------------------------------------------------------------------------ watersgrb2.tex, ApJ, Jan 2006, in press Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.5.10 (nef2.ens.fr [129.199.96.32]); Tue, 17 Jan 2006 15:39:45 +0100 (CET) X-MailScanner-Information: Please contact postmaster@aoc.nrao.edu for more information X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 5, autolearn=disabled) X-MailScanner-From: javier.rodriguez-goicoechea@lra.ens.fr %astro-ph/0601336 \documentclass{emulateapj} %-------------------------------------------------- \newcommand{\vdag}{(v)^\dagger} \newcommand{\myemail}{javier@damir.iem.csic.es} %% MS DEFINICIONES \def\Msun{{\hbox {M$_\odot$}}} \newcommand{\HII}{H\,{\sc ii}} \newcommand{\OIII}{O\,{\sc iii}} \newcommand{\CII}{C\,{\sc ii}} \newcommand{\NII}{N\,{\sc ii}} \newcommand{\OI}{O\,{\sc i}} \newcommand{\NIII}{N\,{\sc iii}} \slugcomment{Pre-print version, accepted in ApJ main journal, 2006 January 12} \shorttitle{Water Vapor around Sgr B2} \shortauthors{Cernicharo, Goicoechea, Pardo \& Asensio Ramos} \begin{document} \title{Warm Water Vapor around Sagittarius B2\altaffilmark{1}} Juan R. Pardo} CSIC, Serrano 121, 28006, Madrid, Spain} \and \author{Andr\'es Asensio-Ramos} \affil{Instituto de Astrof\'{\i}sica de Canarias, E-38205, La Laguna, Tenerife, Spain } \email{cerni@damir.iem.csic.es} \altaffiltext{1}{Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA.} \altaffiltext{2}{Present address: Laboratoire d'\'{E}tude du Rayonnement et de la Mati\`ere, UMR 811, CNRS, Observatoire de Paris et \'{E}cole Normale Sup\'{e}rieure, 24 rue Lhomond, 75231 Paris Cedex 05, France} \begin{abstract} In the region of Sgr B2 there are several condensations heated externally by nearby hot stars. Therefore H$_2$O 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 T$_k$ lower. We have conducted a study combining H$_2$O lines in both spectral regions. First, $\textit{Infrared Space Observatory}$ observations of several H$_2$O thermal lines seen in absorption toward Sgr~B2(M) at a spectral resolution of $\sim$35~km~s$^{-1}$ have been analyzed. Second, an \textit{IRAM}--30m telescope map of the para--H$_2$O $3_{13}-2_{20}$ line at 183.31~GHz, seen in emission, has also been obtained and analyzed. The H$_2$O 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 $\sim$5 to 10 mag. Radiative transfer models indicate that these lines are quite insensitive to temperature and gas density, and that IR photons from the dust play a dominant role in the excitation of the involved H$_2$O 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 \textit{Caltech~Submillimeter~Observatory}, and the lack of emission from the far--IR CO lines, that the gas density has to be lower than $\sim$10$^4$~cm$^{-3}$. Using the values obtained for the kinetic temperature and gas density from OH, CO, and other molecular species, we derive a water column density of (9$\pm$3)$\times$10$^{16}$~cm$^{-2}$ in the absorbing gas. Hence, the water vapor abundance in this region, $\chi$(H$_2$O), is $\simeq$(1-2)$\times$10$^{-5}$. The relatively low H$_2$O/OH abundance ratio in the region, $\simeq$2-4, is a signature of UV photon dominated surface layers traced by far--IR observations. As a consequence the temperature of the absorbing gas is high, T$_K\simeq$300-500 K, which allows very efficient neutral--neutral reactions producing H$_2$O and OH. On the other hand, the 183.31 GHz data provide a much better spatial and spectral resolution than the far-IR ISO data. This maser line allows to trace water deeper into the cloud, i.e., the inner, denser ($n(H_2)$$\ge$10$^{5-6}$~cm$^{-3}$) and colder (T$_k$$\sim$40 K) gas. The emission is very strong toward the cores. The derived water vapor abundance for this component is a few$\times$10$^{-7}$. There is also moderate extended emission around Sgr~B2 main condensations, a fact that supports the water vapor abundance derived from far--IR H$_2$O lines for the outer gas. \end{abstract}