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We report astrometric results of VLBI phase-referencing observations of 22GHz \hho\ masers emission toward the red hypergiant \vxsgr, one of most massiveand luminous red hypergiant stars in our Galaxy, using the Very Long BaselineArray. A background source, \Jtwoze, projected 4\d4 from the target \vxsgr, wasused as the phase reference. For the low declinations of these sources, such alarge separation normally would seriously degrade the relative astrometry. Weuse a two-step method of tropospheric delay calibration, which combines theVLBI geodetic-block (or GPS) calibration with an image-optimizationcalibration, to obtain a trigonometric parallax of $0.64\pm0.04$ mas,corresponding to a distance of 1.56$^{+0.11}_{-0.10}$ kpc. The measured propermotion of \vxsgr\ is $0.36\pm0.76$ and $-2.92\pm0.78$ \masy\ in the eastwardand northward directions. The parallax and proper motion confirms that \vxsgr\belong to the Sgr OB1 association. Rescaling bolometric luminosities in theliterature to our parallax distance, we find the luminosity of \vxsgr~is $(1.95\pm 0.62) \times 10^5$ \Lsun, where the uncertainty is dominated by differingphotometry measurements.

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