0 Assigned ID: 967 ----- Attempting Submission 1 (Fri Apr 05 23:40:17 GMT 2019) ----- HST Phase I Proposal 967 (M101UV_RIA_DitheringWFC3only.apt) successfully submitted. Receipt: # 967-1 ----- Attempting Submission 2 (Fri Apr 05 23:56:51 GMT 2019) ----- HST Phase I Proposal 967 (M101UV_RIA_DitheringWFC3only.apt) successfully submitted. Receipt: # 967-2 0 Towards a 1% local determination of the Hubble constant: quantifying stellar association bias in the supernova-host galaxy M101 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. 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 - 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. PI Dr. Richard I. Anderson has extensive experience in investigating classical Cepheid variable stars using observations (spectroscopy, photometry, optical interferometry, Gaia) and stellar evolution models (specifically concerning the effects of rotation). Dr. Anderson has contributed to the SH0ES project (PI Adam Riess) on several occasions and led the HST/WFC3 based recent study by Anderson & Riess (2018) that examined the effect of stellar association bias as it applies to the measurement of H0 in the SH0ES project. Co-I Prof. Adam Riess initiated and leads the SH0ES project that has improved the accuracy of the distance ladder-based Hubble constant measurement by a factor of 5 compared to the Hubble key project. Prof. Riess is also a WFC3 Instrument Scientist and has led (and in many cases carried out in person) a large number of improvements in the analysis and characterization of HST data, both for SH0ES and for the general community. Co-I Dr. Stefano Casertano has participated in the SH0ES project since the beginning and has been responsible for many of the improvements that have been implemented to improve the accuracy of the H0 measurement. Dr. Casertano is an experienced HST user, has been Instrument Scientist and Data Quality Project Scientist for HST, and has worked deeply with WFC3/UVIS data (e.g. to measure Cepheid parallaxes using the spatial scanning mode on HST/WFC3). Stellar Populations Stellar Populations 6 false true true false false false false SUPPORTED GALAXY DISK DUST LANE SPIRAL SPIRAL ARM This target is centered on the same position as ACS/WFC observations for field M101-F1 in Proposal ID 10918 J2000 J2000 GALAXY DISK DUST LANE SPIRAL SPIRAL ARM This target is centered on the same position as ACS/WFC observations for field M101-F2 in Proposal ID 10918 J2000 J2000 WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X WFC3/UVIS Imaging No No F300X POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false /solaris/user1/dctaylor/proposals2/OP-cache/15946-v01.xml Timing link suitability includes constraints ([03 After 01 by 45.0 to 120.0 days]) The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. 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