Water maser motions in W3(OH) and a determination of its distance

We report phase-referencing VLBA observations of H2O masers near the star-forming region W39(OH) to measure their parallax and absolute proper motions. The measured annual parallax is 0: 489 +/- 0: 017 mas (2: 04 +/- 0: 07 kpc), where the error is dominated by a systematic atmospheric contribution. This distance is consistent with photometric distances from previous observations and with the distance determined from CH3OH maser astrometry presented in a related paper. We also find that the source driving the H2O outflow, the TW-object, moves with a three-dimensional velocity of > 7 km s(-1) relative to the ultracompact H (II) region W39(OH).