1 Introduction

The first Arcetri Catalog of $\rm H_2O$ maser sources was published in July 1990 (Comoretto et al. 1990). A revised version of it was given by Palagi et al. (1993), who also discussed the statistical properties of the sources. The Catalog gives the observational parameters of all 500 $\rm H_2O$masers with $\delta >-30^{\circ}$ reported in the literature up to January 1989, and re-observed with the Medicina 32-m radio telescope (203 were detected at Medicina). After the publication of the Arcetri Catalog, a large number of new water masers were found in our own observing programs at Medicina and elsewhere. This development has prompted the compilation of a first update of the Catalog (Brand et al. 1994, hereafter U1), which contains all the sources reported in the literature from March 1989 to December 1992 as observed at Medicina. U1 gives the parameters of 213 sources in tabular form (141 of these were detected at Medicina), bringing the size of the Arcetri Catalog to 712 masers (a 42% increase).

The basic motivation of the Arcetri Catalog is to provide a homogeneous and complete list of all H₂O maser "centers'' observed as separate sources with the 1.9 arcmin HPBW of the 32-m Medicina telescope, and to present at least one spectrum for each detected maser or an upper limit to the peak flux density for those not detected by us. Single dish and interferometric observations of higher spatial resolution have revealed the existence of many distinct components around a maser center reported in our Catalog. However, tabulation of all these finer components is beyond the scope and usefulness of our study.

There are two types of H₂O masers, those that occur in star forming regions, and those that originate in the envelopes of evolved stars. From the scientific viewpoint, the importance of studying water masers is readily understood. Masers of the first type are beacons of star formation sites, and allow one to explore the environment of deeply embedded sources, completely inaccessible at optical and near-infrared wavelengths. Knowledge of maser emission having been observed towards an IRAS source pinpoints the direction in which more detailed searches for newly formed stars should be made (e.g. Schreyer et al. 1996; Plume et al. 1997; Launhardt et al. 1998; Zinchenko et al. 1998). Stellar masers are observed to obtain knowledge of the spatial- and velocity structure of the stellar envelopes and to study their variability. When used in combination with other (optical, IR, and high-resolution radio) observations these studies yield important information on the stellar mass loss rate, the physical conditions in the circumstellar shell, and the maser pumping mechanism (e.g. Benson & Little-Marenin 1996; Lewis 1999; Colomer et al. 2000).

The availability of a catalog, in which all known water masers are brought together is an important aid to these studies, in that it greatly facilitates the selection of objects on which to perform more detailed studies. The Arcetri catalog is such a data base. Moreover, because all masers in this catalog have been re-observed with the same telescope and receiver, the data presented therein allow statistical studies (e.g. Palagi et al. 1993). The Arcetri catalog is, so to speak, only the tip of the iceberg of the complete Arcetri archive of water masers: for many sources we have continued monitoring the maser emission over the years, resulting in a coverage of more than 10 years in several cases. Thus, maser variability studies on long time scales can also be performed.

The second update of the Catalog presented here (hereafter U2) contains 300 sources which satisfy the criteria established in Comoretto et al. (1990) and in U1. These are: the distance between two maser centers should be larger than 1 arcmin and the source must have a declination $\delta >-30^{\circ}$. We note that such a separation represents a minimum value: when strong masers are present with intensities up to $\sim$10⁵ Jy, as in the case of Orion KL and W3OH, the appropriate distance for two sources to be considered independent becomes much larger. Work is in progress to be more quantitative. Preliminary results from a large region mapped around Orion KL indicate that the emission of the strong maser (at the same velocity) can be seen in the sidelobes of the beam pattern with an intensity above our mean noise up to distances of 30 HPBWs.

The majority of the sources contained in U2 are associated with late-type stars, while the rest are IRAS sources selected for having colors typical of star forming regions (SFR). We have detected water emission in 83 sources, and their spectra, including multiple observations of the same source, are presented in Sect. 3. In total, U2 brings the total number of sources contained in the Arcetri Catalog to 1013 (423 detected at Medicina), a 42% increase with respect to U1. The global properties of the Catalog are discussed in Sect. 4.