18414( 2) - 18-Aug-2026 18:54:12 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18414 Version: 2 Check-in Time: 18-Aug-2026 18:54:12 STScI Edit Number: 0 Title Resolving the Nature of the qWR Star in HD 45166 with High-Resolution FUV Spectroscopy ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Peter Nemeth Astroserver.org Y CoI: Dr. Elisson Saldanha da Gama de Observatorio Nacional N Almeida Elisson Saldanha da Gama de Almeida ------------------------------------------------------------------------------------ Abstract The nature of the quasi-Wolf-Rayet (qWR) primary in the binary HD 45166 remains fundamentally uncertain: it may be either a low-mass stripped helium star analogous to hot subdwarfs or a genuinely massive Wolf-Rayet star. Resolving this ambiguity requires a precise mass determination, which in this non-eclipsing system depends critically on composite spectral modeling. Current optical analyses are limited by degeneracies in wind structure, chemical abundances, and flux contributions. We propose high-resolution far-ultraviolet spectroscopy with STIS to constrain the atmospheric properties of the qWR component. Resonance lines such as C IV 1548A/1550A, N V 1238A/1242A, and Si IV 1393A/1402A directly probe the wind kinematics, ionization structure, and mass-loss rate, providing diagnostics inaccessible from optical data. By anchoring the wind and abundance structure, the FUV data will remove the dominant systematic uncertainties in the spectral modeling and enable a robust mass determination. These observations also serve as a feasibility test for a planned ultraviolet survey of massive stars in the PLATO fields, complementing previous surveys focused on low-metallicity and low-mass populations. HD 45166 is the key benchmark system for establishing this methodology and distinguishing between massive-star and stripped-star evolutionary channels. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will obtain far-UV spectroscopy of HD 45166 with HST/STIS using the E140H grating and the 31X0.05NDA aperture. The observations are divided into three single-orbit visits, each using TIME-TAG mode, with no special scheduling constraints. The three E140H settings together provide the required FUV spectral coverage at a resolving power of approximately R = 14, 000. The neutral-density aperture is required because of the high UV brightness of HD 45166. Each orbit will provide an exposure of approximately 2300 s, yielding an expected S/N of 23 near 1550 Agstrom. The resulting spectra will resolve the narrow photospheric and wind features of the qWR component and provide key diagnostics of its ionization structure, chemical abundances, wind velocity field, and mass-loss properties. ------------------------------------------------------------------------------------ 18414( 2) - 18-Aug-2026 18:54:12 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD-45166 STAR, B6-B9.5 V-IV, RA=06H26M19.1552S, J2000 V = 9.88 COMPOSITE SPECTRAL DEC=+07D58'28.06" TYPE, EMISSION LINE STAR, WOLF RAYET Reference Frame: ICRS 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.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 -2.6186557208962696E-5 -3.390000983927166E-4 9.955E-4 ------------------------------------------------------------------------------------------------------------------------------------ 18414( 2) - 18-Aug-2026 18:54:12 - [ 3] 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 HD-45166 STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD-45166 STIS/CCD ACQ/PEA 31X0.05ND MIRROR 1 5 S (5 S) K A ------------------------------------------------------------------------------------------------------------------------------------ 3 HD-45166 STIS/FUV TIME-TA 31X0.05ND E140H 1416 WAVECAL=NO, BUFFER-TIME=114 1 1694 S -MAMA G A (1694 S) Comments: Exposure time (seconds) = 1, 694.0000 at wavelength 1400.00 A gives: SNR = 25.2033 (per resolution element) https://etc.stsci.edu/etc/results/STIS.sp.236 6731/ ------------------------------------------------------------------------------------------------------------------------------------ 4 WAVE STIS/FUV ACCUM 0.2X0.09 E140H 1416 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 5 HD-45166 STIS/FUV TIME-TA 31X0.05ND E140H 1234 BUFFER-TIME=124, WAVECAL=NO 1 2395 S -MAMA G A (2395 S) Comments: Exposure time (seconds) = 2, 395.0000 at wavelength 1200.00 A gives: SNR = 19.8204 (per resolution element) https://etc.stsci.edu/etc/results/STIS.sp.236 6736/ ------------------------------------------------------------------------------------------------------------------------------------ 6 WAVE STIS/FUV ACCUM 0.2X0.09 E140H 1234 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 7 HD-45166 STIS/FUV TIME-TA 31X0.05ND E140H 1598 BUFFER-TIME=295, WAVECAL=NO 1 2395 S -MAMA G A (2395 S) Comments: Exposure time (seconds) = 2, 395.0000 at wavelength 1600.00 A gives: SNR = 19.6822 (per resolution element) https://etc.stsci.edu/etc/results/STIS.sp.236 6737/ ------------------------------------------------------------------------------------------------------------------------------------ 8 WAVE STIS/FUV ACCUM 0.2X0.09 E140H 1598 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 18414( 2) - 18-Aug-2026 18:54:12 - [ 4] 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 | ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HD-45166 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A HD-45166 2 STIS/CCD ACQ/PEA 31X0.05N MIRROR none none none none 1 N/A K DA HD-45166 3 STIS/FUV TIME-TA 31X0.05N E140H 1416 none none none none 1 N/A -MAMA G DA WAVE 4 STIS/FUV ACCUM 0.2X0.09 E140H 1416 none none none none 1 N/A -MAMA HD-45166 5 STIS/FUV TIME-TA 31X0.05N E140H 1234 none none none none 2 N/A -MAMA G DA WAVE 6 STIS/FUV ACCUM 0.2X0.09 E140H 1234 none none none none 2 N/A -MAMA HD-45166 7 STIS/FUV TIME-TA 31X0.05N E140H 1598 none none none none 3 N/A -MAMA G DA WAVE 8 STIS/FUV ACCUM 0.2X0.09 E140H 1598 none none none none 3 N/A -MAMA 18414( 2) - 18-Aug-2026 18:54:12 - [ 5] Summary Form for Proposal 18414 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.09, 31X0.05NDA, 50CCD ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, ACQ/PEAK, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=114, BUFFER-TIME=124, BUFFER-TIME=295, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements E140H, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD-45166, WAVE ------------------------------------------------------------------------------------------------------------------------------------