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

TL;DR

This study precisely measures the distance and motion of the red hypergiant VX Sgr using VLBI observations and advanced tropospheric delay calibration, confirming its association with Sgr OB1 and refining its luminosity estimate.

Contribution

It introduces a two-step tropospheric delay calibration method combining VLBI geodetic and image-optimization techniques for accurate astrometry of low-declination sources.

Findings

Parallax of 0.64±0.04 mas for VX Sgr

Distance of approximately 1.56 kpc

Luminosity estimated at (1.95±0.62)×10^5 Lsun

Abstract

We report astrometric results of VLBI phase-referencing observations of 22 GHz \hho\ masers emission toward the red hypergiant \vxsgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, \Jtwoze, projected 4\d4 from the target \vxsgr, was used as the phase reference. For the low declinations 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 GPS) calibration with an image-optimization calibration, 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 proper motion of \vxsgr\ is $0.36\pm0.76$ and $-2.92\pm0.78$ \masy\ in the eastward and northward directions. The parallax and proper motion confirms that \vxsgr\ belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature 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 differing photometry measurements.