15946( 10) - 07-Feb-2020 15:09:58 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15946 Version: 10 Check-in Time: 07-Feb-2020 15:09:58 STScI Edit Number: 9 Title Towards a 1% local determination of the Hubble constant: quantifying stellar association bias in the supernova-host galaxy M101 ------------------------------------------------------------------------------------ Type Cycle GO 27 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Richard I Anderson European Southern Observatory - Germany Y CoI: Dr. Adam Riess The Johns Hopkins University N CoI: Dr. Stefano Casertano Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract The growing tension among recent measurements of the present-day value of the Hubble Constant (H0) is one of the potentially most consequential open problems in astrophysics. The value of H0 inferred from early-Universe Cosmic Microwave Background measurements assuming the lambdaCDM Cosmological Concordance Model differs by 9.4 +/- 2.1 % (4.4 sigma) from H0 measured directly in the late Universe using a well-calibrated distance ladder that is free from such assumptions. It is now crucial to push the measurement accuracies to the extreme in order either to identify previously unrecognized or underestimated systematic observational error, or to demonstrate a need to modify lambdaCDM, which would place fundamental physics at the verge of a breakthrough. This proposal seeks to exploit HST/WFC3's unique UV capabilities to increase the accuracy of the late-Universe H0 measurement based on Cepheid variable stars and type-Ia supernovae (SNIa). The requested F300X passband observations of Cepheids in the closest SNIa host galaxy M101 will enable: a) direct measurements of the blending frequency of Cepheids with physically associated hot, early-type stars; b) direct measurements of the photometric bias incurred by such blending; c) improved selection criteria for extragalactic Cepheid searches that will help to further reduce the impact of this bias in upcoming late-Universe H0 measurements. The proposed observations are particularly urgent since a) Gaia astrometry and HST photometry of Local Group Cepheids increase the need to correct for this bias and b) the currently applied bias correction, based on SNIa host analog M31, requires a sanity check as well as greater precision. ------------------------------------------------------------------------------------ Observations Description ------------------------ We plan to observe two fields known to host Cepheid variable stars in the nearby galaxy M101 using WFC3/UVIS. The F300X observations are intended to detect the presence of hot, UV-bright cluster stars that are not otherwise detectable. The two fields have been selected to match the positions of previous optical and near-infrared observations made using ACS/WFC and WFC3/UVIS and WFC3/IR. In summary, the idea is to: - observe two fields of M101 using WFC3/UVIS F300X passband during two visits per field, 1 orbit per visit 15946( 10) - 07-Feb-2020 15:09:58 - [ 2] - leave between 69 days and 120 days between visits of the same field in order to average over Cepheid variability - apply a box dithering pattern during each visit - use the same orientation for both visits of each field - use orientations that avoid the chip gap falling onto priority objects of interest More details as follows. We seek to detect the presence of (unknown) star clusters near (known) Cepheids and to measure the average contrast between Cepheids and their host clusters in the closest SNIa host galaxy M101. To improve the SNR and measure an averaged cluster-Cepheid contrast, we adopt a two-visit observing strategy. The Cepheids in M101 have mean variability period of 27 d, with 68% of Cepheids having P between 16 d and 46 d (cf. Fig. 6 in Riess et al. 2016, ApJ 826, 56 and Table 5 of Hoffmann et al. 2016, ApJ 830, 10). In F300X passband, a Cepheid may vary by more than 2 mag (peak-to-peak) throughout the cycle. We request to observe each field during two visits, as this will provide two random variability phase measurements per Cepheid. For the Cepheids of interest (periods less than 46 days), we estimate that a duration between 69 days and 120 days between two visits of the same field achieves a suitable balance between scheduling flexibility and variability phase diversity (maximized variance). We require maximum spatial resolution (hence best possible PSF sampling) in order to detect and characterize the light contribution from the nearly unresolved host star clusters. Host clusters are detectable via the high UV flux emitted by their hot B-star members and we plan to apply a curve-of-growth following Anderson & Riess (2018, ApJ, 861, 36). To this end, we will apply a 4-point box dithering to densely sample the PSF (UVIS PSF FWHM is 1.87 pixel at 300nm). The exposure times are adopted such as to divide up the orbital visibility into four equally long exposures. We must avoid certain orientations in order to prevent many known Cepheids from falling onto the chip gap. Suitable orientations have been determined using the known positions of Cepheids in M101. Maximizing the number of Cepheids in the FOV of WFC3 is crucial, since only a few percent of Cepheids are expected to reside in clusters. Assuming the 2.5% clustered Cepheid fraction from M31, we expect detecting on the order of 13 host clusters associated with the 538 Cepheids in M101. F300X will provide a sensitive diagnostic for the hot (T > 10000K) cluster stars. The filter's red leak is no cause for concern, since most stars cooler than 6000K have negligible flux levels in our observational setup. The rare exception, such as yellow hypergiant stars, will be easily discernible from the existing multi-band photometry (F555W, F814W, F160W). There are no bright sources in the field of view and there are no limitations for using WFC3/UVIS. ------------------------------------------------------------------------------------ 15946( 10) - 07-Feb-2020 15:09:58 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M101-F1 GALAXY, DISK, DUST RA=14H03M29.9190S, J2000 V = 7.86 LANE, SPIRAL, SPIRAL DEC=+54D21'7.02" ARM Reference Frame: ICRS Comments: This target is centered on the same position as ACS/WFC observations for field M101-F1 in Proposal ID 10918 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 2 M101-F2 GALAXY, DISK, DUST RA=14H02M55.5934S, J2000 V = 7.86 LANE, SPIRAL, SPIRAL DEC=+54D20'13.69" ARM Reference Frame: ICRS Comments: This target is centered on the same position as ACS/WFC observations for field M101-F2 in Proposal ID 10918 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 15946( 10) - 07-Feb-2020 15:09:58 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15946( 10) - 07-Feb-2020 15:09:58 - [ 5] Visit: 01 (Visit 1 Field 1) Visit Requirements: ORIENT 142.0D TO 180 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 M101-F1 WFC3/UVI ACCUM UVIS-CENT F300X FLASH=10 1 640 S SEQ 1 NON-INT; PATTERN 2 S ER (2560 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 1 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 2 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 3 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 4 none none none 1 N/A S TER 15946( 10) - 07-Feb-2020 15:09:58 - [ 6] Visit: 02 (Visit 1 Field 2) Visit Requirements: ORIENT 197.22D TO 199.58 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 M101-F2 WFC3/UVI ACCUM UVIS-CENT F300X FLASH=10 1 600 S SEQ 1 NON-INT; PATTERN 2 S ER (2400 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 1 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 2 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 3 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 4 none none none 1 N/A S TER 15946( 10) - 07-Feb-2020 15:09:58 - [ 7] Visit: 03 (Visit 2 Field 1) Visit Requirements: SAME ORIENT AS 01; AFTER 01 BY 45 D TO 120 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 M101-F1 WFC3/UVI ACCUM UVIS-CENT F300X FLASH=10 1 640 S SEQ 1 NON-INT; PATTERN 2 S ER (2560 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 1 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 2 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 3 none none none 1 N/A S TER M101-F1 1 WFC3/UVI ACCUM UVIS-CEN F300X 4 none none none 1 N/A S TER 15946( 10) - 07-Feb-2020 15:09:58 - [ 8] Visit: 04 (Visit 2 Field 2) Visit Requirements: SAME ORIENT AS 02; AFTER 02 BY 12 D TO 120 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 M101-F2 WFC3/UVI ACCUM UVIS-CENT F300X FLASH=10 1 600 S SEQ 1 NON-INT; PATTERN 2 S ER (2400 S) 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 1 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 2 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 3 none none none 1 N/A S TER M101-F2 1 WFC3/UVI ACCUM UVIS-CEN F300X 4 none none none 1 N/A S TER 15946( 10) - 07-Feb-2020 15:09:58 - [ 9] Summary Form for Proposal 15946 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 45 D TO 120 D, AFTER 02 BY 12 D TO 120 D, ORIENT 142.0D TO 180 D, ORIENT 197.22D TO 199.58 D, PATTERN 2 1, SAME ORIENT AS 01, SAME ORIENT AS 02, SEQ 1 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300X ------------------------------------------------------------------------------------------------------------------------------------ Target Names M101-F1, M101-F2 ------------------------------------------------------------------------------------------------------------------------------------