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Astron. Astrophys. 325, 282-294 (1997)

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1. Introduction

Water masers at 22.2 GHz have been traditionally associated with the process of high mass star formation. Indeed, they are often observed close to typical signposts of massive stars, namely ultracompact (UC) HII regions, far-infrared (FIR) point sources, OH masers, molecular outflows. However, such an association does not imply a physical relation between these phenomena and, to some extent, could be due to a bias in the method used to select the targets to search for H2 O maser emission. In fact, the first surveys in the 22 GHz H2 O line were conducted towards well known HII regions (e.g. Genzel & Downes 1977) and gave ambiguous results: in particular, although the detection rate was quite high, unlike OH masers H2 O masers did not seem to be distributed over the continuum emission. A typical example of this situation is W3(OH) (see e.g. Fig. 15 of Cohen  1989), where the OH masers surround the UC HII region, whereas the H2 O maser spots are located [FORMULA] ([FORMULA]  pc) to the east of it. Analogous conclusions were reached by Forster & Caswell (1989), who studied the spatial association between H2 O and OH masers and, to a lesser extent, HII regions. Also, in a recent H2 O maser survey towards UC HII regions, Hofner & Churchwell (1996) conclude that the median projected distance between masers and continuum peak is [FORMULA]  pc, too large to justify a physical relationship between the two.

With the advent of the IRAS era, it became easy to identify selected samples of sources from the IRAS Point Source Catalog (PSC, IRAS 1985), mostly on the basis of their FIR colours. Several surveys in the H2 O maser line were thus performed towards candidates with suitable FIR colours, resulting in high detection rates (Wouterloot & Walmsley 1986; Braz & Epchtein 1987; Scalise et al. 1989; Palla et al.  1991; Henning et al.  1992; Palla et al.  1993). Although the association of H2 O masers with cold IRAS point sources turned out to be stronger than that with HII regions, nevertheless the spatial resolution of IRAS was too poor (a few minutes of arc) to ensure that a positional (and hence physical) relationship did exist. The "statistical" association suggested by the high detection rates and the striking correlation between the FIR luminosity of the IRAS source and the luminosity in the H2 O lines were commonly interpreted with the idea that more energetic environments give rise to more powerful maser emission (see e.g. Palagi et al. 1993).

What is the environment where H2 O masers form? The first detection of a line tracing the molecular environment of H2 O masers is to be ascribed to Turner & Welch (1984), who mapped the HCN(1-0) transition around W3(H2 O) with high spatial resolution. Such detection was later confirmed in other lines by Wink et al. (1994). Later on, Codella & Felli (1995) and Codella et al. (1996) have investigated the nature of several star forming regions through single dish observations and found that water masers probably occur at the earliest evolutionary stages of high mass stars, much before the development of an ionised region detectable in the radio continuum. By using high spatial resolution, other authors (Cesaroni et al.  1994) have found that H2 O masers are positionally coincident with hot dense cores, most of which do not present free-free continuum emission and are believed to be the site of birth of one or more massive stars. Similarly, Tofani et al. (1995) have performed VLA observations of the water maser line at 22.2 GHz and of the 22.2 GHz and 8.4 GHz continuum emission of 22 molecular outflows and found no continuum emission on scales [FORMULA] in 13 objects, and only weak emission from ionised envelopes in 9 sources. In a near infrared (NIR) survey toward H2 O maser sources Testi et al. (1994b; 1995) found that most of them ([FORMULA]) are associated with [FORMULA] m sources with strong NIR excess, usually not associated with radio continuum. A few objects observed in detail with a high resolution multiwavelength approach (Felli et al.  1997; Hunter et al.  1995; Palla et al.  1995; Persi et al.  1996), showed that the sources that are likely to be powering the H2 O masers are probably the youngest in the region and are never associated with optically thin radio continuum. These findings suggest that H2 O maser emission is strictly related to the very first stages of the evolution of a massive star, when no HII region has yet developed. However, this conclusion is based on a sample of a few objects and hence needs further high resolution observations to become more significant on a statistical ground.

The purpose of this paper is to confirm the previous interpretation, namely to demonstrate that H2 O maser spots are positionally coincident with molecular cores which do not show continuum emission from ionised gas, namely that they switch on close to a massive (proto)star prior to the appearance of an UC H II region. For this purpose we needed a tracer of hot dense gas and an instrument with spatial resolution comparable to the typical size of the cores (a few seconds of arc): the (2,2) and (3,3) inversion transitions of ammonia and the Very Large Array interferometer in its C configuration satisfy these requirements. We have thus performed a small survey towards five sources selected from the list of Forster & Caswell (1989), observing at the same time the ammonia lines and the 1.3 cm continuum. The criteria used to select the sources observed are given in Sect.  2, while the observations are described in Sect.  3. In Sect.  4 we discuss the results for the continuum and the line, dedicating Sect.  5 to the most complex and interesting object observed. Finally, in Sect.  6 the conclusions are drawn.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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