18365( 2) - 14-Aug-2026 12:58:40 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18365 Version: 2 Check-in Time: 14-Aug-2026 12:58:40 STScI Edit Number: 0 Title Luminous O(He) stars - merger products or stripped stars ejected by a supernova? ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Nicole Reindl Universitat Heidelberg Y CoI: Prof. Klaus Werner Eberhard Karls Universitat Tubingen N CoI: Stephan Geier Universitat Potsdam N CoI: Max Pritzkuleit Universitat Potsdam N CoI: Mr. Semih Filiz Universitat Heidelberg N ------------------------------------------------------------------------------------ Abstract H-deficient stars are key probes of non-canonical stellar evolution, tracing pathways such as late He-shell flashes, double-degenerate mergers, and supernova (SN) ejections. They provide direct constraints on extreme physical processes driven by binary interaction and stellar explosions. Among them, O(He) stars represent a short-lived phase in He-dominated stellar evolution and are commonly interpreted as products of double He white dwarf mergers. However, we have identified two luminous O(He) stars that likely follow a different evolutionary pathway. They are the most massive known members of their class, appear single, and exhibit extreme halo kinematics, placing them at the boundary between CO+He white dwarf merger products and SN-ejected stripped He stars. We propose high-resolution UV spectroscopy, which uniquely enables us to derive their metal abundances and thereby distinguish between these formation channels. CO+He white dwarf mergers predict strongly sub-solar iron-group abundances, while SN-ejected stripped stars are expected to retain near-solar composition. By establishing the formation channel of these objects, the program will provide empirical constraints on the relative importance of double-degenerate mergers and SN ejection channels in producing He-dominated stars. Confirming a CO+He white dwarf merger origin provides constraints on merger nucleosynthesis and establishes luminous O(He) stars as the long-sought evolutionary link between cooler He-rich pre-white dwarfs and massive white dwarfs. Alternatively, a SN ejection origin would significantly expand the currently very small sample of such objects and place new constraints on their birthrate. ------------------------------------------------------------------------------------ Observations Description ------------------------ For EC-21049-1330 a FUV spectrum with STIS/E140M(1425) shall be obtained within one orbit. For GALEX-J172854.3+361958 a FUV spectrum with COS/G130M(1291) shall be obtained within one orbit. ------------------------------------------------------------------------------------ 18365( 2) - 14-Aug-2026 12:58:40 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 EC-21049-133 STAR, SDO RA=21H07M42.3949S, J2000 V = 13.759 +/- 0.03 2.2E-12 @ 1100A 0 DEC=-13D18'22.48" (predicted by SED fit of best fit NLTE model to photometry from FUV till W2) Reference Frame: ICRS Extended: NO Comments: 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. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -1.7811504642414955E-6 -0.008515999911651306 1.8920000000000002E-4 ------------------------------------------------------------------------------------------------------------------------------------ 2 GALEX-J17285 STAR, SDO RA=17H28M54.3429S +/- 5.0E-5", J2000 V = 16.84 1.0E-13 @ 1100A (predicted by 4.3+361958 DEC=+36D19'58.62" +/- 5.0E-5" SED fit of best fit NLTE model to photometry from FUV till W1) Reference Frame: ICRS Extended: NO Comments: 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. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -1.773446495634043E-4 -9.310000450568623E-4 5.28E-5 ------------------------------------------------------------------------------------------------------------------------------------ 18365( 2) - 14-Aug-2026 12:58:40 - [ 3] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: STIS E140M(1425) spectrum for EC-21049-1330 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 EC-21049-13 STIS/CCD ACQ F28X50LP MIRROR 1 1 S (1 S) 30 ------------------------------------------------------------------------------------------------------------------------------------ 2 EC-21049-13 STIS/FUV ACCUM 0.2X0.2 E140M 1425 1 2167 S 30 -MAMA (2167 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 | ------------------------------------------------------------------------------------------------------------------------------------ EC-21049-1330 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A EC-21049-1330 2 STIS/FUV ACCUM 0.2X0.2 E140M 1425 none none none none 1 N/A -MAMA 18365( 2) - 14-Aug-2026 12:58:40 - [ 4] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: FUV spectrum with COS/G130M(1291) for GALEX-J172854.3+361958. 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 GALEX-J1728 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7 1 5 S (5 S) 54.3+361958 KXD ------------------------------------------------------------------------------------------------------------------------------------ 2 GALEX-J1728 COS/FUV ACQ/PEA PSA G130M 1291 LIFETIME-POS=LP7, 1 5 S (5 S) 54.3+361958 KD CENTER=FLUX-WT-FLR, STEP-SIZE=0.9 ------------------------------------------------------------------------------------------------------------------------------------ 3 GALEX-J1728 COS/FUV ACCUM PSA G130M 1291 FP-POS=3, LIFETIME-POS=LP7, 1 821 S (821 54.3+361958 SEGMENT=BOTH S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GALEX-J1728 COS/FUV ACCUM PSA G130M 1291 FP-POS=4, LIFETIME-POS=LP7, 1 821 S (821 54.3+361958 SEGMENT=BOTH 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 | ------------------------------------------------------------------------------------------------------------------------------------ GALEX-J172854.3+361 1 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A 958 KXD GALEX-J172854.3+361 2 COS/FUV ACQ/PEA PSA G130M 1291 none none none none 1 N/A 958 KD GALEX-J172854.3+361 3 COS/FUV ACCUM PSA G130M 1291 none none none none 1 N/A 958 GALEX-J172854.3+361 4 COS/FUV ACCUM PSA G130M 1291 none none none none 1 N/A 958 18365( 2) - 14-Aug-2026 12:58:40 - [ 5] Summary Form for Proposal 18365 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.2, F28X50LP, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, ACQ/PEAKD, ACQ/PEAKXD ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CENTER=FLUX-WT-FLR, FP-POS=3, FP-POS=4, LIFETIME-POS=LP7, SEGMENT=BOTH, STEP-SIZE=0.9 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements E140M, G130M, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names EC-21049-1330, GALEX-J172854.3+361958 ------------------------------------------------------------------------------------------------------------------------------------