Small PAHs in the Red Rectangle

Abstract

Following our initial discovery of blue luminescence in the spectrum of the Red Rectangle (RR) and its identification as fluorescence by small three- to four-ringed polycyclic aromatic hydrocarbon (PAH) molecules, we report on the spatial correlation between the blue luminescence and the 3.3 micron emission, commonly attributed to small, neutral PAH molecules, and on the newly-derived UV/optical attenuation curve for the central source of the RR, HD 44179. Both results provide strong additional evidence for the presence of small PAH molecules with masses of less than 250 a.m.u. in the RR, which supports the attribution of the blue luminescence to fluorescence by the same molecules. We contrast the excellent spatial correlation of the two former emissions with the distinctly different spatial distribution of the extended red emission (ERE) and of the dust-scattered light within the RR. The UV/optical attenuation curve of the central star is unlike any interstellar extinction curve and is interpreted as resulting from circumstellar opacity alone. Major contributions to this opacity are absorptions in broad bands in the mid-UV, contributing to the electronic excitation of the luminescing PAH molecules, and a sharp ionization discontinuity near 7.5 eV in the far-UV, which places a sharp upper limit on the masses of the PAH molecules that are responsible for this absorption. The strength of the far-UV absorption leads to an abundance of the PAH molecules of 10^{-5} relative to hydrogen in the RR. Such small PAHs are perhaps unique to the environment in the RR, where they are shielded from harsh radiation by the dense circmstellar disk.