5. The kinetic temperature structure of the Sgr B2 molecular cloud
The kinetic temperature distribution has been determined for the three lower velocity (44-54 , 55-66 , 67-78 ) molecular clouds in Sgr B2. The lines arising in the highest velocity molecular cloud () are very weak and prevented a reliable determination of the kinetic temperature. The results are presented in Fig. 4, panels a, b and c. The gray scale has been selected in each case to stress the lack of uniformity on a large scale. In the hot core region there is saturation. Fig. 4 also shows, in panels d) and e), the density and column density for the 55-66 cloud.
High temperatures ( K) are found for the three molecular clouds. The molecular clouds at 44-54 and 67-78 show kinetic temperatures which range between 40 K and 120 K, and a mean density of . The 44-54 cloud shows a high temperature region (120 K) bow-shaped to the south of Sgr B2M while the 67-78 has several hot clumps ( 100 K) close to star formation regions.
For the largest molecular cloud with velocities of 55-66 , the kinetic temperature reaches 400 K in the central sources associated with the recent star formation regions. Farther from the center, where the intensity is lower, the kinetic temperature is 40 K. In general the kinetic temperature of the molecular gas in Sgr B2 is at least a factor of two larger than the dust temperature derived by Gordon et al. (1993). The kinetic temperature map for this molecular cloud shows three different components: the hot cores, a warm envelope and a hot ring. These, plus a very hot component which presumably originates the absorption line, will be discussed in detail in the following sections.
5.1. The hot cores
The highest temperatures in Sgr B2 are found towards the continuum sources Sgr B2M and Sgr B2N. Our complete set of data in the J=5-4, J=6-5, J=8-7 and J=12-11 transitions of allows a determination of the thermal structure on a scale of 1 pc (corresponding to at 7.5 kpc) in these regions. As seen in Fig. 5, four hot cores were detected. Two of these are extended, with deconvolved FWHP sizes of and . Both have a of 300 K, and are associated with Sgr B2M and Sgr B2N respectively. The other hot cores, with kinetic temperatures of 200 K and angular sizes of , are newly found hot spots. We have labelled these as and , as an indication of their positions relative to Sgr B2N ( to the west and south of Sgr B2N respectively). These two hot cores have different positions than the clumps HNO(NW) and HNO(E) discovered by Kuan & Snyder (1994), and should not be confused with them.
The line profiles towards Sgr B2N and Sgr B2M are very complex because more than one region is seen along the line of sight. Towards Sgr B2M, the J=5-4 and J=6-5 transitions peak at 56 , while the J=8-7 and J=12-11 peak at 64 . The 64 feature towards Sgr B2M probably arises from denser and hotter gas than that at 56 . Towards Sgr B2N the J=5-4, J=6-5 and J=8-7 transitions peak at 64 , while the J=12-11 peaks at 68 . In this source the gas at 68 is denser than that at 64 . This behaviour clearly shows that several components with different physical conditions are present even in the hot cores themselves.
Though it is very likely that the material close to these sources shows density and temperature gradients, we have used, as a first approximation, a two component model, with different physical conditions and radial velocities to fit the profiles. Two different cases have been considered: a) the J=12-11 transition was smoothed to the angular resolution of the J=8-7 line and b) the J=12-11, J=8-7 and J=6-5 lines were smoothed to the angular resolution of the J=5-4 line. The kinetic temperatures and densities are different for both cases and this is an indication that there is a kinetic temperature and hydrogen density gradient in the hot cores.
In Sgr B2M the densest () and hottest ( K) component produces the emission observed in the J=12-11 and J=8-7 lines at 64 (Fig. 6). The J=5-4 and J=6-5 lines at 54 arise from colder ( K) and lower density gas (). The spectra towards Sgr B2N show that most lines (K=0, K=1, K=2 and K=3) have large opacities () making the analysis more uncertain. However the two velocity component model used previously for Sgr B2M with higher kinetic temperature (400 K) and hydrogen density () is consistent with all of the emission observed in J=12-11 profile and some of the J=8-7 and the J=5-4 and J=6-5 profiles.
The values of we obtain are a factor 2 higher than those obtained by Vogel et al. (1987) using towards the hot cores Sgr B2M and Sgr B2N while the densities are similar for Sgr B2N and one order of magnitude lower for Sgr B2M. Similar differences are also found for the hot core in Orion A where Loren & Mundy (1984) derived a kinetic temperature of 275 K using while Hermsen et al. (1988) obtained a temperature of 165 K using several transitions of . Further observations and models are needed to establish if comes from the hottest gas in the cores.
5.2. The warm envelope
Our results show a warm envelope ( pc) around the star forming region. Apart from the hot ring which will be discussed in the next section, the kinetic temperature for 1 pc is rather uniform, between 40-60 K, independently of the radius. This envelope is relatively dense with densities of . The density shows, however, a systematic change with radius, up to a distance of 8 pc, where r is the distance to Sgr B2M. This dependence was obtained by averaging densities from 8 radial cuts and using a least square mean analysis to the averaged curve. The dependence on the radius we obtain is different from that derived by Lis & Goldsmith (1989). However Lis & Goldsmith (1989) result was obtained from the integrated intensity of 1-0. Thus, this must be an average for all three molecular clouds, while our relation has been obtained only for the 55-66 cloud.
5.3. The diffuse and hot envelope
In addition to the dense and warm envelope the absorption lines indicate the presence of another component with lower density. We have estimated the physical conditions of this component by considering the presence of a third layer of molecular gas surrounding the hot cores. However there are two possible scenarios: a) all three velocity components absorb the continuum of Sgr B2M b) only the layer of lower density gas absorbs the continuum radiation while the other two, of higher density, only contribute to the emission (see Hüttemeister et al. 1993). For both cases our model generates similar profiles for the lines. For this analysis we considered a continuum source with a size of (3.2 mm) and (1.2 mm) and a main beam temperature of 3.1 K (3.2 mm) and 4.4 K (1.2 mm) (Martín-Pintado et al. 1990). Table 2 contains the results of the fits. Our profiles can only be explained by a diffuse and hot envelope with a density of and a kinetic temperature of 200-300 K. If only this layer were in front of the continuum sources all the K components of the J=5-4 and J=6-5 lines would be observed in absorption. However, the denser hot cores cause that all K components except the 5(4)-4(4) and 6(5)-5(5) are seen in emission. Larger kinetic temperatures of the hot and diffuse envelope could also explain the absorption.
Table 2. Physical properties derived for the hot cores using a 3 component model. We used continuum main beam temperatures of 3.1 and 1.9 K at 3.2 mm towards Sgr B2M and Sgr B2N respectively and a linewidth of 15 . a) the 12-11 transition was smoothed to the 8-7 angular resolution, b) the 12-11, 8-7 and 6-5 transitions were smoothed to the 5-4 angular resolution
Although there is no absorption observed towards Sgr B2N we have also performed fits to the observed profiles using both, a two component and a three component model. In the latter case we considered the same physical conditions for the hot and low density envelope as those determined for Sgr B2M. The results for a three component model are given in Table 2 and show that the data are consistent with both, the diffuse and hot density envelope and the warm envelope.
This diffuse and very hot envelope probably comes from low density material located in the outer parts of the molecular cloud, and might be heated by additional mechanisms, as for example shocks, or UV radiation.
In summary, we find a warm (40-80 K) dense () envelope surrounding the active star forming region in Sgr B2 and probably surrounded by hotter (300 K) and more diffuse () molecular material.
5.4. The hot ring
Fig. 4 shows that the kinetic temperature decreases abruptly from 300-400 K at the positions of Sgr B2M and Sgr B2N to 60-80 K at a distance of 1.3 pc (), in all directions. At a distance of 2 pc from the central sources, the kinetic temperature increases again up to 100-120 K in a relatively thin region approximately 1.4 pc wide and decreases again at larger distances to 40-60 K. In there is a ring like structure with a radius of 2 pc and a width of 1.4 pc. In the following we will refer to this remarkable feature as the hot ring. The presence of such a hot ring with values of 100-120 K, surrounded by material with kinetic temperature of approximately 60 K, is clearly illustrated by the spectra in Fig. 7; Here we show four spectra taken towards the hot ring and four outside it. Superimposed on the observed lines we show the expected line profiles for two kinetic temperatures of 80 and 100-130 K. It is clear that the spectra from the hot ring cannot be explained by lower kinetic temperatures, because the observed intensities of the high K transitions are larger than would be predicted by a model with 80 K. We stress that the model also fits the intensity and thus takes opacity effects into account. The opposite occurs for the spectra taken outside the hot ring. In this case, the observed intensities for the high K transitions are smaller than would be predicted by a model that uses a kinetic temperature of 120 K. Therefore, we conclude that the bulk of the molecular gas at 55-65 shows a hot ring with a radius of 2 pc, a thickness of 1.4 pc and a kinetic temperature of 100-120 K around Sgr B2M and Sgr B2N.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998