Warm C₂H₂ toward NGC 7538 IRS9: Grain Surface Origin

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 × 10 16 cm -2 , or that N × f ~ 2 × 10 15 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 C 2 H 2 arises very near a region of 100 K. Chemically, this is in consistent with a model where the C 2 H 2 is formed in the gas phase. It is however consistent with a scenario where the C 2 H 2 is evaporated at 100 K from the grain surface, suggesting either a grain-surface origin or earlier origin followed by condensation. Finally, the C 2 H 2 column density is consistent with a disk geometry.