On the band-to-continuum intensity ratio in the infrared spectra of interstellar carbonaceous dust

Abstract

Published interpretations of the relative intensity variations of the Unidentified Infrared Bands (UIBs) and their underlying continuum are discussed. An alternative model is proposed, in which a single carrier for both emits a) mostly a continuum when it is electronically excited by photons (visible or UV), or b) exclusively the UIBs, when only chemical energy is deposited by H capture on its surface, inducing only nuclear vibrations. The bands will dominate in atomic H regions but will be overcome by thermal continuum radiation when the ambient field is strong but lacks dissociating photons (900-1100 Angstroms). The model applies to PDRs as well as to limbs of molecular clouds in the ISM and agrees quantitatively with recent satellite observations. It gives indications on atomic H density and UIB intensity provided the ambient radiation field is known. It invokes no chemical, electronic, structural or size change in order to interpret the observed intensity variations.

18 pages, 2 figures

References in corpus (7)

• The rich 6 to 9 um spectrum of interstellar PAHs
• The CH out-of-plane bending modes of PAH molecules in astrophysical environments
• The aromatic infrared bands as seen by ISO-SWS: probing the PAH model
• Infrared Emission of Normal Galaxies from 2.5 to 12 Microns: ISO Spectra, Near-Infrared Continuum and Mid-Infrared Emission Features
• Do the Infrared Emission Features Need Ultraviolet Excitation? The PAH Model in UV-Poor Reflection Nebulae
• Unidentified Infrared Bands in the Interstellar Medium across the Galaxy
• Classical, non-linear, internal dynamics of large, isolated, vibrationally excited molecules

Cited by in corpus (1)

• Observations of the Near- to Mid-Infrared Unidentified Emission Bands in the Interstellar Medium of the Large Magellanic Cloud