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

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... result, we can place constraints on Trot, Tvib, NC2H2, 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 Tvib < 200 K, with a best fit of Tvib < 20 K. We find NC2H2 = 2.4 +/− 0.6 × 10 16 cm −2 , or that N × f ~ 2 × 10 15 cm −2 . Lastly, we find 80 < Trot < 140 K, with a best fit of Trot ~ 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.