3 Summary of results for Update2

We have collected all the information available in the literature on 300 new H₂O maser sources discovered after the completion of the first update of the Arcetri Catalog (U1). All the entries of the observed sources are listed in Table 1, which gives:

• Columns 1 and 2:  Source names: IRAS and/or another designation;
• Columns 3 and 4:  Equatorial Coordinates (1950 equinox);
• Column 5:  Color-based classification of the IRAS source associated with the H₂O maser (see the Appendix);
• Column 6:  Date of observation;
• Column 7:  Spectral resolution in km s^-1;
• Column 8:  Scan number;
• Column 9:  1$\sigma$ rms noise level in the spectrum;
• Columns 10 to 14:  Parameters of the H₂O maser emission. We give the minimum and maximum velocity of the emission, the velocity of the peak signal, the peak flux density, and the integrated flux, respectively. In the case of no detection, the minimum and maximum velocities indicate the total velocity extent of the spectrum. The parameters were estimated by visual inspection of the spectra. Obviously, this is a subjective method, and we suggest the reader to consult the spectra of the detections, all of which are shown in Fig. A.2. The uncertainty in the peak flux density is taken to be equal to the maximum between the rms given in Col. 9 and the calibration error discussed in Sect. 2. The error in the integrated flux density is dominated by the uncertainty in the calibration. The error in velocity can be assumed equal to the corresponding spectral resolution, given in Col. 7;
• Column 15:  Each entry is the coded reference to the publication in which the maser was first reported. No attempt was made to list all publications pertaining to a particular source. The reference codes are a continuation of those used in Comoretto et al. (1990) and U1, and are specified at the bottom of Table 1.

As in U1, we have classified each maser source following the scheme introduced by Palagi et al. (1993). A full explanation of the criteria adopted to distinguish objects based on the IRAS colors is given in the Appendix. Based on the fluxes in the four IRAS bands, two classes are readily identified: late-type stars (STAR) and star forming regions (SFR). In case of no assignment, the source is labeled as unknown (UNKN) or strange (STRN) (see Appendix).

Almost all sources in Table 1 have an IRAS point source as a counterpart (289 out of 300 entries). This is not surprising, since the input source lists for water maser studies are typically derived from the IRAS PSC. For the 11 sources without counterpart within 1 arcmin, we have searched in the IRAS catalog doubling the radius around the maser position, with no success. Overall, the classification criteria yield the following results: 201 STAR, 75 SFR, 10 UNKN, 3 STRN. The large number of STAR masers reflects the fact that recent studies have concentrated on H₂O masers associated with late-type stars.

Of the 300 sources, 83 (28%) were detected at least once at Medicina during the period 01/1993-04/2000. About two thirds of the 300 sources were observed only once (199); of these, 59 (30%) were detected. More than half (33) of these 59 are associated with long-period variables, the rest are SFR (20) and UNKN (6). Spectra of all the detections are shown in Fig. A.2. The identification is by scan number (upper left) and source name (upper right). Note that, in order to make the spectra readable, only part of the velocity range covered in our observations is shown in the figure.

Table 1 contains 7 new water maser sources discovered during the course of a dedicated project aimed at studying the frequency of maser occurrence among 91 bright IRAS sources ( $F_{60~\mu{\rm m}}\ge 100$ Jy) with colors corresponding to those of ultracompact H II regions (Wood & Churchwell 1 1989). These 91 sources had been already observed in 1989-90 and were reported in Palla et al. (1991) as non-detections. The newly detected sources are: IRAS 18372-0541, IRAS 19368+2239, IRAS 19560+3135, IRAS 20307+3749, IRAS 22267+6244, IRAS 22480+6002, IRAS 23545+6508. Another new detection is also listed in Table 1 as G23.27+0.08 (Codella & Moscadelli 2000).

Figure 2 shows the distribution of the peak flux densities (taken from the literature) of the sources that were not detected in the present survey. We see that the distribution is unusually bimodal. The peak below $\sim$1 Jy is expected, as it corresponds to the average 1 $\sigma$ rms of our observations (see Table 1). The second peak at $\sim$30 Jy, instead, is totally unexpected: however, this is determined by the non-detections obtained towards the sample of Han et al. (1995), as clearly demonstrated by the fact that such a peak disappears once these sources are removed from the distribution (dashed histogram in Fig. 2). We thus conclude that the large majority of the detections obtained by Han et al. (1995) are likely to be spurious. Such a conclusion is reinforced by the fact that out of the 13 sources of Han et al. detected at Medicina, only 7 show emission within 50 km s^-1 from the velocity quoted by Han et al., and for 4 of these the velocity difference is $\ge$19 km s^-1.

  

Table 1: Arcetri update 2000

 

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