2022ApJ...931...99T

We have analyzed Atacama Large Millimeter/submillimeter Array Band 6 data of the hypercompact H II region G24.78+0.08 A1 (G24 HC H II) and report the detection of vibrationally excited lines of HC₃N (v₇ = 2, J = 24 - 23). The spatial distribution and kinematics of a vibrationally excited line of HC₃N (v₇ = 2, J = 24 - 23, l = 2e) are found to be similar to the CH₃CN vibrationally excited line (v₈ = 1), which indicates that the HC₃N emission is tracing the disk around the G24 HC H II region previously identified by the CH₃CN lines. We derive the ¹³CH₃CN/HC¹³CCN abundance ratios around G24 and compare them to the CH₃CN/HC₃N abundance ratios in disks around Herbig Ae and T Tauri stars. The ¹³CH₃CN/HC¹³CCN ratios around G24 (∼3.0-3.5) are higher than the CH₃CN/HC₃N ratios in the other disks (∼0.03-0.11) by more than 1 order of magnitude. The higher CH₃CN/HC₃N ratios around G24 suggest that the thermal desorption of CH₃CN in the hot dense gas and efficient destruction of HC₃N in the region irradiated by the strong UV radiation are occurring. Our results indicate that the vibrationally excited HC₃N lines can be used as a disk tracer of massive protostars at the HC H 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.