Warm Water Vapor around Sagittarius B2

Abstract

Several condensations heated externally by nearby hot stars are present in the Sgr B2 region for which H2O far-IR lines are expected to probe only an external low-density and high temperature section. Millimeter-wave lines can penetrate deeper into them (higher densities and lower Tk). We have conducted a study combining H2O lines in both spectral regions using the ISO (far-IR lines) and the IRAM 30m telescope (183 GHz line). The far-IRH2O lines, seen in absorption, are optically thick. They form in the outermost gas in front of the far-IR continuum sources, probing a maximum visual extinction of ~5–10 mag. IR photons from the dust play a dominant role in their excitation. We conclude, based on observations of the COJ = 7-6 line at 806.65 GHz, and the lack of emission from the far-IR CO lines, that the gas density has to be below ~10^4 cm^-3. Using the gas kinetic temperature and density derived from OH, CO, and other molecular species, we derive a water column density of (9 ± 3) x 10^16 cm^-2 in the absorbing gas, implying an abundance of ~(1-2) x 10^-5 in this region. The resulting relatively low H2O/OH abundance ratio, ~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. Finally, the 183.31 GHz data allow one to trace the inner, denser (n(H2) > 10^5-10^6 cm^-3), and colder (Tk ~ 40K) gas. The emission is very strong toward the cores with an estimated water vapor abundance of a few x 10^-7. There is also moderate extended emission around Sgr B2 main condensations, in agreement with the water vapor abundance derived from far-IR H2O lines.