THE SPITZER IRS SPECTRUM OF SMP LMC 11

摘要

We present the first mid-infrared spectra of SMP LMC 11 in the Large Magellanic Cloud. While this object resembles a planetary nebula in the optical, its infrared properties are more similar to an object in transition from the asymptotic giant branch to the planetary nebula phase. A warm dust continuum dominates the infrared spectrum. The peak emission corresponds to a mean dust temperature of 330 K. The spectrum shows overlapping molecular absorption bands from 12 to 17 μm corresponding to acetylene and polyacetylenic chains and benzene. This is the first detection of C_4H_2, C_6H_2, C_6H_6, and other molecules in an extragalactic object. The infrared spectrum of SMP LMC 11 is similar in many ways to that of the preplanetary nebula AFGL 618. The IRS spectrum shows little evidence of nitrogen-based molecules that are commonly seen in Galactic asymptotic giant branch stars. Polycyclic aromatic hydrocarbons are also absent from the spectrum. The detection of the [Ne II] 12.8 μm line in the infrared and other forbidden emission lines in the optical indicates that an ionized region is present.