HOCO+ toward the Galactic Center

Abstract

Abstract We have identified a weak thermal line, U42.767, which has been detected only in the directions toward Sgr A and Sgr B2, as the $\mathrm{HOCO}^{+} 2_{02} \hbox{--}1_{01}$ transition. Because of the proximity of this line to the SiO maser line at 42.821 GHz ($J = 1 \hbox{--} 0 v = 2$), it was observable simultaneously in $\sim 43 \,\mathrm{GHz}$ SiO maser searches at Nobeyama. From the past data of SiO maser surveys of infrared objects in the Galactic center, we created a map of emission distribution of $\mathrm{HOCO}^{+}$ in the Sgr A molecular cloud as well as maps of the ${{29\atop} \mathrm{SiO}} J = 1 \hbox{--} 0 v=0$ thermal emission and H53$\alpha$ emission. The emission distribution of $\mathrm{HOCO}^{+}$ was quite similar to that of ${{29\atop} \mathrm{SiO}}$ emission. This suggests that an enhancement of the $\mathrm{HOCO}^{+}$ abundance in the galactic center is induced by shock activities, which release $\mathrm{CO}_2$ molecules frozen on grains into gases.