0 Assigned ID: 7400 ----- Attempting Submission 1 (Thu Apr 16 14:28:56 GMT 2026) ----- HST Phase I Proposal 7400 successfully submitted. Receipt: # 7400-1 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. 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 (Tue Aug 18 15:37:10 GMT 2026) ----- The warnings related to the FP-POS of each visit. We followed online guidelines for the FP-POS choice. Stellar X-ray and UV (XUV) irradiation is believed to sculpt short-period exoplanets through photoevaporation. In the Solar System, such atmospheric escape varies along with the Sun's 11-year activity cycle. The atmospheric evolution of young exoplanets may therefore be linked to their host stars' activity cycles, which are likely shorter than the Sun's. V1298 Tau, a 23 Myr multi-planet system with the only confirmed detections of atmospheric escape from young exoplanets, presents an ideal laboratory for revealing this star--planet connection. A series of detections and non-detections of escape in this system hint at variable escape and a 1.5 year stellar activity cycle, and upcoming JWST observations will search for more. We request simultaneous joint observations with HST/COS (primary, 9 orbits) and XMM-Newton (72 ks) to measure the XUV flux of V1298 Tau at the same times and activity phases of ongoing JWST escape measurements. Our data will unravel why atmospheric escape appears time-variable in this benchmark system, and provide a critical missing link between exoplanet atmospheric evolution and their host star's activity cycle. Our program consists of three visits with identical observing setups. Each visit consists of four exposures (one target acquisition and three spectroscopic science exposures) across three orbits. We will collect HST/COS G130M spectra of the exoplanet host star V1298 Tau. Such measurements have previously been made successfully for this target (HST GO 16163), and we follow the same instrument setup. Exposure times were vetted using the online ETC. Our target was previously observed using the same instrument configuration as our program as well as other similar configurations. We downloaded the observed COS spectrum, which is presented in Duvvuri et al. (2023), and used it as input to the ETC. The exposure durations that maximized signal-to-noise ratio (SNR) would exceed the orbit visibility window and overlap with occultation, so for each exposure we instead opted for the maximum time that avoided overlap as determined by the APT Orbit Planner. The exposure times listed for each exposure reflect these adjusted values. We verified these times would still achieve acceptable SNR using the ETC. The buffer times follow the ETC recommendations. The primary science goal is to measure the time-variable XUV flux of the exoplanet host star V1298 Tau. These observations are complementary to ongoing JWST observations studying atmospheric escape from several planets in this system. As such, we aim to measure the stellar flux contemporaneously with these JWST observations. These data must be taken as close in time as possible, otherwise the stellar flux cannot be connected to planetary atmospheric escape due to unknown time variability. Visit 1 should be scheduled as close in time as possible to the first observation of JWST GO 11831, which is curretly planned for September 7, 2026. There are abundant visibility windows from September 2026 into the beginning of Cycle 34 in November 2026. We therefore request this visit be scheduled as soon as possible after the start of Cycle 34. Visit 2 should be scheduled as close in time as possible to the second observation of JWST GO 11831, currently planned for January 23, 2027. The visit planner lists no possible simultaneous HST windows. The earliest and closest possible window is around the end of July 2027 or the start of August 2027. We request this visit be scheduled as early as possible during this window. Visit 3 is meant to measure the stellar flux at an analogous phase in the star's activity cycle as previous atmospheric escape observations, assuming a hypothesized stellar activity cycle. The optimal times for this is July - September 2027. However, this overlaps with the possible window for Visit 2. Having two of our visits stacked directly on top of each other hampers our ability to probe time variability. The minimum useful cadence for our science is approximately one month. Therefore, we request Visit 3 be scheduled around this analogous activity phase, so long as it is one month after Visit 2 executes. PI Matthew Murphy has extensive experience in space-based spectroscopic observations of transiting exoplanets, including investigation into atmospheric escape. Co-PI Adina Feinstein is an expert in transit observations of young planets, stellar activity, and is the lead software developer for several time-series photometry extraction and analyses, and JWST data reduction pipelines. Co-PI Feinstein is the PI of JWST GO 5311, 5959, 8597 - three programs to observe nine young transiting exoplanet systems. PI Feinstein is an expert in identifying and characterizing stellar flares on young stars at both ultraviolet and optical wavelengths. Co-I Darryl Seligman has broad theoretical expertise including dynamics, plasma physics, fluid dynamics and nonlinear dynamics. He has relevant expertise on solar and stellar physics. Co-I Benjamin Rackham is an expert in observations of exoplanet transmission spectra from space and the ground, as well as modeling and correcting for the impact of stellar photospheric heterogeneity on transmission spectra. He recently co-led Study Analysis Group 21 of NASA's ExoPAG, which was focused on understanding the challenge of stellar photospheric heterogeneity in space-based transmission spectra and identifying mitigation approaches. He is also the PI of the HST Stellar Treasure Trove Legacy Program (AR17551), which is focused on understanding the limits of stellar heterogeneity inferences and corrections from precise, space-based spectra. Co-I Saugata Barat has expertise in space-based spectroscopic observations of transiting exoplanets using HST and JWST, and is leading ongoing analyses of atmospheric escape using HST and JWST data of planets in the V1298 Tau system. Co-I Kevin France is the PI of the HST MUSCLES Treasury (GO13650), MEATS (GO16166), and MALWARE (GO18030) Surveys to study the UV and X-ray spectral and temporal properties of exoplanet host stars (France et al. 2016, 2018, 2020; Loyd et al. 2016; Youngblood et al. 2016; Behr et al. 2023). He is the co-PI of the follow-on Mega-MUSCLES (GO15071; Brown et al. 2022; Wilson et al. 2025a) and Mega-MEATS (GO16701; France et al. 2025; Wilson et al. 2025b) Surveys that focused on activity in stars optimal for planetary characterization with JWST. He was a member of the COS instrument and science teams; he is intimately familiar with HST-COS and -STIS instrument, data, and analysis procedures. His research group has developed numerous community tools for spectral and temporal analysis of HST and X-ray observatory observations of cool stars. Dr France has authored over 100 refereed publications on exoplanets, cool stars, and planet-forming disks using UV spectra from HST. Co-I Luis Welbanks is an expert in atmospheric modeling and the interpretation of atmospheric observations using inference tools known as retrievals. He is an expert in the interpretation of ground and space based observations at low and high resolution. Co-I Sydney Petz is a graduate student working with Co-PI Feinstein, training in the reduction and fitting of space-based observations of young exoplanets. Co-I Girish Duvvuri has scientific and technical expertise in the time-series spectroscopy of cool dwarfs observed at X-ray + ultraviolet + optical wavelengths to characterize the variability of stellar activity and in modeling stellar X-ray and extreme ultraviolet radiation. Co-I Matthew Nixon is an expert in the modeling and the interpretation of atmospheric observations using inference tools known as retrievals. Exoplanets and Exoplanet Formation 6 false true false false false false false true PRIMARY 72 true SUPPORTED STAR EXTRA-SOLAR PLANETARY SYSTEM T TAURI STAR 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 Sloan u=14.97, g=14.617, r=14.19, J=8.687 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G130M 1291 1787067156302 The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Visit contemporaneous or as close as possible with Obs.1 of JWST GO 11831, which will execute on either September 7, 2026 or October 1, 2026. (Tic-Visit :proposal-id 18344 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 11:32:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 165632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18344 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18344 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 2405 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 2405 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 497 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 3 0 1787066968408 Timing link suitability includes constraints ([03 After 02 by 28.0 to 90.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Visit close in time as possible to Obs.2 of JWST GO 11831, currently scheduled for January 23, 2027 (Tic-Visit :proposal-id 18344 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 11:29:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 165632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18344 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18344 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 2405 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 2405 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "02" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2484 :al-overhead-at-end 0 :internal-dur-at-end 497 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 3 0 1787067159071 Timing link suitability includes constraints ([03 After 02 by 28.0 to 90.0 days]) The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18344 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 11:32:38.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 165632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18344 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18344 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2483 :al-overhead-at-end 0 :internal-dur-at-end 2406 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2483 :al-overhead-at-end 0 :internal-dur-at-end 2406 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18344 :visit-id "03" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2483 :al-overhead-at-end 0 :internal-dur-at-end 497 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 3 0