Hot expanding shells in the envelope of the Sagittarius B2 molecular cloud

We present high-resolution (3 ") maps of the (3, 3) and (4, 4) lines of NH3 toward the southern part of the molecular envelope of the Sagittarius B2 s tar-forming region. These maps reveal, for the first time, that the morphol ogy of the hot gas in the Sgr B2 envelope is dominated by at least six ring s, two arcs, and a filament. The sizes of the rings are between 1 and 2.6 p c and their thicknesses between 0.2 and 0.4 pc. Most of the gas in the ring s is warm, with kinetic temperatures, T-k, of 40-70 K, although some parts of the low-velocity rings reach temperatures larger than 100 K. These hot r ings and arcs represent regions of enhanced H-2 density and/or enhanced NH3 abundance in the Sgr B2 envelope. Some of the hot rings show radial veloci ty gradients, which suggests that the rings and arcs correspond to three-di mensional shells expanding at velocities of similar to 6-10 km s(-1). The w alls of the hot shells are highly clumped and contain unresolved condensati ons (less than or similar to 1 ") in the line maps. There are two kinds of unresolved condensations: those appearing in the (3, 3) line maps with only weak emission in the (4, 4) line and those with rather strong emission in the (4, 4) line compared with that of the (3, 3) line. For the first kind, we identify six condensations in the NH3 (3, 3) line maps that have brightn ess temperatures larger than the kinetic temperatures. It is likely that th ese are newly found NH3 masers. We also find that other masers in the regio n such as class II CH3OH and H2CO masers are very well correlated in veloci ty and position with the hot shells. The large number of masers observed in the hot shells can be explained as a result of the combination of high abu ndance of volatile molecules like NH3, CH3OH, and H2CO (produced by hot tem perature and/or shock chemistry) and sufficient velocity coherence at the e dges of the expanding shells. For the second kind of condensations, we have identified three compact sources in the walls of the shells that are hot ( T-k greater than or similar to 300 K) and have high H-2 densities (greater than or similar to 10(6)-10(7) cm(-3)). These exhibit characteristics simil ar to those of the hot cores associated with newly formed massive stars. Th e inferred dust luminosity for the hot cores is at least 10(5) L., similar to the Orion A hot core. The high luminosity of the hot cores and the lack of associated radio continuum emission indicates that these are internally heated and very likely associated with the dense circumstellar material sur rounding newly formed stars. The detection of hot cores suggests that massi ve star formation is not restricted to only Sgr B2N and Sgr B2M but is also taking place in the envelope of Sgr B2. Very likely, this star formation h as been triggered by the expanding bubbles that produce the shells seen in the NH3 inversion lines. We discuss the possible origin of these hot shells . We find that a wind-blown bubble driven by typical Galactic Wolf-Rayet st ars could account for the kinetic energies and the momenta observed in the hot NH3 shells. A cluster containing massive evolved stars such as the Quin tuplet could easily explain the large concentration of hot shells, the heat ing of the warm envelope, and the peculiar chemistry observed in the envelo pe of Sgr B2. The implication of these findings for the heating of Galactic center molecular clouds is briefly discussed.