0 Assigned ID: 7549 ----- Attempting Submission 1 (Thu Apr 16 19:02:53 GMT 2026) ----- HST Phase I Proposal 7549 successfully submitted. Receipt: # 7549-1 ----- Attempting Submission 2 (Thu Apr 16 19:12:50 GMT 2026) ----- FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser 0 ----- Attempting Submission 1 (Fri Jul 31 06:33:19 GMT 2026) ----- We are repeating the observing strategy of PID 13034 to be able to directly compare our data to see the evolution of the nebula. 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. 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 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. Dr. Samaporn Tinyanont is a staff researcher at National Astronomical Research Institute of Thailand. He is an expert in infrared observations of supernovae, especially for those that show signs of interaction like SN 2014C. His part works include the analysis of Spitzer light curves of all nearby supernovae of all types observed by the SPitzer InfraRed Transients Survey to characterize their IR evolution. He is the PI of the JWST Cycle 1 program to observe SN 2014C. Kittipong Wangnok is an expert on NaSt1 who led the analysis of high-resolution spectroscopy and integral field spectroscopy to confirm the binary nature of NaSt1 and constrain the age of the nebula. Dr. Ryan Lau is a leading expert on dust-formation from massive stars and is highly familiar with ground-based, space-based, and airborne infrared observatories. He is the PI of the WR-DustERS program (ERS 1349), which observed the nested dust shells around WR140. HE also led the study that uncovers the variability of NaSt1. Prof. Ryan Foley is an expert transient observer who has extensively used HST and other space-based observatories. He has preferred access to telescopes at Keck observatory, with which he will support this program. He was also PI of the KCWI program that obtained the IFU data on NaSt1. Kyle Davis is an expert transient observer. They have led multiple JWST and ground-based programs to characterize dust and molecule formation in supernovae with dense circumstellar media. He obtained the IFU data on NaSt1. Prof. Nathan Smith is an expert on massive stars mass loss in binary systems and the resulting extreme supernovae. He led multiple paper studying RY Scuti and NaSt1, the two subjects of this proposal. He is also the world expert on Luminous Blue Variables, Eta Car variable, and extreme interacting supernovae. Stellar Physics and Stellar Types 6 false false false false false false false SUPPORTED STAR CIRCUMSTELLAR MATTER EMISSION LINE NEBULA EMISSION LINE STAR INTERACTING BINARY 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. J2000 J2000 WFC3/UVIS Imaging No No F645N WFC3/UVIS Imaging No No F658N POS-TARG DITHER 0.145 LINE false false 5 46.84 WFC3-UVIS-DITHER-LINE 1 false 1785128804262 /Users/samaporn/work/proposals/HST/C34/phast2/OP-cache/NaSt1_phase2-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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