The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023

Abstract

The carrier of the dust-associated photoluminescence process causingERE in many dusty interstellar environments remains unidentified.Several competing models are more or less able to match the observedbroad, unstructured ERE band. We now constrain the character of theERE carrier further by determining the wavelengths of the radiationthat initiates the ERE. Using the imaging capabilities of the HST,we have resolved the width of narrow ERE filaments appearing on thesurfaces of externally illuminated molecular clouds in the brightreflection nebula NGC 7023 and compared them with the depth of penetrationof radiation of known wavelengths into the same cloud surfaces. Weidentify photons with wavelengths shortward of 118 nm as the sourceof ERE initiation, not to be confused with ERE excitation, however.There are strong indications from the well-studied ERE in the RedRectangle Nebula and in the high-vertical bar b vertical bar Galacticcirrus that the photon flux with wavelengths shortward of 118 nmis too small to actually excite the observed ERE, even with 100%quantum efficiency. We conclude, therefore, that ERE excitation resultsfrom a two-step process. The first, involving far-UV photons withE > 10: 5 eV, leads to the creation of the ERE carrier, most likelythrough photoionization or photodissociation of an existing precursor.The second, involving more abundant near-UV/optical photons, consistsof the optical pumping of the previously created carrier, followedby subsequent deexcitation via photoluminescence. The latter processcan occur many times for a single particle, depending upon the lifetimeof the ERE carrier in its active state. While none of the previouslyproposed ERE models can match these newconstraints, we note thatunder interstellar conditions most PAH molecules are ionized to thedication stage by photons with E > 10: 5 eV and that the electronicenergy level structure of PAH dications is consistent with fluorescencein the wavelength band of the ERE. Therefore, PAH dications deservefurther study as potential carriers of the ERE.