Abstract
Abstract We conducted astrometry of H2O masers in Galactic star-forming regions G48.99−0.30 and G49.19−0.34 with VLBI Exploration of Radio Astrometry (VERA). The observational results yield the parallaxes of G48.99−0.30 and G49.19−0.34 to be 0.178 ± 0.017 mas ($5.62^{+0.59}_{-0.49}\:$kpc) and 0.211 ± 0.016 mas ($4.74^{+0.39}_{-0.33}\:$kpc), respectively. We discuss in detail the sources of astrometric errors based on the observations of the two sources. The positional systemic errors in RA and Dec of (23, 31) μas for G49.19−0.34 are dominated by a tropospheric contribution from the Earth's atmosphere. Those of (23, 53) μas for G48.99−0.30 are attributed to a tropospheric contribution and a maser structure effect. The formal positional error from thermal noise is significantly smaller than these systemic positional errors. VERA phase-referencing observations could achieve a positional accuracy of a few tens of μas in both RA and Dec directions for point-like and bright sources at high elevation.
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