18391( 2) - 10-Aug-2026 18:49:48 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18391 Version: 2 Check-in Time: 10-Aug-2026 18:49:48 STScI Edit Number: 0 Title The First Direct Measurement of the Extreme-UV Spectra of Very Massive Stars ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Claudia Scarlata University of Minnesota - Twin Cities Y CoI: Prof. Matthew James Hayes Stockholm University N CoI: Rui Marques-Chaves University of Geneva, Department of N Astronomy ------------------------------------------------------------------------------------ Abstract Very Massive Stars (VMS ; M*>~100Msol) are increasingly invoked to explain the hard ionizing spectra, strong HeII emission, and elevated ionizing photon production efficiencies observed in early galaxies, yet their extreme-ultraviolet (EUV) spectra remain essentially unconstrained. We propose ACS/SBC imaging of nine UV-luminous star-forming galaxies at z=2.3-3.6 with spectroscopic signatures consistent with VMS populations, and high escape fraction of ionizing radiation (~40-90%). At these redshifts, rest-frame 300-500 AA emission is shifted into the HST far-UV window, enabling the first direct constraints on escaping stellar radiation between the H I and He II ionization edges. Combining new SBC photometry with existing rest-UV and optical spectroscopy and HST measurements near rest-frame 800 AA, we will infer the hardness and shape of the ionizing continuum using forward modeling of stellar populations and IGM transmission. These observations will test whether VMS dominate the EUV output of extreme starbursts and provide an empirical calibration of ionizing spectra crucial for interpreting JWST galaxies and cosmic reionization. ------------------------------------------------------------------------------------ Observations Description ------------------------ Imaging in F140L ------------------------------------------------------------------------------------ 18391( 2) - 10-Aug-2026 18:49:48 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 J1220+0842 GALAXY, STARBURST RA=12H20M40.7200S, J2000 V = 20 DEC=+08D42'38.14" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 J1316+2614 GALAXY, STARBURST RA=13H16M29.6100S, J2000 V = 20 DEC=+26D14'7.05" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 3 J0817+3241 GALAXY, STARBURST RA=08H17M38.7700S, J2000 V = 20 DEC=+32D41'8.10" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18391( 2) - 10-Aug-2026 18:49:48 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.179 Line_Spacing 0.116 Coordinate_Frame POS-TARG Pattern_Orient 20.02 Angle_Between_Sides 63.65 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18391( 2) - 10-Aug-2026 18:49:48 - [ 4] Visit: 01 Visit Requirements: 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 J0817+3241 ACS/SBC ACCUM SBC-LODAR F140LP 1 580 S POS TARG -1.5000, 0; K (2320 S) PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 J0817+3241 ACS/SBC ACCUM SBC-LODAR F140LP 1 587 S POS TARG -1.5000, 0; K (2348 S) PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 J0817+3241 ACS/SBC ACCUM SBC-LODAR F140LP 1 587 S POS TARG -1.5000, 0; K (2348 S) PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ J0817+3241 1 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 1 N/A RK J0817+3241 1 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 1 N/A RK J0817+3241 1 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 1 N/A RK J0817+3241 1 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 1 N/A RK J0817+3241 2 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 2 N/A RK J0817+3241 2 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 2 N/A RK J0817+3241 2 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 2 N/A RK J0817+3241 2 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 2 N/A RK J0817+3241 3 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 3 N/A RK J0817+3241 3 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 3 N/A RK J0817+3241 3 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 3 N/A RK J0817+3241 3 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 3 N/A RK 18391( 2) - 10-Aug-2026 18:49:48 - [ 5] Visit: 02 Visit Requirements: 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 J1220+0842 ACS/SBC ACCUM SBC-LODAR F140LP 1 580 S POS TARG -1.5000, 0; K (2320 S) PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 J1220+0842 ACS/SBC ACCUM SBC-LODAR F140LP 1 587 S POS TARG -1.5000, 0; K (2348 S) PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 J1220+0842 ACS/SBC ACCUM SBC-LODAR F140LP 1 587 S POS TARG -1.5000, 0; K (2348 S) PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ J1220+0842 1 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 1 N/A RK J1220+0842 1 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 1 N/A RK J1220+0842 1 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 1 N/A RK J1220+0842 1 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 1 N/A RK J1220+0842 2 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 2 N/A RK J1220+0842 2 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 2 N/A RK J1220+0842 2 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 2 N/A RK J1220+0842 2 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 2 N/A RK J1220+0842 3 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 3 N/A RK J1220+0842 3 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 3 N/A RK J1220+0842 3 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 3 N/A RK J1220+0842 3 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 3 N/A RK 18391( 2) - 10-Aug-2026 18:49:48 - [ 6] Visit: 03 Visit Requirements: 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 J1316+2614 ACS/SBC ACCUM SBC-LODAR F140LP 1 580 S POS TARG -1.5000, 0; K (2320 S) PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 J1316+2614 ACS/SBC ACCUM SBC-LODAR F140LP 1 587 S POS TARG -1.5000, 0; K (2348 S) PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 J1316+2614 ACS/SBC ACCUM SBC-LODAR F140LP 1 595 S POS TARG -1.5000, 0; K (2380 S) PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ J1316+2614 1 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 1 N/A RK J1316+2614 1 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 1 N/A RK J1316+2614 1 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 1 N/A RK J1316+2614 1 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 1 N/A RK J1316+2614 2 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 2 N/A RK J1316+2614 2 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 2 N/A RK J1316+2614 2 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 2 N/A RK J1316+2614 2 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 2 N/A RK J1316+2614 3 ACS/SBC ACCUM SBC-LODA F140LP 1 none none none 3 N/A RK J1316+2614 3 ACS/SBC ACCUM SBC-LODA F140LP 2 none none none 3 N/A RK J1316+2614 3 ACS/SBC ACCUM SBC-LODA F140LP 3 none none none 3 N/A RK J1316+2614 3 ACS/SBC ACCUM SBC-LODA F140LP 4 none none none 3 N/A RK 18391( 2) - 10-Aug-2026 18:49:48 - [ 7] Summary Form for Proposal 18391 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC-LODARK ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2, PATTERN 1 3, POS TARG -1.5000, 0 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F140LP ------------------------------------------------------------------------------------------------------------------------------------ Target Names J0817+3241, J1220+0842, J1316+2614 ------------------------------------------------------------------------------------------------------------------------------------