0 Assigned ID: 7798 ----- Attempting Submission 1 (Thu Apr 16 23:33:19 GMT 2026) ----- HST Phase I Proposal 7798 successfully submitted. Receipt: # 7798-1 ----- Attempting Submission 2 (Thu Apr 16 23:43:25 GMT 2026) ----- Could not connect to APT Status Server For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser. For the best data quality, it is generally required to use all four FP-POS positions at a given COS cenwave (or 2 positions for certain exception cases). See extended explanation in the diagnostic browser. 0 ----- Attempting Submission 1 (Mon Aug 17 18:19:46 GMT 2026) ----- can't use all FP-POS since we have only two science exposures in those cases Catching Carbon Outflow: A New FUV Probe of Escaping Exoplanet Atmospheres 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. 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 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. L. Ramos-Rosado is a PhD student at JHU advised by Dr. David Sing and Dr. Dos Santos and is focused on the observations of transiting exoplanets. Dr. Dos Santos is an expert spectroscopist with experience on data reduction and analysis of IR, optical and FUV stellar spectra, as well as the development of new methods to interpret stellar spectra; L. dos Santos is leading studies in the rotational and activity evolution of solar-type stars, as well as transmission spectra of exoplanets around M dwarfs using HST data. Prof. Sing is an expert in the analysis and interpretation of transiting exoplanets with space-based observatories, leading many HST observing programs targeting transiting planets over the last 10 cycles including small, large, and Treasury programs. He has published 43 refereed papers over the last 12 years on space-based transiting exoplanet data. Prof. Sing has additional expertise in retrieval analyses of exoplanet atmosphere data. J. Lothringer is an expert in both observing exoplanet atmospheres with HST, as well as atmosphere modeling with PHOENIX with extensive experience studying ultra-hot Jupiters. 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