18348( 2) - 03-Aug-2026 20:39:27 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18348 Version: 2 Check-in Time: 03-Aug-2026 20:39:27 STScI Edit Number: 0 Title Watching a rare massive binary mass loss in real time in NaSt1 ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Samaporn Tinyanont National Astronomical Research Institute Y of Thailand CoI: Mr. Kittipong Wangnok National Astronomical Research Institute N of Thailand CoI: Dr. Ryan M Lau California Institute of Technology N CoI: Prof. Ryan Foley University of California - Santa Cruz N CoI: Kyle Davis University of California - Santa Cruz N CoI: Dr. Nathan Smith University of Arizona N ------------------------------------------------------------------------------------ Abstract Massive stellar evolution has profound effects on galaxy evolution. Massive stars provide the first chemical enrichment via their mass loss and their different types of supernovae. Their explosive deaths provide stellar feedback governing star formation, is a key uncertainty in galaxy evolution modeling. Binary-induced mass loss is likely the primary mechanism through which massive stars lose mass. Yet, it is poorly understood because massive stars undergoing binary mass loss are extremely rare. We propose to obtain narrowband images of NaSt1, one such rare Galactic post mass transfer massive binary systems to capture the expansion of its known spatially resolved circumstellar medium, and distinguish between eruptive and stable mass loss. The CSM around NaSt1 was imaged by HST in 2013, showing an extended nitrogen-enriched nebula with a radius of around 1000 AU. Recent spectroscopic observations from 2025 measured the CSM expansion speed to 31 km/s, which put the age of the nebula to only about 150 yrs. Provided the time gap of 14 years between the previous HST observation and Cycle 34, a detectable (~1 pixel) proper motion of the CSM is expected in the new image. Space-based image quality is key to resolving any asymmetry arising in the CSM expansion, in particular to determine whether the speed is roughly the same across the nebula as expected in steady mass loss, or grows outward as expected in eruptive mass loss. ------------------------------------------------------------------------------------ Observations Description ------------------------ We request to repeat observations of NaSt1 in the same manner as in the 2013 observations (PID 13034; PI Mauerhan); thus, the description of observations is based on the public information from that program. Images in two narrow band filters are required: F658N to detect the [N II] emission from the extended nebula and F645N to detect the continuum from the scattered light from the central star (Fig. 3 in the proposal). We will perform the same image subtraction to remove the stellar contribution and isolate the CSM. To mitigate the CTE issue and minimize overhead we request the 1Kx1K subarray (UVIS2-C1K1C-SUB), and FLASH = 12. (The SUB subarray is selected to put the nebula in the middle of the subarray, instead of at the corner for the -CTE 18348( 2) - 03-Aug-2026 20:39:27 - [ 2] subarray). To prevent saturation and improve spatial sampling, we request the 5-point subpixel WFC3-UVIS-DITHER-LINE dither pattern, with 176-sec exposure time each, totaling 880 sec of exposure in each filter. We verified the raw data from 2013 that the central star did not saturate the detector. We did not repeat the ETC calculation since this setup has proven to work and even though the source is variable, the amplitude is only 0.13 mag in the R band (Lau et al, 2021). Since the nebula is well resolved and only fills a small fraction of the field of view, we do not restrict the position angle of the telescope. We are performing source subtraction using the F645N data, so the diffraction spikes orientation is not concerning either. Fig. 3 shows that the subtraction can effectively remove them. We verified using Phase II tools that observations fit within one orbit. ------------------------------------------------------------------------------------ 18348( 2) - 03-Aug-2026 20:39:27 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LS-IV-+00-5 STAR, CIRCUMSTELLAR RA=18H52M17.5478S, J2000 V = 15.4 MATTER, EMISSION LINE DEC=+00D59'44.30" NEBULA, EMISSION LINE STAR, INTERACTING BINARY 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. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -1.700256703629849E-5 -6.379999376804335E-4 3.332E-4 ------------------------------------------------------------------------------------------------------------------------------------ 18348( 2) - 03-Aug-2026 20:39:27 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 5 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18348( 2) - 03-Aug-2026 20:39:27 - [ 5] 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 LS-IV-+00-5 WFC3/UVI ACCUM UVIS2-C1K F645N FLASH=12 1 176 S PATTERN 1 1 S 1C-SUB see below (880 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 LS-IV-+00-5 WFC3/UVI ACCUM UVIS2-C1K F658N FLASH=12 1 176 S PATTERN 1 2 S 1C-SUB see below (885 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 | ------------------------------------------------------------------------------------------------------------------------------------ LS-IV-+00-5 1 WFC3/UVI ACCUM UVIS2-C1 F645N 1 none none none 1 172.0 S S K1C-SUB LS-IV-+00-5 1 WFC3/UVI ACCUM UVIS2-C1 F645N 2 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 1 WFC3/UVI ACCUM UVIS2-C1 F645N 3 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 1 WFC3/UVI ACCUM UVIS2-C1 F645N 4 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 1 WFC3/UVI ACCUM UVIS2-C1 F645N 5 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 2 WFC3/UVI ACCUM UVIS2-C1 F658N 1 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 2 WFC3/UVI ACCUM UVIS2-C1 F658N 2 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 2 WFC3/UVI ACCUM UVIS2-C1 F658N 3 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 2 WFC3/UVI ACCUM UVIS2-C1 F658N 4 none none none 1 177.0 S S K1C-SUB LS-IV-+00-5 2 WFC3/UVI ACCUM UVIS2-C1 F658N 5 none none none 1 177.0 S S K1C-SUB 18348( 2) - 03-Aug-2026 20:39:27 - [ 6] Summary Form for Proposal 18348 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=12 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F645N, F658N ------------------------------------------------------------------------------------------------------------------------------------ Target Names LS-IV-+00-5 ------------------------------------------------------------------------------------------------------------------------------------