2018ApJ...855..136R

We present new measurements of the parallax of seven long-period (>=10 days) Milky Way (MW) Cepheid variables (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup, and S Vul) using one-dimensional astrometric measurements from spatial scanning of Wide-Field Camera 3 on the Hubble Space Telescope (HST). The observations were obtained at ∼6 month intervals over 4 years. The distances are 1.7-3.6 kpc, with a mean precision of 45 µas (signal-to-noise ratio (S/N) ≃ 10) and a best precision of 29 µas (S/N = 14). The accuracy of the parallaxes is demonstrated through independent analyses of >100 reference stars. This raises to 10 the number of long-period Cepheids with significant parallax measurements, 8 obtained from this program. We also present high-precision mean F555W, F814W, and F160W magnitudes of these Cepheids, allowing a direct, zeropoint-independent comparison to >1800 extragalactic Cepheids in the hosts of 19 SNe Ia. This sample addresses two outstanding systematic uncertainties affecting prior comparisons of MW and extragalactic Cepheids used to calibrate the Hubble constant (H₀): their dissimilarity of periods and photometric systems. Comparing the new parallaxes to their predicted values derived from reversing the distance ladder gives a ratio (or independent scale for H₀) of 1.037 ± 0.036, consistent with no change and inconsistent at the 3.5σ level with a ratio of 0.91 needed to match the value predicted by Planck cosmic microwave background data in concert with ΛCDM. Using these data instead to augment the Riess et al. measurement of H₀ improves the precision to 2.3%, yielding 73.48 ± 1.66 km s^–1 Mpc^–1, and the tension with Planck + ΛCDM increases to 3.7σ. The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 MW Cepheids can support a <1% calibration of H₀.