We present new measurements of the parallax of 7 long-period ( \geq 10 days ) Milky Way 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 ( WFC3 ) on the Hubble Space Telescope ( HST ) .
The observations were obtained at \sim 6 month intervals over 4 years .
The distances are 1.7–3.6 kpc with a mean precision of 45 \mu as [ signal-to-noise ratio ( SNR ) \approx 10 ] and a best precision of 29 \mu as ( SNR = 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 F 555 W , F 814 W , and F 160 W magnitudes of these Cepheids , allowing a direct , zeropoint-independent comparison to > 1800 extragalactic Cepheids in the hosts of 19 Type Ia supernovae .
This sample addresses two outstanding systematic uncertainties affecting prior comparisons of Milky Way and extragalactic Cepheids used to calibrate the Hubble constant ( H _ { 0 } ) : 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 H _ { 0 } ) of 1.037 \pm 0.036 , consistent with no change and inconsistent at the 3.5 \sigma level with a ratio of 0.91 needed to match the value predicted by Planck CMB data in concert with \Lambda CDM .
Using these data instead to augment the Riess et al .
( 2016 ) measurement of H _ { 0 } improves the precision to 2.3 % , yielding 73.48 \pm 1.66 km s ^ { -1 } Mpc ^ { -1 } , and the tension with Planck + \Lambda CDM increases to 3.7 \sigma .
The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 Milky Way Cepheids can support a < 1 % calibration of H _ { 0 } .