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Astron. Astrophys. 320, 957-971 (1997)

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1. Introduction

The Sagittarius B2 molecular cloud is one of the most active regions of massive star formation in the Galaxy. The most luminous regions in the molecular cloud, Sgr B2M and Sgr B2N, are very strong IR emitters (Thronson & Harper 1986; Goldsmith et al. 1987a; Goldsmith et al. 1990) and contain all the signposts of recent massive star formation. Both regions contain several compact and ultracompact HII regions (Martin & Downes 1972; Benson & Johnston 1984; Carlstrom & Vogel 1989; Gaume & Claussen 1990; Mehringer et al. 1993), hot cores (Vogel, Genzel & Palmer 1987; Goldsmith et al. 1987b) and maser emission in OH, [FORMULA], [FORMULA] and SiO (Genzel & Downes 1977; Forster et al. 1978; Hasegawa et al. 1985; Gardner et al. 1986; Elmegreen et al. 1980; Kobayashi et al. 1989; Gaume & Mutel 1987; Mehringer et al. 1993). A third region with less luminosity than the two main cores and located [FORMULA] to the south of Sgr B2M also shows compact HII regions and maser emission. These regions, which contain [FORMULA], are embedded in a giant molecular cloud with a size of [FORMULA] 45 pc (Scoville et al. 1975) and a total mass of [FORMULA] (Lis & Goldsmith 1989).

In spite of the large mass in the "envelope" surrounding the main star forming regions most of the molecular studies have been directed towards studying the physical properties of the star forming cores Sgr B2M and Sgr B2N (Vogel, Genzel & Palmer 1987; Lis & Goldsmith 1991) and little is known about the properties of this massive envelope in Sgr B2. The properties of the molecular envelope have beEn derived from observations of [FORMULA] and [FORMULA] (Scoville et al. 1975; Lis & Goldsmith 1989). These authors have proposed that the Sgr B2 cloud is composed by two components. The first has a uniform [FORMULA] density between 1800 and 3500 [FORMULA] and the second shows a density law varying as [FORMULA], truncated at a value of approximately [FORMULA] for [FORMULA]. Gordon et al (1993) show maps of the dust emission at 1.3 mm and [FORMULA] ; they obtained dust grain temperatures in the range of 15-20 K for the envelope.

On the other hand, several molecules present deep absorption lines towards the continuum sources in Sgr B2M and Sgr B2N (Wilson et al. 1982, Henkel et al. 1983, Greaves et al. 1992). From the analysis of these data it is inferred the presence of a warm ([FORMULA] K) and moderate density envelope ([FORMULA]) around the sources (Wilson et al. 1982, Huttemeister et al. 1993). The presence of this envelope suggests that turbulent heating is operating in the molecular cloud (Wilson et al. 1982). Unfortunately, the absorption lines only sample a very specific region along the line of sight which provide limited information on the extension of the hot material traced by these lines and therefore of the thermal structure in the Sgr B2 molecular cloud.

The goal of the study presented in this paper is to determine the kinetic temperature and density structure of the molecular gas in the Sgr B2 cloud at scales of several parsecs with high angular resolution. To carry out this study we have mapped the J=5-4, J=8-7 and J=12-11 lines of the symmetric top rotor [FORMULA]. These maps, when combined with a model for the excitation of this molecule provides, for the first time, maps of kinetic temperature and [FORMULA] density in the Sgr B2 molecular cloud. From the kinetic temperature and density maps one can make the study of the thermal balance in this prototypical molecular cloud in the Galactic Center and estimate the influence of the central sources, Sgr B2M and Sgr B2N, in the heating of the molecular cloud.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998