Parallax of Galactic Cepheids from Spatially Scanning the Wide Field Camera 3 on the Hubble Space Telescope: The Case of SS Canis Majoris

TL;DR

This paper reports a high-precision parallax measurement of the Cepheid SS Canis Majoris using Hubble's WFC3 with spatial scanning, aiming to refine the period-luminosity relation and improve Hubble constant estimates.

Contribution

It introduces a novel high-precision astrometric method using spatial scanning on HST for Cepheids, enhancing distance measurements and calibration of the period-luminosity relation.

Findings

Parallax of SS CMa is 348 +/- 38 microarcseconds.

Refined geometric distortion of WFC3 achieved with <0.01 pixel accuracy.

Sample of 18 long-period Cepheids targeted for H_0 calibration.

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 muas, 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 eighteen 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 muas, 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.