THREE MILLIMETER MOLECULAR LINE OBSERVATIONS OF SAGITTARIUS B2. II. HIGH-RESOLUTION STUDIES OFC~(18)O, HNCO, NH_2CHO, AND HCOOCH_3

摘要

High-resolution imaging of C~(18)O, HNCO, NH_2CHO, and HCOOCH_3 in Sgr B2 are presented in this study. The C~(18)O emission comes mainly from the Sgr B2(M) and Sgr B2(N) dense cores and the western gas clump HNO(M). Toward Sgr B2(M), the C~(18)O column density is 2 times higher and the fractional abundance is 80 times higher than toward Sgr B2(N). In HNO(M), the narrow line width implies that the C~(18)O emission arises from the diffuse gas. The complex molecules NH_2CHO and HCOOCH_3 were detected only toward the Sgr B2(N) core. The HNCO K_(-1) = 2 emission is detected only in Sgr B2(N) and is attributed to efficient radiative pumping, which indicates the significant presence of far-infrared field and warm dust grains. Only ～4% of the HNCO was found in the K_(-1) = 0 ladders in Sgr B2(N). The nondetection of the K_(-1) = 2 emission toward Sgr B2(M) is caused by excitation and low abundance. In contrast, the HNCO K_(-1) = 0 emission comes mainly from the extended gas component: the far northern region and HNCO(SW). For the K_(-1) = 0 transitions, T_(rot) = ～7 K. The low T_(rot) and the apparent ubiquity of HNCO suggest that abundant HNCO exists in the Sgr B2 envelope. The HNCO K_(-1) = 0 emission unveiled two spatially extended velocity components; the velocity gap between them covers the same LSR velocities of the Sgr B2 dense cores. If HNCO is formed via surface reactions, the pervasive detection of HNCO in the outer edges of Sgr B2 cloud core leads to the cloud-cloud collision postulate.