Submillimeter continuum observations of Sagittarius B2 at subarcsecond spatial resolution

We report the first high spatial resolution submillimeter continuum observations of the Sagittarius B2 cloud complex using the Submillimeter Array (SMA). With the subarcsecond resolution provided by the SMA, the two massive star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact sources. In total, twelve submillimeter cores are identified in the Sgr B2(M) region, while only two components are observed in the Sgr B2(N) clump. The gas mass and column density are estimated from the dust continuum emission. We find that most of the cores have gas masses in excess of 100 M$_{odot}$ and column densities above 10$^{25}$ cm$^{-2}$. The very fragmented appearance of Sgr B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well fitted by a Plummer density distribution. This would lead one to believe that in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms first in an homogeneous core, and the rest of the cluster forms subsequently in the then fragmenting structure.

Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines

We have observed CH absorption lines ($J=3/2, N=1 leftarrow J=1/2, N=1$) against the continuum source Sgr~B2(M) using the textit{Herschel}/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show `spiral arm cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr~B2 envelope. The observations show that each `spiral arm harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H$_2$ column densities found at lower $A_V$ by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.

High-Resolution Observations of Molecular Lines toward the Hot Core G28.20-0.04N

We present the results from arcsecond resolution observations of various line transitions at 1.3 mm toward hypercompact HII region G28.20-0.04N. With the SMA data, we have detected and mapped the transitions in the CH$_{3}$CN, CO, $^{13}$CO, SO$_{2}$, OCS, and CH$_{3}$OH molecular lines as well as the radio recombination line H30$alpha$. The observations and analysis indicate a hot core associated with G28.20-0.04N. The outflow and possible rotation are detected in this region.

Infall and Outflow of Molecular Gas in Sgr B2

Observations of two H$_2$CO ($3_{03}-2_{02}$ and $3_{21}-2_{20}$) lines and continuum emission at 1.3 mm towards Sgr B2(N) and Sgr B2(M) have been carried out with the SMA. The mosaic maps of Sgr B2(N) and Sgr B2(M) in both continuum and lines show a complex distribution of dust and molecular gas in both clumps and filaments surrounding the compact star formation cores. We have observed a decelerating outflow originated from the Sgr B2(M) core, showing that both the red-shifted and blue-shifted outflow components have a common terminal velocity. This terminal velocity is 58$pm$2 km s$^{-1}$. It provides an excellent method in determination of the systematic velocity of the molecular cloud. The SMA observations have also shown that a large fraction of absorption against the two continuum cores is red-shifted with respect to the systematic velocities of Sgr B2(N) and Sgr B2(M), respectively, suggesting that the majority of the dense molecular gas is flowing into the two major cores where massive stars have been formed. We have solved the radiative transfer in a multi-level system with LVG approximation. The observed H$_2$CO line intensities and their ratios can be adequately fitted with this model for the most of the gas components. However, the line intensities between the higher energy level transition H$_2$CO ($3_{21}-2_{20}$) and the lower energy level transition H$_2$CO ($3_{03}-2_{02}$) is reversed in the red-shifted outflow region of Sgr B2(M), suggesting the presence of inversion in population between the ground levels in the two K ladders (K$_{-1}$= 0 and 2). The possibility of weak maser processes for the H$_2$CO emission in Sgr B2(M) is discussed.