Observational Strategy

Typically we will acquire either 4 or 5 observations spanning 12 to 18 months. In Fig. 1, we show three examples of the parallax signature for a source at 4 kpc distance; the three examples are for sources at Galactic longitudes 134 deg. (i.e., Dec = +62 deg.), 33 deg. (Dec = 0 deg), and 0 deg. (Dec = -30 deg.). For the high Dec example, the amplitude of the parallax effect is comparable for the RA and Dec axes. For sources like this, we propose to observe every 3 months, near the maxima of the parallax excursion as shown in the figure. This is what we did for W3OH. This requires 5 observations to symmetrically sample the excursions and minimize correlations between the parallax and proper motion parameters.

Fig. 1: Parallax signatures for 3 hypothetical sources at different locations in the Milky Way. The Galactic longitude is indicated in the upper right hand corner of each panel. For each source, the solid lines indicates the eastward motion and the dashed lines the northward motion. For all examples the source was assumed to be at 4 kpc distance. The dots indicate proposed observing epochs, which are different for high and low Dec sources (see text). Intermediate Dec sources could be observed either way.

For the low Dec example, the amplitude of the parallax effect in Dec gets very small. Also, because of the southerly Dec of the source, the VLBA beam degrades considerably in this direction. The combination of these effects make observation of the RA effect the only useful observable (for parallax) and we propose an alternative observing scheme with 4 epochs at 6 month intervals spanning 18 months. This is the optimal approach to determining parallax and proper motion for low Dec sources. For sources at intermediate Declinations, we could use either technique.