The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration

Abstract

We report astrometric results of VLBI phase-referencing observations of 22 GHz H₂O masers emission toward the red hypergiant VX Sgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, J1820-2528, projected 4.°4 from the target VX Sgr, was used as the phase reference. For the low decl. of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or Global Positioning System) calibration with an image-optimization calibration, to obtain a trigonometric parallax of 0.64 ± 0.04 mas, corresponding to a distance of {1.56}_-0.10^+0.11 kpc. The measured proper motion of VX Sgr is 0.36 ± 0.76 and -2.92 ± 0.78 mas yr^-1 in the eastward and northward directions. The parallax and proper motion confirms that VX Sgr belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature to our parallax distance, we find that the luminosity of VX Sgr is (1.95 ± 0.62) × 10⁵ L _⊙, where the uncertainty is dominated by differing photometry measurements.