18063( 5) - 16-Oct-2025 17:30:00 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18063 Version: 5 Check-in Time: 16-Oct-2025 17:30:00 STScI Edit Number: 0 Title UV Spectroscopy of the Stingray Nebula Central Star: Stellar Evolution in Real Time ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kailash C. Sahu Eureka Scientific Inc. Y CoI: Dr. Mudumba Parthasarathy Indian Institute of Astrophysics N CoI: Dr. W. Van Dyke Dixon Space Telescope Science Institute N CoI: Dr. Howard E. Bond The Pennsylvania State University N ------------------------------------------------------------------------------------ Abstract V839 Ara, the central star of the Stingray planetary nebula (PN), evolved from a B1-type post-AGB star in the 1970s to become hot enough to ionize the surrounding ejecta and reveal itself as a new PN by the 1990s. It provided the first-ever opportunity to study the transition of a star from the post-AGB to the PN phase, prompting studies with ground- and space-based telescopes. However, several aspects of this rapid evolution have remained enigmatic. After the initial fast rise in temperature, high-resolution HST UV spectra obtained in 2015 showed a decrease in Teff, suggesting that V839 Ara is already returning toward the AGB (a "born-again AGB star" event). HST images in 2016 showed that the nebular emission had faded precipitously, in keeping with an observed decrease in the central star's Teff. This has led to the speculation that the star is undergoing a late thermal pulse (LTP). However, known LTP stars are hydrogen-poor and rich in He, C and s-process elements, while V839 Ara shows solar abundances. And the current LTP models do not reproduce its observed rapid evolution. V839 Ara provides a unique opportunity to study the transition of a post-AGB star to a PN, and then apparently back toward the AGB. Since 2015, there have been no further UV spectra of the central star. We will obtain new HST UV spectra and, in conjunction with spectral-synthesis models, make a direct and accurate measurement of its present Teff, log g, and chemical composition. Our measurements will provide important and much-needed constraints on models of post-AGB evolutionary tracks, leading to a better understanding of this brief and fast-evolving phase of stellar evolution. ------------------------------------------------------------------------------------ Observations Description ------------------------ Visit 1: The exposure time is exactly that required to reach S/N = 27 at 1310 A. Visit 2: The nebula is smaller than the COS aperture, and at longer wavelengths, the nebular emission lines dominate the stellar continuum. So we use STIS and observe through the 52x0.1 aperture, which will minimize the nebular contamination. The exposure time is sufficient to achieve the required S/N ~ 100. ------------------------------------------------------------------------------------ 18063( 5) - 16-Oct-2025 17:30:00 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 V839-ARA STAR, BE, PLANETARY RA=17H16M21.0646S +/- J2000 V = 12.6 V = 15.4 G=15.35 NEBULA CENTRAL STAR, 0.0005683", DEC=-59D29'23.52" POST-AGB STAR +/- 0.0005407" Reference Frame: ICRS Extended: NO Comments: SIMBAD reports V = 10.75, but this includes the nebula. Bobrowsky et al. (1998) estimate that stellar mag is V = 15.4. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016.0 3.427283291527243E-4 -0.008338 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18063( 5) - 16-Oct-2025 17:30:00 - [ 3] Visit: 01 (COS Visit) 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 V839-ARA COS/NUV ACQ/IMA PSA MIRRORB 1 5 S (5 S) GE Comments: S/N calculation is based on the STIS G230L spectrum from program 7653, which was obtained in 1999. The ETC recommends an exposure time of 1.1 seconds to achieve S/N = 20 through the PSA, but the nebular emission raises the background, so we increase the exposure time to 5 seconds. (Note that requesting S/N = 40 yields an exposure time of 4.5 seconds, so this exposure time is not unreasonable.) The ETC returns a warning about "Partial overlap between instrument throughput band and target spectrum." Using a model scaled to match the COS spectrum at 2000 A yields the same exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 2 V839-ARA COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=511, FP-POS=3 1 621 S (621 G S) Comments: Exposure time of 1242 seconds chosen to achieve S/N = 27 (per resolution element) at 1310 A. S/N calculation is based on the COS G130M/1222 spectrum from program 13708, which was obtained in 2015. ------------------------------------------------------------------------------------------------------------------------------------ 3 V839-ARA COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=511, FP-POS=4 1 621 S (621 G S) Comments: Exposure time of 1242 seconds chosen to achieve S/N = 27 (per resolution element) at 1310 A. S/N calculation is based on the COS G130M/1222 spectrum from program 13708, which was obtained in 2015. ------------------------------------------------------------------------------------------------------------------------------------ 18063( 5) - 16-Oct-2025 17:30:00 - [ 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 | ------------------------------------------------------------------------------------------------------------------------------------ V839-ARA 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none 1 N/A GE V839-ARA 2 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 N/A G V839-ARA 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 N/A G 18063( 5) - 16-Oct-2025 17:30:00 - [ 5] Visit: 02 (STIS Visit) 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 V839-ARA STIS/CCD ACQ 50CCD MIRROR 1 1 S (1 S) Comments: S/N calculation based on Castelli & Kurucz model with Teff = 45, 000 K, E(B-V) = 0.18, V = 15.6. Time rquired for S/N=40 is 0.22 sec, and time for saturation is 24.5 sec. Given that the target has some surrounding nebulosity, we use exp time of 1 sec to be safe. ------------------------------------------------------------------------------------------------------------------------------------ 2 V839-ARA STIS/NUV TIME-TA 52X0.1 G230L 2376 BUFFER-TIME=488, WAVECAL=NO 1 2224 S -MAMA G (2224 S) Comments: S/N calculation based on the STIS 52X0.05 G230L spectrum from program 7653, which was obtained in 1999. ------------------------------------------------------------------------------------------------------------------------------------ 3 WAVE STIS/NUV ACCUM 52X0.1 G230L 2376 1 DEF (0 S) -MAMA Comments: Moved WAVECAL to the end to maximize on-target exposure time. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ V839-ARA 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A V839-ARA 2 STIS/NUV TIME-TA 52X0.1 G230L 2376 none none none none 1 N/A -MAMA G WAVE 3 STIS/NUV ACCUM 52X0.1 G230L 2376 none none none none 1 N/A -MAMA 18063( 5) - 16-Oct-2025 17:30:00 - [ 6] Summary Form for Proposal 18063 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD, 52X0.1, PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV, STIS/CCD, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=488, BUFFER-TIME=511, FP-POS=3, FP-POS=4, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, G230L, MIRROR, MIRRORB ------------------------------------------------------------------------------------------------------------------------------------ Target Names V839-ARA, WAVE ------------------------------------------------------------------------------------------------------------------------------------