TITLE:
Warm C2H2 toward NGC 7538 IRS9: Grain Surface Origin
AUTHORS:
Steven D. Doty, Sandra L. Doty, Jeff Cochran, John Lacy, John Barentine, Richard Field
KEYWORDS:
Stars: Formation, Stars: High Mass, ISM: Molecules, Molecular Processes, Stars: Individual (NGC 7538 IRS9)
JOURNAL NAME:
International Journal of Astronomy and Astrophysics,
Vol.4 No.3,
August
22,
2014
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 Trot, Tvib, NC2H2,
and the filling factor, f. In particular we find 0.03 vib vib C2H2 = 2.4 +/- 0.6 × 1016 cm-2, or that N × f ~ 2 ×
1015 cm-2.
Lastly, we find 80 rot 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.