2. Selection criteria
An initial sample of 15 targets was selected on the basis of the following criteria: (i) H2 O maser positions known from the literature (Forster & Caswell 1989) with an accuracy of and (ii) association with a NIR source with strong NIR excess (Testi et al. 1994b; 1995). Note that in this paper we are treating only masers in star forming regions: no maser in late type stars is considered. As previously discussed, we wish to investigate the compact molecular emission around the H2 O masers and for this purpose we have used the VLA interferometer in its C configuration, which ensures resolution comparable to that of the maser and NIR data, as well as sensitivity to structures in size. However, in order to get an estimate of the line strength and properly center the bandwidth of the VLA observations for each source, we have also performed single dish observations by means of the Medicina antenna. NH3 (1,1) emission was detected in 13 sources out fo 15 (Testi et al. 1994a), confirming the expectation that molecular emission is virtually present in all star formation type H2 O masers.
Given the time restriction for our VLA observations, we had to limit our sample to 5 sources, chosen among the strongest NH3 emitters, according to the Medicina spectra. Table 1 lists names, coordinates, local standard of rest (LSR) velocities, and the calculated kinematic distances, d, computed using the observed at Medicina and the rotation curve of Brand (1986), which assumes for the Sun an orbital velocity of 220 km s-1 and a galactocentric distance of 8.5 kpc. The coordinates correspond to the H2 O maser reference feature in Forster & Caswell (1989).
Table 1. List of observed sources
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998