Polycyclic Aromatic Hydrocarbon (PAH) molecules are thought to be present widely in the interstellar medium and in nebulae. However, the observational evidence, consisting largely of observations of near-infrared vibrational and bending transitions at wavelengths of 3.3, 6.2, 7.7, 8.6, 11.3, 12.7, and 16.4 μ m, provides very few constraints on the sizes or the charge state of the PAH structures involved. By contrast, the fluorescence spectra of PAH molecules, resulting from electronic downward transitions, are are highly size- and species-specific. Here we are reporting the first detection of near-UV/blue fluorescence by neutral PAH molecules consisting of three to four aromatic rings, such as anthracene (C14H10) and pyrene (C16H10), in an environment outside the solar system, the Red Rectangle nebula. These are the largest molecules so far specifically identified in the interstellar medium. We obtained the observations for this work through long-slit spectroscopy with the 1.5-m telescope at CTIO and the 2.3-m Bok telescope at Steward Observatory. The intensity of the fluorescence in the Red Rectangle nebula was measured at the wavelengths of the Hydrogen Balmer lines, using the line-depth technique. The identification of the source of emission is supported by spectral comparisons with existing laboratory spectra, through spatial correlations with existing surface brightness distributions of the 3.3 μ m PAH feature, and through an analysis of the FUV PAH ionization discontinuity in the spectrum of the exciting star, HD 44179. This work was supported by NASA Grant NAG5-9262 and NSF Grant AST0307307 to The University of Toledo.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2003