0 Assigned ID: 7560 ----- Attempting Submission 1 (Thu Apr 16 19:19:48 GMT 2026) ----- HST Phase I Proposal 7560 successfully submitted. Receipt: # 7560-1 ----- Attempting Submission 2 (Thu Apr 16 23:37:35 GMT 2026) ----- HST Phase I Proposal 7560 successfully submitted. Receipt: # 7560-2 ----- Attempting Submission 3 (Fri Apr 17 00:00:43 GMT 2026) ----- Phase II first version; here using D1 aperture, but noting that centering may require further adjustment to ensure key targets are entirely on-detector 1457371364 ----- Attempting Submission 1 (Wed Aug 19 10:16:39 GMT 2026) ----- Dissecting a newly unveiled He II nebula at the edge of the Local Group with STIS For decades, the routine detection of nebular He II emission in unresolved metal-poor star forming galaxies has evaded explanation. The failure of population synthesis models to explain this emission suggests that these models are missing or mismodeling sources of hard ionizing radiation that are relatively commonplace at low metallicities; and spells trouble for our ability to confidently interpret the spectra of the growing population of distant galaxies with high-ionization line detections. Recently, a rare spatially resolved He II nebula has been identified within a bright HII region in the dwarf irregular galaxy WLM. Follow-up optical spectroscopy confirms this narrow high-ionization nebular emission, but reveals no signatures of any source hot enough to provide the requisite >54 eV photons. This is consistent with a picture in which a very hot atmosphere is present, but outshined by its companion or other neighbors in the optical. Such an object can only be identified unambiguously in the far ultraviolet. Here we propose STIS FUV spectroscopy designed to dissect the cluster underlying this emission. These observations will detect or place stringent limits on any hot atmospheres present which would be effectively invisible at other wavelengths beyond the EUV and UV; provide access to key diagnostics of these and all UV bright sources within the region; and map high-ionization UV nebular lines at pc-scales. This object represents a unique opportunity that only HST can take advantage of to directly tie hard ionizing radiation in a metal-poor dwarf galaxy to its source, providing crucial input to models of massive stars and their ionizing feedback in the early Universe. This program will obtain a set of deep FUV / G140L spectroscopic observations of the gas and UV-bright stars underlying a resolved He II emitting region in the galaxy WLM. In order to capture the brightest stars which are resolved and separated at few-arcsecond scales, we have carefully chosen 2 position angles that when employed with the 52x0.5 slit will cover the bulk of these while also dissecting the nebular emission covering this region. Because of the complex nature of this central knot and the requirements on slit centering to capture these stars, a direct acqusition is infeasible. Instead, we have identified a nearby (~30'') isolated and reasonably bright star (~19 AB) with negligible proper motion. We will perform a simple ACQ on this star for each visit, and offset to our primary target positions. Both the offset and primary target coordinates are GAIA-derived and checked against aligned HST imaging, and the point source acquisiton is simulated with existing F814W and F625W imaging. The remainder of each visit is devoted to spectroscopic integrations in TIME-TAG mode. Even in an extremely optimistic ETC estimate for a combined AB=27 F275W O3V source, we do not expect to encounter high countrates that would approach the readout limits; but to guard against slightly brighter sources than expected, we conservatively set the buffer time to just less than half each integration length. Each PA is assigned 3 visits (3+3+2 orbits) for a total of 18 orbits across both pointings. The team is composed of experts in observing and modeling resolved massive star populations, photoionization modeling, and integrated galaxy population modeling. Members have a strong track record of successfully designing, executing, and publishing the results of HST programs, particularly of UV spectroscopy, and including STIS spectroscopy of massive stars; including PI programs GO:12465, 13052, 15794, 15921, 16155, 16717, 16767, 16920, 17123, 17198, 17438, 17475, 17491, 17526, 17771, 18040, 18087; and e.g. Senchyna+17,19,20,22, Telford+20,21,23,24, Gull+22,25,26, Erba+21, Lazzarini+18,21,22,23, Crowther+16,19. Stellar Physics and Stellar Types Galaxies 6 false true false false false false false SUPPORTED EXT-CLUSTER EMISSION LINE NEBULA KNOT OB ASSOCIATION STAR FORMING REGION J2000 J2000 UNIDENTIFIED UNDESIGNATED NO J2000 The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 The scheduling data was generated by CASM 34.0. CASM was run in Phase I mode. STIS/FUV-MAMA Spectroscopic No No G140L 1425 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:38:09.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 678800 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 363 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 3 0 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v02.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:38:56.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 678800 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 363 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "02" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 3 0 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v03.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:39:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 452672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 2 0 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v04.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "04" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:41:00.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 678800 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 363 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "04" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 3 0 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v05.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "05" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:41:01.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 678800 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "05" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 363 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "05" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 3 0 /Users/peter/Documents/hst_apts/hst18393/phaseii/OP-cache/go18393-v06.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18393 :visit-id "06" :proposal-version -1 :trans-version "20260618" :date "19-Aug-2026 02:41:02.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 452672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18393 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18393 :visit-id "06" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18393 :visit-id "06" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 625 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2976 :pcs-gap 0 ) true 2 0