Warm Water Vapor around Sagittarius B2

In the region of Sgr B2 there are several condensations heated externally by nearby hot stars. Therefore H2O far–IR lines are expected to probe only an external low–density and high temperature section of these condensations, whereas millimeter-wave lines can penetrate deeper into them where the density is higher and Tk lower. We have conducted a study combining H2O lines in both spectral regions. First, Infrared Space Observatory observations of several H2O thermal lines seen in absorption toward Sgr B2(M) at a spectral resolution of ∼35 km s have been analyzed. Second, an IRAM –30m telescope map of the para–H2O 313 − 220 line at 183.31 GHz, seen in emission, has also been obtained and analyzed. The H2O lines seen in absorption are optically thick and are formed in the outermost gas of the condensations in front of the far–IR continuum sources. They probe a maximum visual extinction of ∼5 to 10 mag. Radiative transfer models indicate that these lines are quite insensitive to temperature and gas density, and that IR photons from the dust play a dominant role in the excitation of the involved H2O rotational levels. In order to get the physical conditions of the absorbing gas we have also analyzed the CO emission toward Sgr B2(M). We conclude, based on the observed CO J=7–6 line at 806.65 GHz with the Caltech Submillimeter Observatory, and the lack of emission from the far–IR CO lines, that the gas density has to be lower than ∼10 cm. Using the values obtained for the kinetic temperature and gas density from OH, CO, and other molecular species, we derive a water column density of (9±3)×10 cm in the absorbing gas. Hence, the water vapor abundance in this region, χ(H2O), is ≃(1-2)×10 . The relatively low H2O/OH abundance ratio in the region, ≃2-4, is a signature of UV photon dominated surface layers traced by far–IR observations. As a consequence the temperature of the absorbing gas is high, TK ≃300-500 K, which allows very efficient neutral–neutral reactions producing H2O and OH. On the other hand, the 183.31 GHz data provide a much better spatial and spectral resolution than the far-IR ISO data. This maser line allows to trace water deeper into the cloud, i.e., the inner, denser (n(H2)≥10 5−6 cm) and colder (Tk∼40 K) gas. The emission is very strong toward the cores. The derived water vapor abundance for this component is a few×10. There is also moderate extended emission around Sgr B2 main condensations, a fact that supports the water vapor abundance derived from far–IR H2O lines for the outer gas. Subject headings: infrared: ISM: lines and bands — ISM: individual (Sgr B2) — ISM: molecules