Warm C₂H₂ toward NGC 7538 IRS9: Grain Surface Origin

Steven D. Doty, Sandra L. Doty, Jeff Cochran, John Lacy, John Barentine, Richard Field

Department of Astronomy, University of Texas, Austin, USA.
Department of Physics and Astronomy, Denison University, Granville, USA.
Department of Physics, Ohio University Lancaster, Lancaster, USA.

DOI: 10.4236/ijaa.2014.43044                

Abstract

We consider models for the observed ro-vibrational absorption lines of acetylene toward NGC 7538 IRS9. The data are fit with multiple screens, each having separate column densities, rotational and vibrational excitation temperatures, and filling factors. The best fit was determined using a chi-squared minimization scheme. We find that only one screen is necessary—multiple screens gave rise to either making one of the screens transparent, or very occasionally making the two screens the same. As a result, we can place constraints on T_rot, T_vib, N_C2H2, and the filling factor, f. In particular we find 0.03 < f < 0.3 with a best fit of f ~ 0.1. We also find T_vib < 200 K, with a best fit of T_vib < 20 K. We find N_C2H2 = 2.4 +/- 0.6 × 10¹⁶ cm^-2, or that N × f ~ 2 × 10¹⁵ cm^-2. Lastly, we find 80 < T_rot < 140 K, with a best fit of T_rot ~ 100 K. Physically, this can be interpreted as: (1) no vibrational excitation, (2) the warm region only fills a small fraction of the beam, (3) the C₂H₂ arises very near a region of 100 K. Chemically, this is in consistent with a model where the C₂H₂ is formed in the gas phase. It is however consistent with a scenario where the C₂H₂ is evaporated at 100 K from the grain surface, suggesting either a grain-surface origin or earlier origin followed by condensation. Finally, the C₂H₂ column density is consistent with a disk geometry.

Keywords

Stars: Formation, Stars: High Mass, ISM: Molecules, Molecular Processes, Stars: Individual (NGC 7538 IRS9)

Conflicts of Interest

The authors declare no conflicts of interest.

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