16459( 4) - 01-Dec-2020 18:10:42 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16459 Version: 4 Check-in Time: 01-Dec-2020 18:10:42 STScI Edit Number: 0 Title The color-magnitude diagram of an extremely metal-poor globular cluster ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Soeren S. Larsen Radboud Universiteit Nijmegen Y CoI: Prof. Aaron Romanowsky San Jose State University N CoI: Prof. Jean P. Brodie University of California - Santa Cruz N ------------------------------------------------------------------------------------ Abstract We propose to obtain a color-magnitude diagram (CMD) of the globular cluster (GC) EXT8 in M31. With a metallicity of [Fe/H]=-2.9, EXT8 is by far the most metal-poor GC yet identified, and as such it provides us with unique information on CMD morphology at this low metallicity. At the same time, it challenges theories for the formation of GCs in the context of hierarchical galaxy formation. We will image the cluster with WFC3 in the F300X, F606W, and F814W filters and obtain a CMD that reaches below the horizontal branch. The observations will thus provide the first empirical insight into horizontal branch morphology in a GC near [Fe/H]=-3. Photometry of stars on the red giant branch will provide an independent verification of the spectroscopic metallicity determination and will constrain any metallicity spread present in the cluster (that might be suggestive of an origin as a stripped nucleus). ------------------------------------------------------------------------------------ Observations Description ------------------------ The extremely metal-poor globular cluster EXT8 in M31 will be imaged with WFC3 for two orbits in F606W, three orbits in F814W, and two orbits in F300X in order to produce a color-magnitude diagram that reaches below the horizontal branch. Each orbit will be split into two dithered sub-exposures. The cluster will be positioned on the UVIS2 detector (which is more sensitive in the UV) at a POS-TARG offset of (-40, 0) arcsec in order to place the cluster closer to the read-out amplifier C and minimize CTE losses. The observations are divided into three visits, one for each filter. The two-orbit visits (F606W and F300X) will use a four-point dither pattern and the three-orbit visit (F814W) will use a six-point dither patterns. We are using the offsets in ISR WFC 2020-07 and specifying the dither points via POS-TARG offsets (in addition to the (-40, 0) offset mentioned above), rather than predefined dither patterns. The WFC3 ETC predicts a background level of 10 e- (sky + dark), to which we add FLASH=10 to get to a total of 20. We note that the WFC3 ETC predicts a slightly higher background level than the value computed by APT. Impact of reduced-gyro operations: As each visit consists of WFC3 imaging for 2-3 orbits in a single filter, the 16459( 4) - 01-Dec-2020 18:10:42 - [ 2] slight increase in target acquisition time is expected to have a negligible impact on the S/N of our observations. The observations are not time critical and our program requires no specific orientation (apart from all three visits having the same orientation). We therefore expect that the impact of reduced-gyro operations would be small. ------------------------------------------------------------------------------------ 16459( 4) - 01-Dec-2020 18:10:42 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M31-EXT8 STELLAR CLUSTER, RA=00H53M14.5300S, J2000 V = 15 GLOBULAR CLUSTER DEC=+41D33'24.50" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 16459( 4) - 01-Dec-2020 18:10:42 - [ 4] Visit: 01 (M31-EXT8 F606W) 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 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F606W 1 1350 S POS TARG -40, 0.0 S see below (1359 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F606W 1 1350 S POS TARG -39.8215, S see below 0.1908 (1359 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F606W 1 1350 S POS TARG -39.6827, S see below 0.3591 (1355 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F606W 1 1350 S POS TARG -39.5042, S see below 0.5101 (1355 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 | ------------------------------------------------------------------------------------------------------------------------------------ M31-EXT8 1 WFC3/UVI ACCUM UVIS2-FI F606W none none none none 1 1359.0 S S X M31-EXT8 2 WFC3/UVI ACCUM UVIS2-FI F606W none none none none 1 1359.0 S S X M31-EXT8 3 WFC3/UVI ACCUM UVIS2-FI F606W none none none none 2 1355.0 S S X M31-EXT8 4 WFC3/UVI ACCUM UVIS2-FI F606W none none none none 2 1355.0 S S X 16459( 4) - 01-Dec-2020 18:10:42 - [ 5] Visit: 02 (M31-EXT8 F814W) 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 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -40, 0.0 S see below (1359 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -39.9073, S see below 0.3376 (1359 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -39.8149, S see below 0.1978 (1355 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -39.7618, S see below 0.5130 (1355 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -39.6695, S (1350 S) 0.3732 ------------------------------------------------------------------------------------------------------------------------------------ 6 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F814W 1 1350 S POS TARG -39.4976, S (1350 S) 0.5171 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ M31-EXT8 1 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 1 1359.0 S S X M31-EXT8 2 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 1 1359.0 S S X M31-EXT8 3 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 2 1355.0 S S X M31-EXT8 4 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 2 1355.0 S S X M31-EXT8 5 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 3 N/A S X M31-EXT8 6 WFC3/UVI ACCUM UVIS2-FI F814W none none none none 3 N/A S X 16459( 4) - 01-Dec-2020 18:10:42 - [ 6] Visit: 03 (M31-EXT8 F300X) 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 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F300X FLASH=10 1 1350 S POS TARG -40, 0.0 S see below (1356 S) Comments: ETC gives a background of 10 e-/pixel, to which a FLASH of another 10 is added to reach 20 e-/pixel ------------------------------------------------------------------------------------------------------------------------------------ 2 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F300X FLASH=10 1 1350 S POS TARG -39.8215, S see below 0.1908 (1356 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F300X FLASH=10 1 1350 S POS TARG -39.6827, S see below 0.3591 (1351 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 M31-EXT8 WFC3/UVI ACCUM UVIS2-FIX F300X FLASH=10 1 1350 S POS TARG -39.5042, S see below 0.5101 (1351 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 | ------------------------------------------------------------------------------------------------------------------------------------ M31-EXT8 1 WFC3/UVI ACCUM UVIS2-FI F300X none none none none 1 1356.0 S S X M31-EXT8 2 WFC3/UVI ACCUM UVIS2-FI F300X none none none none 1 1356.0 S S X M31-EXT8 3 WFC3/UVI ACCUM UVIS2-FI F300X none none none none 2 1351.0 S S X M31-EXT8 4 WFC3/UVI ACCUM UVIS2-FI F300X none none none none 2 1351.0 S S X 16459( 4) - 01-Dec-2020 18:10:42 - [ 7] Summary Form for Proposal 16459 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG -39.4976, 0.5171, POS TARG -39.5042, 0.5101, POS TARG -39.6695, 0.3732, POS TARG -39.6827, 0.3591, POS TARG -39.7618, 0.5130, POS TARG -39.8149, 0.1978, POS TARG -39.8215, 0.1908, POS TARG -39.9073, 0.3376, POS TARG -40, 0.0, SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300X, F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M31-EXT8 ------------------------------------------------------------------------------------------------------------------------------------