On the Hubble Space Telescope Trigonometric Parallax of the Dwarf Nova SS Cygni

TL;DR

This study reanalyzed HST/FGS data for SS Cygni, finding a parallax consistent with VLBI measurements, thus resolving previous discrepancies and confirming the reliability of HST astrometry with proper data reduction.

Contribution

The paper demonstrates that careful reanalysis of HST/FGS data can eliminate systematic errors, aligning its parallax measurements with VLBI results for SS Cygni.

Findings

Revised HST/FGS parallax: 8.30+/-0.41 mas

Proper motion consistent with VLBI

HST/FGS can produce reliable parallaxes with proper data handling

Abstract

SS Cygni is one of the brightest dwarf novae (DNe), and one of the best-studied prototypes of the cataclysmic variables. Astrometric observations with the Fine Guidance Sensors (FGS) on the Hubble Space Telescope (HST), published in 2004, gave an absolute trigonometric parallax of 6.06+/-0.44 mas. However, recent very-long-baseline interferometry (VLBI), obtained during radio outbursts of SS Cyg, has yielded a significantly larger absolute parallax of 8.80+/-0.12 mas, as well as a large difference in the direction of the proper motion compared to the HST result. The VLBI distance reduces the implied luminosity of SS Cyg by about a factor of two, giving good agreement with predictions based on accretion-disk theory in order to explain the observed DN outburst behavior. This discrepancy raises the possibility of significant systematic errors in FGS parallaxes and proper motions. We have reanalyzed the archival HST/FGS data, including (1) a critical redetermination of the parallaxes of the background astrometric reference stars, (2) updated input values of the reference-star proper motions, and (3) correction of the position measurements for color-dependent shifts. Our new analysis yields a proper motion of SS Cyg that agrees well with the VLBI motion, and an absolute parallax of 8.30+/-0.41 mas, also statistically concordant with the VLBI result at the ~1.2 sigma level. Our results suggest that HST/FGS parallaxes are free of large systematic errors, when the data are reduced using high-quality input values for the astrometry of the reference stars, and when instrumental signatures are properly removed.