PARALLAX of GALACTIC CEPHEIDS from SPATIALLY SCANNING the WIDE FIELD CAMERA 3 on the HUBBLE SPACE TELESCOPE: The CASE of SS CANIS MAJORIS

We present a high-precision measurement of the parallax for the 12-day Cepheid SS Canis Majoris, obtained via spatial scanning with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). Spatial scanning enables astrometric measurements with a precision of 20-40 μas, an order of magnitude better than pointed observations. SS CMa is the second Cepheid targeted for parallax measurement with HST and is the first of a sample of 18 long-period (10 days) Cepheids selected in order to improve the calibration of their period-luminosity relation and eventually permit a determination of the Hubble constant H ₀ to better than 2%. The parallax of SS CMa is found to be 348 ± 38 μas, corresponding to a distance of 2.9 ± 0.3 kpc. We also present a refinement of the static geometric distortion of WFC3 obtained using spatial scanning observations of calibration fields, with a typical magnitude ≲0.01 pixels on scales of 100 pixels.