The Line-of-Sight Distribution of Water in SgrB2

The SgrB2 cloud is one of the most massive star forming regions in our Galaxy. It has many unique characteristics, among them an exceptional chemistry. Several species (FeO, [21]; NH_2, [20]; HF, [15]; C_3, [7]) have, in spite of searches elsewhere, been detected only toward this source. One possible explanation for this enigmatic chemistry is the existence of a layer of hot gas, which is thought to be produced by a shock. Since this layer has the same velocity as the ambient gas in SgrB2, it has been proven difficult to assess its importance for the chemistry of many species, with a few exceptions. In particular its importance for the water chemistry has been a matter of recent debate ([3]; [17]). While its temperature and density are well known (cf. [9]), its column density and width have remained elusive. In this work (illustrated in detail in [5]) we have been able to determine its importance for water chemistry and give constraints for column density and spatial width, by modeling the HDO and H_2^(18)O emission and absorption.