Vibrationally-excited Lines of HC$_{3}$N Associated with the Molecular Disk around the G24.78+0.08 A1 Hyper-compact H$_{\rm {II}}$ Region

Abstract: We have analyzed Atacama Large Millimeter/submillimeter Array Band 6 data of the hyper-compact H${\rm {II}}$ region G24.78+0.08 A1 (G24 HC H${\rm {II}}$) and report the detection of vibrationally-excited lines of HC${3}$N ($v{7}=2$, $J=24-23$). The spatial distribution and kinematics of a vibrationally-excited line of HC${3}$N ($v{7}=2$, $J=24-23$, $l=2e$) are found to be similar to the CH${3}$CN vibrationally-excited line ($v{8}=1$), which indicates that the HC${3}$N emission is tracing the disk around the G24 HC H${\rm {II}}$ region previously identified by the CH${3}$CN lines. We derive the $^{13}$CH${3}$CN/HC$^{13}$CCN abundance ratios around G24 and compare them to the CH${3}$CN/HC${3}$N abundance ratios in disks around Herbig Ae and T Tauri stars. The $^{13}$CH$_{3}$CN/HC$^{13}$CCN ratios around G24 ($\sim 3.0-3.5$) are higher than the CH${3}$CN/HC${3}$N ratios in the other disks ($\sim 0.03-0.11$) by more than one order of magnitude. The higher CH${3}$CN/HC${3}$N ratios around G24 suggest that the thermal desorption of CH${3}$CN in the hot dense gas and efficient destruction of HC${3}$N in the region irradiated by the strong UV radiation are occurring. Our results indicate that the vibrationally-excited HC${3}$N lines can be used as a disk tracer of massive protostars at the HC H${\rm {II}}$ region stage, and the combination of these nitrile species will provide information of not only chemistry but also physical conditions of the disk structures.