2 The FC89 database

The VLA observations and data reduction are discussed in FC89 and will not be repeated here. The pertinent parameters are summarized in Table 1.

  

Table 1: VLA observations

The entire OH and H$_{\rm 2}$O maser database is presented in two tables and one figure. Table 2 contains a list of reference positions for each field observed. These are H$_{\rm 2}$O maser positions, with the exception of the eight instances where no H$_{\rm 2}$O maser was detected and the OH maser positions are used. Both B1950 and J2000 coordinates are given, along with the kinematical distance and the synthesized beams (major and minor axes and position angle of the restoring Gaussian). The near and far distances are both listed unless there is no ambiguity. If the ambiguity has not been resolved but one distance is preferred, the other is enclosed in parentheses. For the 23 GHz continuum detections and the sense of circular polarization used for the OH observations, Table 1 of FC89 should be consulted.

  

Table 2: FC89 database - reference positions

The source names used are the same as in FC89. These are derived from the galactic coordinates of the OH maser reference positions, and may differ slightly from the galactic coordinates of the H$_{\rm 2}$O masers listed in Table 2. There is one additional field in Table 2 which was not listed in FC89. 8.68-0.37 is an isolated OH maser approximately 1 arcmin from the OH/H$_{\rm 2}$O maser association 8.67-0.36. This specific OH maser site has been referred to in several publications (e.g. Caswell 1998) and has therefore been given a separate listing here. There are three other fields which contain maser sites with large separations (348.70-1.04, 12.22-0.12 and 23.01-0.41), but these are listed as single entries for consistency with FC89.

While a position for 19.61-0.13 is given in Table 2, this weak, isolated OH maser is not included in Table 3 or in Fig. 1 since only one spectral channel was positioned and there is no associated H$_{\rm 2}$O maser.

  

Table 3: FC89 database (this is a sample only; the full table containing all sources is available electronically)

Table 3 lists the amplitude, velocity and position of each OH and H$_{\rm 2}$O maser with spectral channel amplitude greater than 0.3 Jy in each field. The position of the peak of the 23 GHz continuum emission is also listed if detected. The complete database contains over 1900 lines and is therefore only available electronically. However, a representative field (351.42+0.64) is printed here as an example. The HII and maser positions are given as offsets in arc seconds relative to the reference position listed in Table 2. Note that the offsets are in celestial coordinates, so a +1$^{\prime\prime}$ offset in RA and Dec moves the maser spot to the left (east) and up (north) on the sky. The velocity is the center velocity of the VLA channel map imaged. No attempt at fitting velocities of individual spectral features was made.

Spectra and positions for the OH and H$_{\rm 2}$O masers in each field are plotted in Fig.1. There are a total of 74 spot maps in the complete figure, which is only available in electronic form. An example for the field 351.42+0.64 (NGC6334N) is printed here as Fig. 1. The source name is given at the top left of the sky plot, and the OH and H$_{\rm 2}$O beam sizes are shown schematically to the right (OH beam cross-hashed). Note that the beams are not to scale, but have the correct position angle and aspect ratio. Table 2 should be consulted for the size of the beams in arc seconds. Linear size scales are given to the right of each sky plot, with the near and far distances (if ambiguous) printed at the top of each scale. The symbols in the sky plot are crosses ($\times$) for OH and circles ($\bigcirc$) for H$_{\rm 2}$O. The corresponding grayed symbols represent the mean position of the maser spots for that species (note that the grayed crosses become gray squares). The grayed symbols do not necessarily correspond to an actual maser position. The star symbol, if present, represents the position of the continuum peak detected at 23 GHz.

Below the sky plots are the OH and H$_{\rm 2}$O maser spectra measured at the VLA. The two maser species are plotted on the same velocity scale, and the scale for each field spans a fixed range of 80 km s^-1 for ease of comparison. Each spectrum has one filled (darker) symbol denoting the median velocity of each species. For fields in which only one species was detected, only one spectrum is shown.

Below the spectra are listed the following values derived from the spot maps. If no continuum source or only one maser species was detected some items are not displayed. Linear sizes in mpc are given for both near and far distances.

1.

The total extent (in arcsec, mpc and km s^-1) of the OH and H$_{\rm 2}$O masers in the field.

2.

The separation of the mean positions and the median velocity of the OH and H$_{\rm 2}$O masers in the field.

3.

The separation between the position of the peak continuum emission and the mean positions of the OH and H$_{\rm 2}$O masers in the field.

Several minor modifications have been made to the original FC89 database. First, the assumed distance has been revised from 2.0 to 5.7 kpc for 9.62+0.20 (Hofner et al. 1994). Second, deep imaging of the continuum emission at 9 GHz has been obtained for fields between 353$^\circ$ and 25$^\circ$ galactic longitude (Forster & Caswell, in preparation). The new observations show that three of the 23 GHz continuum detections listed in FC89 (359.14+0.03, 359.62-0.25, and 12.91-0.26) are spurious. These have therefore been removed from the FC89 database. In addition, new observations of OH and H$_{\rm 2}$O masers and Hii regions toward some of the fields in FC89 have provided independent measurements of the absolute positions. These are discussed in the next section. Finally, a misprint in FC89 for the reference position of the OH maser 11.90-0.14 has been corrected.