Three Millimeter Molecular Line Observations of Sagittarius B2. II. High-Resolution Studies of C 18O, HNCO, NH 2CHO, and HCOOCH 3

Abstract

High-resolution imaging of C<SUP>18</SUP>O, HNCO, NR<SUB>2</SUB>CHO, and RCOOCH<SUB>3</SUB> in Sgr B2 are presented in this study. The C<SUP>18</SUP>O emission comes mainly from the Sgr B2(M) and Sgr B2(N) dense cores and the western gas clump RNO(M). Toward Sgr B2(M), the C<SUP>18</SUP>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<SUP>18</SUP>O emission arises from the diffuse gas. The complex molecules NR<SUB>2</SUB>CHO and HCOOCR<SUB>3</SUB> were detected only toward the Sgr B2(N) core. The HNCO K<SUB>-1</SUB> = 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<SUB>-1</SUB> = 0 ladders in Sgr B2(N). The nondetection of the K<SUB>-1</SUB> = 2 emission toward Sgr B2(M) is caused by excitation and low abundance. In contrast, the HNCO K<SUB>-1</SUB> = 0 emission comes mainly from the extended gas component: the far northern region and HNCO(SW). For the K<SUB>-1</SUB> = 0 transitions, T<SUB>rot</SUB> = ∼7 K. The low T<SUB>rot</SUB> and the apparent ubiquity of RNCO suggest that abundant HNCO exists in the Sgr B2 envelope. The HNCO K<SUB>-1</SUB> = 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. <P />A north-south C<SUP>18</SUP>O bipolar structure was seen in Sgr B2(M) centered at the compact H II region F. The bipolar structure appears asymmetric and thus favors the outflow interpretation. The sharp outer edges of the C<SUP>18</SUP>O line profiles of the two lobes further support the outflow picture. The estimated outflow age is ∼2±1 x 10<SUP>4</SUP> yr, and the total mass is ∼1700 M<SUB>sun</SUB>. The outflow masses for the blue and red lobes are &lt;360 M<SUB>sun</SUB> and &lt;410 M<SUB>sun</SUB>, respectively. The mass-loss rate is thus &lt;0.037 M<SUB>sun</SUB> yr<SUP>-1</SUP>. The detection of outflows in Sgr B2(M) supports the gas dispersal picture and subsequent chemical variations disclosed by the HNO and HC<SUP>13</SUP>CCN emission void. Three distinct velocity components toward Sgr B2(N) were seen from the HNCO K<SUB>-1</SUB> = 2 emission. The broad component is centered at the H II region K2 with a north-south velocity gradient, which is probably due to rotation. The mass of the rotating cloud is between ∼630 and 1570 M<SUB>sun</SUB>. The two narrow components are located on the opposite sides of the K1-K2 ridge and are in fact the two lobes of a gas outflow. The estimated outflow age of Sgr B2(N) is ∼6 x 10<SUP>3</SUP> yr, which is a factor of 3 younger than Sgr B2(M). The outflow masses are ≤200 and ≤300 M<SUB>sun</SUB> for the red and blue lobes, respectively. This yields a mass-loss rate &lt;0.08 M<SUB>sun</SUB> yr<SUP>-1</SUP>, about 2 times higher than that of Sgr B2(M). All these suggest that Sgr B2(N) is much younger than Sgr B2(M). Finally, high-resolution imaging of the radiatively excited HNCO K<SUB>-1</SUB> = 2 transition allows the separation of an apparent bipolar structure into a gas outflow and a rotating disk cloud.