Nitrile chemistry in a disk structure around the G24.78+0.08 hyper-compact HII region
K. Taniguchi,1 K. Tanaka,2,1 Y. Zhang,3 R. Fedriani,4 J. C. Tan,4,5 S. Takakuwa,6F. Nakamura,1,7 M. Saito,1,7 L. Majumdar,8 and E. Herbst5,91

National Astronomical Observatory of Japan, Japan
Center for Astrophysics and Space Astronomy, University of Colorado Boulder, USA
3Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Japan
4Department of Space, Earth & Environment, Chalmers University of Technology, Sweden
5Department of Astronomy, University of Virginia, USA
6
Department of Physics and Astronomy, Kagoshima University, Japan
7Department of Astronomical Science, SOKENDAI, Japan
8
School of Earth and Planetary Science, NISER, India
9Department of Chemistry, University of Virginia, USA
2Complex nitrile species, such as CH3CN and HC3N, have been usually detected in disks
around T Tauri stars and Herbig Ae stars (e.g., [1]). The Molecules with ALMA at Planetforming Scale (MAPS) [2] Large Program reveals that CH3CN and HC3N trace different layers of these disks; HC3N traces upper and warmer layers compared to CH3CN [3]. Thus, it isuseful for investigating physical conditions of disks to observe different nitrile species. However, only CH3CN lines have been used as typical disk tracers around massive stars (M > 8Msun) so far, and the nitrile chemistry in disks around such massive stars is still unrevealed yet.
We have analyzed Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 6 Band 6
data toward the hyper-compact HII region G24.78+0.08 A1 (hereafter G24 HC HII region). A
current protostellar mass is derived to be ~20 Msun by the SED fitting. Two vibrationallyexcited lines of HC3N (v7=2, J=24-23, l=0 and l=2e) have been detected from this HC HIIregion. The spatial distribution and position-velocity (P-V) diagram of the HC3N line (l=2e)
are similar to those of the vibrationally-excited CH3CN line (v8=1, JK,l =126,1-116,1). The
CH3CN lines were confirmed to trace the molecular disk in the G24 HC HII region [4]. Theseresults suggest that the HC3N emission traces the molecular disk around the G24 HC HII region previously identified by CH3CN.
Using the 13CH3CN (J=13K-12K, K=0-6) and HC13CCN (J=26-25) lines, we have derived theN(13CH3CN)/N(HC13CCN) column density ratios at the two positions around the G24 HC HIIregion, which we name “HC HII region” and “Molecular Peak”, respectively. The
N(13CH3CN)/N(HC13CCN) ratios are derived to be ~3.0-3.5. If there are no effects of the 13C
isotopic fractionation in each molecule (e.g., [5]) and the selective photodissociation of13CH3CN and HC13CCN, the N(13CH3CN)/N(HC13CCN) ratio is expected to reflect theCH3CN/HC3N abundance ratio. We compare these N(13CH3CN)/N(HC13CCN) ratios to theCH3CN/HC3N abundance ratio in disks around T Tauri stars and Herbig Ae stars [3], asshown in Figure 1. The N(13CH3CN)/N(HC13CCN) ratios around the G24 HC HII region are
more than an order of magnitude higher than the CH3CN/HC3N abundance ratios in the otherdisks (~0.03-0.11). The higher N(13CH3CN)/N(HC13CCN) ratios around G24 suggest that theCH3CN/HC3N abundance ratios in the disk around the massive star are higher than thosearound the other disks.
Another possible interpretation of the much high N(13CH3CN)/N(HC13CCN) ratios around theG24 HC HII region is that the column density ratios do not reflect the CH3CN/HC3N abun-dance ratios. In fact, the integrated-intensity ratios of the vibrationally-excited lines ofCH3CN and HC3N, which have almost same upper-state energies (~770 K), are derived to be~ 0.5-1.5 around the G24 HC HII region. These integrated-intensity ratios are still higher than
the abundance ratios in disks around T Tauri stars and Herbig Ae stars. Thus, it is most likelythat the CH3CN/HC3N abundance ratios around the G24 HC HII region are higher than thosein the other disks around the lower-mass stars.
The high CH3CN/HC3N ratios around the G24 HC HII region imply the thermal desorption ofCH3CN in the hot dense region, and efficient destruction of HC3N by the UV radiation andreactions with ions (e.g., H3+) in the ionized region. The former is supported by the fact thatthe derived excitation temperatures of CH3CN (~ 330 K) around the G24 HC HII region aremuch higher than its sublimation temperature. This is different from a suggestion that thephoto-evaporation is important in disks around T Tauri stars and Herbig Ae stars. The higher
UV photodissociation rate of HC3N compared to CH3CN [6] can destroy both HC3N andHC13CCN efficiently, which should increase the CH3CN/HC3N abundance ratios and theN(13CH3CN)/N(HC13CCN) ratios. In summary, the nitrile chemistry around the massive star islikely different from that in the disks around T Tauri stars and Herbig Ae stars.

Figure 1: Comparisons of the CH3CN/HC3N abundance ratio among disks with different stellar masses.

References[1]
[2]
[3]
[4]
[5]
[6]

J. B. Bergner, V. G. Guzman, K. I. Öberg, et al., 2018, ApJ, 857, 69.
K. I. Öberg, V. V. Guzman, C. Walsh, et al., 2021 (in press) arXiv:2109.06268
J. D. Ilee, C. Walsh, A. S. Booth, et al., 2021 (in press) arXiv:2109.06319
L. Moscadelli, R. Cesaroni, M. T. Beltrán, 2021, A&A, 650, A142.
K. Taniguchi, H. Ozeki, M. Saito, 2017, ApJ, 846, 46.
R. Le Gal, M. T. Brady, K. I. Öberg, et al., 2019, ApJ, 886, 86.