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 HNC18O lines around Sagittarius B2(Sgr B2)with the IRAM 30 m telescope at the 3 mm wavelength.HNCO 404-303 and HOCN 404-303 are used to obtain the abundance ratio of HNCO to HOCN.The ratio of HNCO 404-303 to HNC18O 404-303 is used to calculate the optical depth of HNCO 404-3o3.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～0.9％.The reason for that still needs further investigation.Based on the intensity ratio of HNCO and HNC18O lines,we updated the isotopic ratio of 16O/18O to be 296±54 in Sgr B2.