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4 Global properties of the Arcetri Catalog


  \begin{figure}
\par\includegraphics[width=8.1cm,clip]{H2385F2.ps}
\end{figure} Figure 2: The distribution (thick histogram) of the peak H2O fluxes taken from the literature for the sources not detected in our survey. The thin solid line represents the distribution of the peak fluxes published by Han et al. (1995). The dashed curve is the difference of the thick and thin histograms. At peak fluxes lower than $\sim $15 Jy, this curve coincides with the thick histogram

A computer-readable version of Table 1, plus the amended version of Table 2 of Comoretto et al. (1990) and Table 1 of U1 is published electronically. The combined Table 2 contains all the 1013 sources observed at Medicina (423 detected) and lists one entry per source, usually the one with the highest flux density observed at Medicina.

Many sources have multiple observations in our database, in some cases covering a period of more than 10 years. An analysis of the variability of the maser emission for a subset of sources with the longest time coverage and largest sampling is under way (Valdettaro et al., in preparation). Information on multiple observations of selected sources can be obtained upon request to palagi@arcetri.astro.it.

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{H2385F3.ps}
\end{figure} Figure 3: The distribution in the u4 vs. u5 plane of the 937 sources of the Arcetri Catalog with an IRAS PSC counterpart. Water masers associated with SFR and STAR are clearly separated in this diagram

Of the 1013 masers listed in the Arcetri catalog, 937 have an associated IRAS source. According to the classification criteria introduced in the previous section and in Appendix A, we find that the 937 masers are partitioned in the following way: 410 SFR, 460 STAR, 61 UNKN, 6 STRN. In Fig. 3 we show the IRAS color-color plot using the variables u4 and u5 defined in the Appendix. Note that the advantage of this classification over the more conventional two-color IRAS diagrams is that it exploits the information provided by the four far-infrared bands simultaneously. Also, sources with upper limits can be more properly treated (see Appendix). Water masers associated with SFR and STAR are well separated in this diagram. Figure 3 also indicates that the distribution of the UNKN sources overlaps that of the SFR-type, indicating a similar nature. On the other hand, the six STRN sources are equally distributed between STAR and SFR.

The distribution of the 937 sources with IRAS counterpart in the [60-12] vs. [25-12] diagram is displayed in Fig. 4. IRAS sources associated with SFR are distributed in the upper part of the plane. The box in the upper right corresponds to the color criteria adopted by Wood & Churchwell (1989) to identify ultracompact H II regions. The majority of the SFR-type sources are located within these boundaries, consistent with the idea that H2O masers are active preferentially during the earliest phases of the evolution of bright, massive stars.

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{H2385F4.ps}\end{figure} Figure 4: Distribution in the [60-12], [25-12] plane of the 937 sources of the Arcetri Catalog with IRAS counterpart. The symbols have the same meaning as in Fig. 3. The box in the upper right corner delimits the colors for ultracompact H II regions, as suggested by Wood & Churchwell (1989)

The Medicina radiotelescope has been extensively used for water maser searches. In Fig. 5 we show the distribution of the 5074 independent positions in our database, using the same criterion of considering two positions independent if separated by more than 1 arcmin. The total sky coverage is small, with a noticeable higher coverage toward the galactic plane. Of the 5074 centers observed with the Medicina antenna, 1013 are known to be associated with H2O maser emission. This corresponds to a high fraction (20%), which is simply the result of the biased searches conducted with the Medicina antenna: in fact, the large majority of the observations were carried out towards known H2O masers taken from the literature and/or associated with IRAS point sources, which are expected to show maser emission.

In Fig. 6 we show the distributions of the masers of type SFR (top) and those of type STAR (bottom). The two distributions are clearly different, with SFR concentrated towards the galactic plane and STAR much more uniformly distributed in galactic latitude. Such distributions are consistent with those expected respectively for young stars and late-type stars, thus giving further support to our classification based on color indices.

In Fig. 7 we show the distribution of the H2O integrated fluxes for the 423 maser sources detected by Comoretto et al. (1990), U1, and U2. The percentage of very bright masers is very small compared to the total. While the slope of the right part of the distribution represents the true increase in the number of sources towards lower integrated fluxes, the peak and subsequent decrease are instrumental effects due to the sensitivity limit. In order to inspect whether the brightest masers have all been detected in the first catalogs, we show in the lower panel of Fig. 7 the 203 sources detected by Comoretto et al. (1990) (full histogram), the 137 detected by U1 (dotted), and the 83 detected by U2 (dashed). The indication that emerges from this comparison is that continuing searches of new water masers tend to populate the region of lower integrated fluxes and that very few masers with integrated fluxes above 1000 Jy km s-1 have been detected in more recent surveys.

  \begin{figure}
\par\includegraphics[angle=-90,width=14.5cm,clip]{H2385F5.ps}\end{figure} Figure 5: Distribution in galactic coordinates of the 5074 positions observed at the frequency of the 22 GHz H2O maser line with the Medicina antenna since 1987


  \begin{figure}
\par\includegraphics[angle=-90,width=14cm,clip]{H2385F6a.ps}\par\includegraphics[angle=-90,width=14cm,clip]{H2385F6b.ps}\end{figure} Figure 6: Distributions in galactic coordinates of the H2O maser centers observed in the surveys of Comoretto et al. (1990), U1, and U2. The top panel shows the 410 sources classified as SFR and the bottom the 460 classified as STAR

Finally, Fig. 8 shows the distribution of the integrated fluxes of maser sources detected by Comoretto et al. (1990), U1, and U2, separated into SFR (255) and STAR (133). Clearly, both distributions peak at the same integrated flux, but the SFR distribution is broader towards higher values. This indicates that H2O masers associated with late-type stars are likely to be fainter than those found in star forming regions.

  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{H2385F7.ps}\end{figure} Figure 7: Distributions of the integrated flux of the 423 maser sources detected by Comoretto et al. (1990), U1, and U2. The top panel shows the global distribution, while in the bottom panel a distinction is made between the 203 sources detected by Comoretto et al. (1990) (full histogram), the 137 detected by U1 (dotted), and the 83 detected by U2 (dashed)


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{H2385F8.ps}\end{figure} Figure 8: Distributions of the integrated flux of maser sources detected by Comoretto et al. (1990), U1, and U2, according to their classification: 255 SFR (full histogram) and 133 STAR (dashed)

Acknowledgements
The technical staff at the Medicina station is gratefully acknowledged for the competent and constant assistance during all the observing runs. We also thank the referee for a very useful report which resulted in a substantial improvement of the paper.


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