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

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10. Conclusions

We have used the 30m telescope to map the Sgr B2 molecular cloud in three transitions (J=5-4, 8-7, 12-11) of [FORMULA] and [FORMULA] and the J=11-10 [FORMULA] lines. In certain positions we have also observed the J=6-5 lines of [FORMULA]. All these data have been combined to produce the first large scale map of the kinetic temperature distribution in the Sgr B2 molecular cloud. The main results derived from this work are the following:

  1. The molecular emission in [FORMULA] and [FORMULA] in Sgr B2 comes from an elongated ridge of [FORMULA] (7 [FORMULA] 14 pc) which contains the main three star forming regions, Sgr B2M, Sgr B2N and Sgr B2S. Basically the ridge consists of four molecular clouds with radial velocities of 44-54, 55-66, 67-78 and 90-120 [FORMULA] as observed from the [FORMULA] J=11-10 line, which overlap partially along the line of sight. The bulk of the molecular gas at 55-66 [FORMULA] extends throughout the ridge, while the low velocity gas (44-54 [FORMULA]) and the high velocity gas (67-78 [FORMULA]) appear at different locations in the ridge.
  2. In addition to the emission lines we have detected an absorption feature at 65 [FORMULA] towards the Sgr B2M continuum source in the 5(4)-4(4) and 6(5)-5(5) lines of [FORMULA].
  3. To determine the physical properties of the Sgr B2 molecular cloud we have developed a LVG model which fits simultaneously the A and E species, several velocity components and the main isotope as well as the [FORMULA] lines of [FORMULA]. From the multitransition analysis we have obtained the [FORMULA] density, the kinetic temperature and the [FORMULA] column density maps of the molecular clouds in Sgr B2.
  4. The 44-54 and the 67-78 [FORMULA] clouds, which contain [FORMULA], are warm with typical kinetic temperatures of 60-80 K and [FORMULA] densities of [FORMULA].
  5. The bulk of the mass ([FORMULA]) of the Sgr B2 molecular cloud is in the 55-66 [FORMULA] cloud. The kinetic temperature map of this cloud decreases with distance to Sgr B2M from 300 K to 40 K. From the kinetic temperature distribution we have identified three main features: the hot cores, the warm envelope and the hot ring. The absorption lines also show the presence of a very hot and diffuse component.
  6. Our [FORMULA] maps show two hot cores associated with Sgr B2M and Sgr B2N. In addition we have discovered another two hot cores near Sgr B2N; these have been labelled [FORMULA] and [FORMULA]. The kinetic temperatures of the hot cores Sgr B2M and Sgr B2N are 300-400 K and the densities [FORMULA] to [FORMULA]. For [FORMULA] and [FORMULA], [FORMULA] 200 K and [FORMULA].
  7. Surrounding the hot cores there is a cloud of warm envelope (40-80 K) and dense ([FORMULA]) gas. The density decreases with distance from Sgr B2M as [FORMULA] from 1 to 8 pc.
  8. The most remarkable feature found in the kinetic temperature map is the hot ring. This feature appears in the [FORMULA] map as a ring like structure of higher temperature (120 K versus 60-80 K) surrounding the star forming region Sgr B2M and Sgr B2N. The radius of the ring is [FORMULA] 2 pc and its thickness is 1.4 pc.
  9. The very hot component contains gas at very high temperature (500 K) and low density ([FORMULA]). This component is seen towards the continuum source Sgr B2M from [FORMULA] absorption lines. More observations are needed to determine the extent and physical properties of this envelope.
  10. The kinetic temperature distribution found in the Sgr B2 molecular cloud cannot be explained only by the heating from the OB stars. While the high kinetic temperature distribution found in the hot cores up to a distance of 1 pc can be accounted by heating through grain-gas collisions when the total luminosity of Sgr B2M plus Sgr B2N is considered, the high temperature (40-60 K) at larger distances requires a different heating agent. Turbulent heating given a turbulent velocity of 13 [FORMULA] can account for the observed temperatures in the warm envelope. However an additional heating mechanism is needed to explain the presence of the hot ring in Sgr B2. The hot ring, which surrounds the hot dust emission and the ionized gas, very likely represents the interface between the ionized gas and the molecular cloud. The increase of the temperature is probably related to the presence of shocks originated by the expansion of the ionized gas into the molecular cloud.
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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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