VLBI astrometry of radio stars to link radio and optical celestial reference frames III: 11 radio stars

TL;DR

This study addresses the need to robustly link the radio ICRF3 to the optical Gaia-CRF3, especially at the bright end where Gaia systematics bias alignment. It presents new VLBI astrometry for 11 radio stars using the VLBA, applying a MultiView calibration pipeline to achieve high-precision parallaxes and proper motions; ten stars reach parallax and proper-motion measurements with fractional parallax uncertainties around $\sim$1.5% and sub-percent proper-motion errors in the best cases, with some stars limited by binary motion or calibrator geometry. The results substantially expand the sample of radio stars usable for cross-frame validation, thereby improving the reliability of linking ICRF and Gaia-CRF at optical bright magnitudes. The methods—high-sensitivity C-band VLBI, MultiView phase referencing, and MCMC-based astrometric parameter estimation—lay groundwork for refined frame alignment and will feed into subsequent frame-link studies (e.g., 2025AA...699A). The work demonstrates that expanding the bright-end radio-star sample is crucial for a robust, multi-wavelength celestial reference frame.

Abstract

The alignment between the radio-based International Celestial Reference Frame (ICRF) and the optical Gaia Celestial Reference Frame (Gaia-CRF) is critical for multi-waveband astronomy, yet systematic offsets at the optical bright end (G<13) limit their consistency. While radio stars offer a potential link between these frames, their utility has been restricted by the scarcity of precise Very Long Baseline Interferometry (VLBI) astrometry. In this study, we present new VLBI astrometry of 11 radio stars using the Very Long Baseline Array (VLBA), expanding the existing sample with positions, parallaxes, and proper motions measured. All 11 radio stars were detected, for 10 of which parallaxes and proper motions can be estimated, reaching a precision level of <1% in the best cases. These new samples greatly contribute to the link between ICRF and Gaia-CRF at the optical bright end.

Figures (16)

• Figure 1: The MultiView observing cycle for FF UMa.
• Figure 2: $\texttt{JMFIT}$ error ratio of PR to sMV versus the RA and DEC offsets between the target and the primary calibrator (which also serves as the PR calibrator). The histogram is shown on the right side. Upper panel: RA direction; Lower panel: DEC direction. RA offsets are multiplied by $\cos\delta$ of the targets. Hollow markers in the scatterplots and hollow areas in the histograms denote data points of DM UMa and RS UMi.
• Figure 3: $F_{\mathrm{peak}}$ ratio of PR to sMV versus the angular separation between the target and the primary calibrator. The histogram is shown on the right side. Hollow markers in the scatterplot and hollow areas in the histogram denote data points of DM UMa and RS UMi.
• Figure 4: VLBI images of AR Mon. Left: calibrated with PR; Right: calibrated with sMV. Restored beams are shown at the bottom left corner of each panel. Contours %: -16 16 32 64. All images are cleaned with the AIPS task $\texttt{IMAGR}$.
• Figure 7: Estimated parallax curves of FF Aqr. Upper panel: calibrated with PR; Lower panel: calibrated with sMV. The broadening of the curves represents the probability distribution of estimated astrometric parameters.