18393( 2) - 19-Aug-2026 14:56:33 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18393 Version: 2 Check-in Time: 19-Aug-2026 14:56:33 STScI Edit Number: 0 Title Dissecting a newly unveiled He II nebula at the edge of the Local Group with STIS ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Peter Senchyna Carnegie Institution of Washington Y CoI: Dr. Ylva Gotberg Institute of Science and Technology N Austria CoI: Dr. Grace Telford University of Utah N CoI: Maude Gull Carnegie Institution of Washington N CoI: Dr. Christi Erba Space Telescope Science Institute N CoI: Dr. Margaret Lazzarini California State University - Los Angeles N CoI: Prof. Paul A. Crowther University of Sheffield N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 18393( 2) - 19-Aug-2026 14:56:33 - [ 2] 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. ------------------------------------------------------------------------------------ 18393( 2) - 19-Aug-2026 14:56:33 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 WLM-HEII EXT-CLUSTER, EMISSION RA=00H01M56.4583S +/- 0.05", J2000 V = 20 +/- 1.0 LINE NEBULA, KNOT, OB DEC=-15D26'54.05" +/- 0.05" ASSOCIATION, STAR FORMING REGION Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII-OFF UNIDENTIFIED, RA=00H01M57.2078S +/- 0.05", J2000 V = 18.9 +/- 0.2 SET UNDESIGNATED DEC=-15D27'18.50" +/- 0.05" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 18393( 2) - 19-Aug-2026 14:56:33 - [ 4] Visit: 01 (PA_130) Visit Requirements: ORIENT 355D TO 355 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1262 1 2200 S -MAMA G see below (2526 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G WLM-HEII 4 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 3 2526.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 5] Visit: 02 (PA_130) Visit Requirements: ORIENT 355D TO 355 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1262 1 2200 S -MAMA G see below (2526 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G WLM-HEII 4 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 3 2526.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 6] Visit: 03 (PA_130) Visit Requirements: ORIENT 355D TO 355 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 7] Visit: 04 (PA_25) Visit Requirements: ORIENT 250D TO 250 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1262 1 2200 S -MAMA G see below (2526 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G WLM-HEII 4 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 3 2526.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 8] Visit: 05 (PA_25) Visit Requirements: ORIENT 250D TO 250 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1262 1 2200 S -MAMA G see below (2526 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G WLM-HEII 4 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 3 2526.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 9] Visit: 06 (PA_25) Visit Requirements: ORIENT 250D TO 250 D 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 WLM-HEII-OF STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 30 S (30 FSET S) ------------------------------------------------------------------------------------------------------------------------------------ 2 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=954 1 1800 S -MAMA G see below (1909 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 WLM-HEII STIS/FUV TIME-TA 52X0.5D1 G140L 1425 BUFFER-TIME=1283 1 2200 S -MAMA G see below (2568 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 | ------------------------------------------------------------------------------------------------------------------------------------ WLM-HEII-OFFSET 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A WLM-HEII 2 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 1 1909.0 S -MAMA G WLM-HEII 3 STIS/FUV TIME-TA 52X0.5D1 G140L 1425 none none none none 2 2568.0 S -MAMA G 18393( 2) - 19-Aug-2026 14:56:33 - [ 10] Summary Form for Proposal 18393 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.5D1, F28X50LP ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT, BUFFER-TIME=1262, BUFFER-TIME=1283, BUFFER-TIME=954 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 250D TO 250 D, ORIENT 355D TO 355 D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names WLM-HEII, WLM-HEII-OFFSET ------------------------------------------------------------------------------------------------------------------------------------