0 Assigned ID: 6088 ----- Attempting Submission 1 (Wed Apr 02 19:29:07 GMT 2025) ----- HST Phase I Proposal 6088 successfully submitted. Receipt: # 6088-1 ----- Attempting Submission 2 (Mon Apr 07 14:05:36 GMT 2025) ----- HST Phase I Proposal 6088 successfully submitted. Receipt: # 6088-2 ----- Attempting Submission 3 (Tue Apr 08 21:33:53 GMT 2025) ----- COS G130M exposure time is exactly the value required to achieve S/N = 27 at 1310 A, according to the ETC. STIS G230L exposure time yields S/N of about 100. We are performing an IMAGE/ACQ on the target star. We had originally feared that knots in the nebula might be brigher than the star, but examination of an F555W WFPC2 image revealed that, in white light, the star is the brightest object in the field. We had orignally planned to use the 52x0.05 aperture, but the 52x0.1 aperture gives 40% more throughput with no loss of spectral resolution (because of the low-res grating), allowing us to achieve our S/N goal in the available time. Re-submit to use 50CCD aperture for STIS ACQ/IMAGE. We originally used the F28X50LP filter, but the target is very blue. Because the LP filter transmits only light longward of 5500 Å, there is a possibility that a nearby red star is brighter than the target in this filter. We changed the acquisition filter from F28X50LP to 50CCD, which has more blue coverage. Re-submit to use ACQ/IMAGE for COS target acquisition. Re-submit, increasing exposure time for COS ACQ/IMAGE to 5 seconds to account for nebular emission in background. 2138087726 ----- Attempting Submission 1 (Fri Aug 08 22:59:27 GMT 2025) ----- HST Phase II Proposal 18063 successfully submitted. Receipt: # 18063-1 ----- Attempting Submission 2 (Fri Aug 08 23:24:30 GMT 2025) ----- ----- Attempting Submission 3 (Wed Oct 08 13:50:08 GMT 2025) ----- ----- Attempting Submission 4 (Thu Oct 09 14:27:21 GMT 2025) ----- ----- Attempting Submission 5 (Tue Oct 14 14:35:16 GMT 2025) ----- 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. 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. PI Kailash Sahu (Astronomer, Space Telescope Science Institute) is an experienced user of HST and an authority on late stages of stellar evolution, astrophysical microlensing, and analysis of HST data. He has co-authored several papers on stingray nebula. He has authored or coauthored over 150 refereed publications, with over 10,000 citations. CoI M. Parthasarathy has published more than 200 papers, a good number of them on hot post-AGB stars. He has published several papers on the Stingray Nebula and its central star V839 Ara. He is a retired senior professor from Indian Institute of Astrophysics, Bangalore, India, currently active in research. He was elected as the president of the commission 29 Stellar Spectra of the International Astronomical Union (IAU) for the term 2006 to 2009. CoI Howard E. Bond (Adjunct Professor of Practice, Pennsylvania State University; and Astronomer Emeritus, Space Telescope Science Institute) has extensive experience with HST observing. He specializes in stellar astronomy and has authored or coauthored over 280 refereed papers, with over 12,500 citations. CoI Van Dixon (Scientist, Space Telescope Science Institute) has considerable experience with FUV spectroscopy, having served as an instrument team member for the Hopkins Ultraviolet Telescope (Astro I and II missions), the Berkeley Extreme and Far-UV Spectrometer (BEFS; on the ORFEUS II mission), the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Cosmic Origins Spectrograph (COS). He is an expert on hot post-AGB stars in globular clusters, objects similar to the central star of the Stingray Nebula. Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false true false false false false SUPPORTED STAR BE PLANETARY NEBULA CENTRAL STAR POST-AGB STAR NO SIMBAD reports V = 10.75, but this includes the nebula. Bobrowsky et al. (1998) estimate that stellar mag is V = 15.4. CD-59-6479 STINGRAY-NEBULA J2000 J2000 G=15.35 STIS/NUV-MAMA Spectroscopic No No G230L 2376 COS/FUV Spectroscopic No No G130M 1291 1760451702222 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. /Users/dixon/Support/BOP Checks/Cycle 33/18063/OP-cache/18063_1-v01.xml (Tic-Visit :proposal-id 18063 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "14-Oct-2025 10:21:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 243808 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18063 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18063 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2036 :al-overhead-at-end 0 :internal-dur-at-end 479 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3044 :pcs-gap 0 ) true 1 0 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. 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. 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. 1760451702644 /Users/dixon/Support/BOP Checks/Cycle 33/18063/OP-cache/18063_1-v02.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18063 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "14-Oct-2025 10:21:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 365728 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18063 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18063 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2664 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3044 :pcs-gap 0 ) true 1 0 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. S/N calculation based on the STIS 52X0.05 G230L spectrum from program 7653, which was obtained in 1999. Moved WAVECAL to the end to maximize on-target exposure time.