Vibrationally-excited Lines of HC${}_3$N Associated with the Molecular Disk around the G24.78+0.08 A1 Hyper-compact H${}_{\mathrm{II}}$ Region

Abstract

We have analyzed Atacama Large Millimeter/submillimeter Array Band 6 data of the hyper-compact H${}_{\mathrm{II}}$ region G24.78+0.08 A1 (G24 HC H${}_{\mathrm{II}}$) 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${}_{\mathrm{II}}$ 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${}_{\mathrm{II}}$ region stage, and the combination of these nitrile species will provide information of not only chemistry but also physical conditions of the disk structures.

21 pages, 13 figures, 5 tables, Accepted by The Astrophysical Journal

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