Parallax of Galactic Cepheids from Spatially Scanning the Wide Field Camera 3 on the Hubble Space Telescope: The Case of SS Canis Majoris
Casertano, Stefano; Anderson, Jay; Filippenko, Alexei V.; Riess, Adam G.; Bowers, J. Bradley; Anderson, Richard I.; MacKenty, John W.; Melis, Carl; Tucker, Brad E.; Clubb, Kelsey I.; Graham, Melissa L.; Cukierman, Aviv R.; Upadhya, Gautam
United States, Switzerland, Australia
Abstract
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.