KaVA Array Delivers First Parallax of 0.497 mas for W3(OH) Masers

Six-epoch 22 GHz VLBI observations targeted H2O masers in the W3(OH) star-forming region using two atmospheric calibration approaches and two phase-reference sources. Both GPS-based and JMA mesoscale data corrections produced consistent astrometric solutions, with the first successful JMA-calibrated parallax reported. The combined fit yielded the quoted parallax, corresponding to a distance of 2.01 +0.10/-0.09 kpc. Phase-referenced maps showed a 55% SNR gain over VERA alone, aligning with theoretical expectations for the expanded array baseline coverage.

The result supplies a direct geometric anchor for galactic-scale distance ladders and proper-motion studies. Flux densities varied from 510 to 3900 Jy across the campaign, revealing two features separated by only 64 AU whose differing saturation behaviors illustrate how local velocity structure modulates maser amplification. This level of detail was previously accessible only to the VLBA; KaVA now extends similar capability to fainter northern sources.

KaVA’s demonstrated performance directly addresses the longstanding shortage of precise parallaxes for low-flux 22 GHz masers. Future campaigns can target additional high-mass star-forming regions whose distances remain uncertain by tens of percent, tightening constraints on spiral-arm kinematics and the Galactic rotation curve.

Next steps include expanding the target list to sources below VERA’s reliable detection threshold and integrating simultaneous multi-frequency observations to reduce residual atmospheric errors further.