Search for the 1.67 mu.m PAH emission band : more upper limits.
Abstract
In an attempt to further resolve the origin and excitation mechanism of the unidentified infrared emission bands, we have obtained high resolution spectroscopy in the H-band for three objects with strong 3.28 and 3.4 micron emission features. In the scenario of highly vibrational excited, isolated, polycyclic aromatic hydrocarbons a broad band resonance at about 1.67 microns with an intensity of I3.4/I1.67 less than or = 6 has been predicted, that should be detectable for our three objects: the Orion bar at position 4, He2-77 and the Red Rectangle. However, this prediction has not been confirmed by us. Our upper limits show that the intensity of the second harmonic should be at least 48 times weaker than the intensity of the 3.4 micron C-H stretching mode. We conclude that both the hot band theory and the explanation that the excitation of the 3.4 micron feature is due to molecular side groups is questionable. More laboratory data such as measurements of the first overtone emission of polycyclic aromatic Hydrocarbons (PAH) structures like cyclopropylidene (C3H2) and phenanthereneare (C10H14) are needed.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- November 1993
- Bibcode:
- 1993A&A...279L..45S
- Keywords:
-
- Astronomical Models;
- Emission Spectra;
- Infrared Astronomy;
- Interstellar Extinction;
- Interstellar Matter;
- Orion Constellation;
- Planetary Nebulae;
- Polycyclic Aromatic Hydrocarbons;
- Astronomical Photometry;
- Astronomical Spectroscopy;
- Infrared Spectra;
- Mathematical Models;
- Astronomy