17438( 5) - 28-Mar-2025 18:40:57 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17438 Version: 5 Check-in Time: 28-Mar-2025 18:40:57 STScI Edit Number: 1 Title Emission-line stars in the extremely metal-poor dwarf galaxy Sextans A ------------------------------------------------------------------------------------ Type Cycle GO 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Maude Gull University of California - Berkeley Y CoI: Dr. Daniel R. Weisz University of California - Berkeley N CoI: Dr. Martha L. Boyer Space Telescope Science Institute N CoI: Dr. Yumi Choi NOIRLab - (AZ) N CoI: Dr. Julianne Dalcanton University of Washington N CoI: Dr. Meredith Durbin University of California - Berkeley N CoI: Dr. Kareem El-Badry California Institute of Technology N CoI: Dr. Karoline Gilbert Space Telescope Science Institute N CoI: Dr. Anna Faye McLeod Durham Univ. N CoI: Dr. Alessandro Savino University of California - Berkeley N CoI: Dr. Evan D. Skillman University of Minnesota - Twin Cities N CoI: Dr. Nathan Ross Sandford University of Toronto N ------------------------------------------------------------------------------------ Abstract We propose HST/ACS Halpha (F658N) imaging to build the largest uniform sample of emission-line stars at sub-SMC metallicity by targeting the dwarf galaxy Sextans A. We will identify and characterize ~500 emission-line stars (Be, Oe, etc) at Z~0.1 Zsun, a >10x increase in the number of emission-line stars known at this metallicity. Such emission-line stars are expected to be a common phase of evolution for intermediate mass and massive stars, particularly at the low-metallicities, but there are few data points available to constrain theoretical predictions (fraction of emission-line stars, relationship to age, etc). Our large, homogenous study of these emission-line stars will greatly improve our knowledge of massive star evolution at low-metallicities as a function of age, metallicity, and location on the HR diagram. We will also supply the first constraints on the emission-line star contamination fraction of intermediate mass core blue helium burning stars (BHeBs; Mstar >~2 Msun) at such low metallicities. BHeBs provide critical tests for the physics of the post-main sequence stellar evolution (e.g., mass loss, rotation). This program will provide essential data to the broader community interested in other emission line stars such as symbiotic stars, PNe, luminous M(e) stars, massive young stellar objects, and binary-stripped stars. Observations of such low-metallicity emission line stars can only be made with HST, as they are all too faint and crowded for spectroscopic or ground-based imaging studies. ------------------------------------------------------------------------------------ Observations Description ------------------------ This program will obtain precise F658N ACS photometry of Sextans A and matched F625W imaging.The goal is to obtain all O and B type stars in the field down to F475W ~ 25.5 mag in order to have a complete stellar population study of the emission line phenomena at low metallicity. Furthermore we are also investigating the emission-line star contamination fraction of intermediate 17438( 5) - 28-Mar-2025 18:40:57 - [ 2] mass core blue helium burning stars. In total we have 4 visits planned. The first 3 visits will each consist of 2 orbits acquiring 8 F658N exposures over a 4-point box dither pattern with sub-pixel spacing. The exposure times have been optimized to maximize the SNR. The last visit will consist of 2 orbits where we will acquire 4 F658N exposures and 4 F625W exposures. The exposures in each filter will be taken consecutively of each pointing before moving to the next pointing in a 4-point box dither pattern. This combination was done to maximize F658N exposures, while simultaneously avoiding the dump buffer time of short exposures taking up exposure time. We have specified ORIENTS of 55-65 degrees and 319 degrees, which are all acceptable for our science goals. We require SAME ORIENT for each visit to maximize the SNR of the stars in our sample, and ensure that we reach the necessary depth of the stars in our field. We expect minimal impact from Reduced Gyro Mode (RGM). ------------------------------------------------------------------------------------ 17438( 5) - 28-Mar-2025 18:40:57 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SEX-A GALAXY, STAR FORMING RA=10H11M0.5900S, J2000 V = 11.93 REGION DEC=-04D41'48.73" Reference Frame: Other: SIMBAD Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 17438( 5) - 28-Mar-2025 18:40:57 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG Pattern_Orient 20.7 Angle_Between_Sides 69.02 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17438( 5) - 28-Mar-2025 18:40:57 - [ 5] Visit: 01 Visit Requirements: ORIENT 55D TO 65 D; ORIENT 319D TO 319 D 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 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 1 see below (1928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 2 see below (1992 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 3 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 4 none none none 1 482.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 498.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 6] Visit: 02 Visit Requirements: SAME ORIENT AS 01 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 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 1 see below (1928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 2 see below (1992 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 3 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 4 none none none 1 482.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 498.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 7] Visit: 03 Visit Requirements: SAME ORIENT AS 01 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 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 1 see below (1928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 2 see below (1992 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 3 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 4 none none none 1 482.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 498.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 8] Visit: 04 Visit Requirements: SAME ORIENT AS 01 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 SEX-A ACS/WFC ACCUM WFCENTER F625W 1 340 S PATTERN 1 1-2 see below (918 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 660 S see below (2198 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 1 none none none 1 226.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 2 none none none 1 226.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 3 none none none 2 233.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 4 none none none 2 233.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 546.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 546.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 553.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 553.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 9] Visit: 05 Visit Requirements: ORIENT 319D TO 319 D 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 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 1 see below (1928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 2 see below (1992 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 3 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 4 none none none 1 482.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 498.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 10] Visit: 06 Visit Requirements: SAME ORIENT AS 05 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 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 1 see below (1928 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 516 S PATTERN 1 2 see below (1992 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 3 none none none 1 482.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F658N 4 none none none 1 482.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 498.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 498.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 11] Visit: 07 Visit Requirements: SAME ORIENT AS 05 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 SEX-A ACS/WFC ACCUM WFCENTER F625W 1 340 S PATTERN 1 1-2 see below (918 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 SEX-A ACS/WFC ACCUM WFCENTER F658N 1 660 S see below (2198 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 | ------------------------------------------------------------------------------------------------------------------------------------ SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 1 none none none 1 226.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 2 none none none 1 226.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 3 none none none 2 233.0 S SEX-A 1 ACS/WFC ACCUM WFCENTER F625W 4 none none none 2 233.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 1 none none none 1 546.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 2 none none none 1 546.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 3 none none none 2 553.0 S SEX-A 2 ACS/WFC ACCUM WFCENTER F658N 4 none none none 2 553.0 S 17438( 5) - 28-Mar-2025 18:40:57 - [ 12] Summary Form for Proposal 17438 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFCENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 319D TO 319 D, ORIENT 55D TO 65 D, PATTERN 1 1, PATTERN 1 1-2, PATTERN 1 2, SAME ORIENT AS 01, SAME ORIENT AS 05 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F625W, F658N ------------------------------------------------------------------------------------------------------------------------------------ Target Names SEX-A ------------------------------------------------------------------------------------------------------------------------------------