Vibrationally Excited Lines of HC 3 N Associated with the Molecular Disk around the G24.78+0.08 A1 Hypercompact H ii Region

Abstract

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 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 = 2 e ) 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 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 (∼3.0–3.5) are higher than the CH 3 CN/HC 3 N ratios in the other disks (∼0.03–0.11) by more than 1 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 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.