Spatial distribution of HOCN around Sagittarius B2
  HOCN and HNCO abundance ratio in molecular gas can tell us the information of
their formation mechanism. We performed high-sensitivity mapping observations
of HOCN, HNCO, and HNC$^{18}$O lines around Sagittarius B2 (Sgr B2) with IRAM
30m telescope at 3-mm wavelength. HNCO 4$_{04}$-3$_{03}$ and HOCN
4$_{04}$-3$_{03}$ are used to obtain the abundance ratio of HNCO to HOCN. The
ratio of HNCO 4$_{04}$-3$_{03}$ to HNC$^{18}$O 4$_{04}$-3$_{03}$ is used to
calculate the optical depth of HNCO 4$_{04}$-3$_{03}$. The abundance ratio of
HOCN and HNCO is observed to range from 0.4% to 0.7% toward most positions,
which agrees well with the gas-grain model. However, the relative abundance of
HOCN is observed to be enhanced toward the direction of Sgr B2 (S), with HOCN
to HNCO abundance ratio of $\sim$ 0.9%. The reason for that still needs further
investigation.Based on the intensity ratio of HNCO and HNC$^{18}$O lines, we
updated the isotopic ratio of $^{16}$O/$^{18}$O to be 296 $\pm$ 54 in Sgr B2.
