immediate pre/post-transit baseline. The other two visits are 2-orbit Baseline visits, used to independently characterize short-term stellar variability outside of transit. The phase constraints for the program are set on the acquisition (ACQ) exposure of each Baseline visit: Baseline 1 is constrained to occur 12 hours from its associated transit, and Baseline 2 is further offset from Baseline 1 by one additional HST orbit (~ 45 minutes). This extra offset between the two baseline visits maximizes our phase coverage of out-of-transit stellar variability across the two epochs, rather than sampling 18413( 2) - 17-Aug-2026 19:03:44 - [ 2] the same relative phase twice. The four visits are organized into two independent epochs, each pairing one Baseline visit with one Transit visit (Baseline 1/Transit 1 and Baseline 2/Transit 2), in order to test repeatability of any detected signal and to guard against a single-epoch signal driven by stellar activity or instrumental systematics. All science exposures use TIME-TAG mode at the PSA aperture with 2000 s integrations, allowing us to bin the data into finer sub-exposures for a precise C III time series. Each visit begins with a COS/NUV MIRRORA acquisition at the BOA aperture. There are multiple opportunities to schedule these phase-constrained visits between April 2027 and September 2027. ------------------------------------------------------------------------------------ 18413( 2) - 17-Aug-2026 19:03:44 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD-209458 EXT-STAR, EXTRA-SOLAR RA=22H03M10.7727S +/- J2000 V = 7.63 PLANETARY SYSTEM, G 0.0000213", DEC=+18D53'3.55" +/- V-IV 0.0000227" Reference Frame: ICRS Extended: NO Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.002097288720895362 -0.017976 0.0207694 ------------------------------------------------------------------------------------------------------------------------------------ 18413( 2) - 17-Aug-2026 19:03:44 - [ 4] Visit: 01 (Baseline 1) Visit Requirements: Period 3.52474859 D AND ZERO-PHASE HJD2451659.93742 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 HD-209458 COS/NUV ACQ/IMA BOA MIRRORA 1 2 S (2 S) PHASE 0.8011442096451291 GE TO 0.8129653837240063 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=3, 1 2000 S G LIFETIME-POS=LP11 see below (2261 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=4, 1 2000 S G LIFETIME-POS=LP11 see below (2532 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-209458 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE HD-209458 2 COS/FUV TIME-TA PSA G140L 1280 none none none none 1 2261.0 S G HD-209458 3 COS/FUV TIME-TA PSA G140L 1280 none none none none 2 2532.0 S G 18413( 2) - 17-Aug-2026 19:03:44 - [ 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 HD-209458 COS/NUV ACQ/IMA BOA MIRRORA 1 2 S (2 S) GE ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=1, 1 2000 S G LIFETIME-POS=LP11 see below (2261 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, 1 2000 S G LIFETIME-POS=LP11, FP-POS=2 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=3, 1 2000 S G LIFETIME-POS=LP11 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, 1 2000 S G LIFETIME-POS=LP11, FP-POS=3 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=4, 1 2000 S G LIFETIME-POS=LP11 see below (2532 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-209458 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE HD-209458 2 COS/FUV TIME-TA PSA G140L 1280 none none none none 1 2261.0 S G HD-209458 3 COS/FUV TIME-TA PSA G140L 1280 none none none none 2 2532.0 S G HD-209458 4 COS/FUV TIME-TA PSA G140L 1280 none none none none 3 2532.0 S G HD-209458 5 COS/FUV TIME-TA PSA G140L 1280 none none none none 4 2532.0 S G HD-209458 6 COS/FUV TIME-TA PSA G140L 1280 none none none none 5 2532.0 S G 18413( 2) - 17-Aug-2026 19:03:44 - [ 6] Visit: 03 (Baseline 2) Visit Requirements: Period 3.52474859 D AND ZERO-PHASE HJD2451659.93742 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 HD-209458 COS/NUV ACQ/IMA BOA MIRRORA 1 2 S (2 S) PHASE 0.810010090204287 GE TO 0.8218312642831641 ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=3, 1 2000 S G LIFETIME-POS=LP11 see below (2261 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=4, 1 2000 S G LIFETIME-POS=LP11 see below (2532 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-209458 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE HD-209458 2 COS/FUV TIME-TA PSA G140L 1280 none none none none 1 2261.0 S G HD-209458 3 COS/FUV TIME-TA PSA G140L 1280 none none none none 2 2532.0 S G 18413( 2) - 17-Aug-2026 19:03:44 - [ 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 HD-209458 COS/NUV ACQ/IMA BOA MIRRORA 1 2 S (2 S) GE ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=1, 1 2000 S G LIFETIME-POS=LP11 see below (2261 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=2, 1 2000 S G LIFETIME-POS=LP11 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=3, 1 2000 S G LIFETIME-POS=LP11 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=3, 1 2000 S G LIFETIME-POS=LP11 see below (2532 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 HD-209458 COS/FUV TIME-TA PSA G140L 1280 BUFFER-TIME=12000, FP-POS=4, 1 2000 S G LIFETIME-POS=LP11 see below (2532 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-209458 1 COS/NUV ACQ/IMA BOA MIRRORA none none none none 1 N/A GE HD-209458 2 COS/FUV TIME-TA PSA G140L 1280 none none none none 1 2261.0 S G HD-209458 3 COS/FUV TIME-TA PSA G140L 1280 none none none none 2 2532.0 S G HD-209458 4 COS/FUV TIME-TA PSA G140L 1280 none none none none 3 2532.0 S G HD-209458 5 COS/FUV TIME-TA PSA G140L 1280 none none none none 4 2532.0 S G HD-209458 6 COS/FUV TIME-TA PSA G140L 1280 none none none none 5 2532.0 S G 18413( 2) - 17-Aug-2026 19:03:44 - [ 8] Summary Form for Proposal 18413 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures BOA, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=12000, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4, LIFETIME-POS=LP11 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 11.5 H TO 12.5 H, AFTER 03 BY 11.5 H TO 12.5 H, PHASE 0.8011442096451291 TO 0.8129653837240063, PHASE 0.810010090204287 TO 0.8218312642831641, Period 3.52474859 D AND ZERO-PHASE HJD2451659.93742 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD-209458 ------------------------------------------------------------------------------------------------------------------------------------