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.
Categories: astro-ph.GA astro-ph.SR