New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant
Abstract
We present new measurements of the parallax of seven longperiod (≥10 days) Milky Way (MW) Cepheid variables (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup, and S Vul) using onedimensional astrometric measurements from spatial scanning of WideField Camera 3 on the Hubble Space Telescope (HST). The observations were obtained at ∼6 month intervals over 4 years. The distances are 1.73.6 kpc, with a mean precision of 45 μas (signaltonoise 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 longperiod Cepheids with significant parallax measurements, 8 obtained from this program. We also present highprecision mean F555W, F814W, and F160W magnitudes of these Cepheids, allowing a direct, zeropointindependent 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 (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 ± 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 H _{0} 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 H _{0}.
 Publication:

The Astrophysical Journal
 Pub Date:
 March 2018
 DOI:
 10.3847/15384357/aaadb7
 arXiv:
 arXiv:1801.01120
 Bibcode:
 2018ApJ...855..136R
 Keywords:

 astrometry;
 cosmological parameters;
 distance scale;
 parallaxes;
 stars: distances;
 stars: variables: Cepheids;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 accepted by ApJ, 15 pages of Text, 7 Tables, 13 Figures