We switched rapidly among W3OH and 3 background sources, repeating the following pattern: W3OH, J0235+6216, W3OH, J0231+6250, W3OH, J0230+6209. Sources were changed every 40 s, typically achieving 30~s of on-source data. We used a methanol maser as the phase-reference source, because it is considerably stronger than the background sources and could be detected on individual baselines with signal-to-noise ratios typically more than 100 in the available on-source time.
The data were reduced using NRAO Astronomical Image Processing System (AIPS). The calibration sequence included: 1) parallactic angle, reference source position, and atmospheric delay error correction; 2) correlator bias and system temperature and gain curve corrections; 3) electronic phase-calibration, phase referencing and self-calibration. After applying these corrections and calibrations, we made an image of a strong maser (ie, the reference channel) using a 1.2 mas (CLEAN) restoring beam. We show the first and last epoch images of the reference spectral channel at V(LSR) = -44.2 km/s in Fig.~1. One can see that there is little change in the masers over a year time scale.
|Fig. 1: The first and last epoch maps of the spectral channel at V(LSR) = -44.2 km/s containing the maser reference spot. Note that the maser emission structures change little over the time range of our parallax measurements. The contour levels are multiples of 2 Jy, with the zero contour surpressed. The origin of the maps is at R.A.(J2000) = 02 27 03.8192 and Dec.(J2000) = 61 52 25.230.|
|Fig. 2: Position versus time for a maser spot at V(LSR) = -44.2 km/s relative to three background radio sources. The large difference in position between W3OH and each background source has been removed and the data for the different background sources have been offset for clarity. The top, middle, and bottom plots are for the background sources 0230+621, 0231+625, and 0235+621, respectively. Also plotted are the best fitting models, specified by five parameters: one parameter for the parallax and two parameters for the proper motion in each of the coordinates.|
The proper motions in the RA and Dec coordinates were -1.14 +/- 0.05 and -0.26 +/- 0.05 mas/yr, respectively. Thus, the uncertainties in proper motions translate to an impressive 0.5 km/s (for the 1.93 kpc distance). As a check on this accuracy, we computed the motion of W3OH perpendicular to the Galactic plane to be -0.8 +/- 0.5 km/s (after correction for the 7 km/s motion of the Sun in this direction). This ``null result'' is consistent with what one might expect for the motion of a massive forming star.
Note that the Perseus arm sources, including W3OH, have kinematic distances of about 4 kpc, while the Georgelin & Georgelin model used a luminosity distance of 2.2 kpc for this arm (Humphreys 1978). We have now resolved this significant discrepancy; the luminosity distance is approximately correct and the Perseus arm has strong kinematic anomalies.
Humphreys, R. M. 1978, ApJS, 38, 309