Three millimeter molecular line observations of sagittarius B2. II. High-resolution studies of C18O, HNCO, NH2CHO, and HCOOCH3

Yi Jehng Kuan, Lewis E. Snyder

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    High-resolution imaging of C18O, HNCO, NH2CHO, and HCOOCH3 in Sgr B2 are presented in this study. The C18O 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 C18O 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 C18O emission arises from the diffuse gas. The complex molecules NH2CHO and HCOOCH3 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 = Q transitions, Trot = ∼7 K. The low Trot 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. A north-south C18O 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 C18O line profiles of the two lobes further support the outflow picture. The estimated outflow age is ∼2 ± 1 × 104 yr, and the total mass is ∼1700 M. The outflow masses for the blue and red lobes are ≤360 M and ≤410 M, respectively. The mass-loss rate is thus ≤0.037 M yr-1. The detection of outflows in Sgr B2(M) supports the gas dispersal picture and subsequent chemical variations disclosed by the HNO and HC13CCN emission void. Three distinct velocity components toward Sgr B2(N) were seen from the HNCO K-1 = 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. 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 × 103 yr, which is a factor of 3 younger than Sgr B2(M). The outflow masses are ≤200 and ≤300 M for the red and blue lobes, respectively. This yields a mass-loss rate ≤0.08 M yr-1, 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-1 = 2 transition allows the separation of an apparent bipolar structure into a gas outflow and a rotating disk cloud.

    Original languageEnglish
    Pages (from-to)981-1000
    Number of pages20
    JournalAstrophysical Journal
    Issue number2 PART I
    Publication statusPublished - 1996


    • ISM: abundances
    • ISM: individual (Sagittarius B2)
    • ISM: jets and outflows ISM
    • Molecules
    • Radio lines: ISM
    • Stars: formation

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science

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