Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud

Details Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud

Jul 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...519..667m&link_type=abstract

The Astrophysical Journal, Volume 519, Issue 2, pp. 667-686.

Astronomy and Astrophysics

Astronomy

30

Galaxy: Center, Ism: Bubbles, Ism: Individual: Name: Sagittarius B2, Ism: Molecules, Masers, Stars: Wolf-Rayet

Scientific paper

We present high-resolution (3") maps of the (3, 3) and (4, 4) lines of NH_3 toward the southern part of the molecular envelope of the Sagittarius B2 star-forming region. These maps reveal, for the first time, that the morphology of the hot gas in the Sgr B2 envelope is dominated by at least six rings, two arcs, and a filament. The sizes of the rings are between 1 and 2.6 pc and their thicknesses between 0.2 and 0.4 pc. Most of the gas in the rings 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 rings and arcs represent regions of enhanced H_2 density and/or enhanced NH_3 abundance in the Sgr B2 envelope. Some of the hot rings show radial velocity gradients, which suggests that the rings and arcs correspond to three-dimensional shells expanding at velocities of ~6-10 km s^-1. The walls of the hot shells are highly clumped and contain unresolved condensations (<~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 NH_3 (3, 3) line maps that have brightness temperatures larger than the kinetic temperatures. It is likely that these are newly found NH_3 masers. We also find that other masers in the region such as class II CH_3OH and H_2CO masers are very well correlated in velocity 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 abundance of volatile molecules like NH_3, CH_3OH, and H_2CO (produced by hot temperature and/or shock chemistry) and sufficient velocity coherence at the edges 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>~300 K) and have high H_2 densities (>~10^6-10^7 cm^-3). These exhibit characteristics similar to those of the hot cores associated with newly formed massive stars. The inferred dust luminosity for the hot cores is at least 10^5 L_solar, 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 surrounding newly formed stars. The detection of hot cores suggests that massive 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 has been triggered by the expanding bubbles that produce the shells seen in the NH_3 inversion lines. We discuss the possible origin of these hot shells. We find that a wind-blown bubble driven by typical Galactic Wolf-Rayet stars could account for the kinetic energies and the momenta observed in the hot NH_3 shells. A cluster containing massive evolved stars such as the Quintuplet could easily explain the large concentration of hot shells, the heating of the warm envelope, and the peculiar chemistry observed in the envelope of Sgr B2. The implication of these findings for the heating of Galactic center molecular clouds is briefly discussed.

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