Gas-phase polycyclic aromatic hydrocarbons in absorption toward protostellar sources?

One of the major criticisms of identifying the infrared emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the lack of a mat ch between laboratory spectra of individual PAHs and the emission features. Part of the difficulty arises from the complexity of modeling the emission mechanism with an a priori unknown mixture of ionized and neutral PAHs. A direct comparison between laboratory spectra of PAHs and astronomical sourc es is possible for absorption spectra. However, because of poor atmospheric transmission, ground-based spectra of the PAH absorption band in the C-H s tretch region are too noisy to make a detailed comparison with laboratory s pectra. In this paper we show that ISO Short Wavelength Spectrometer spectr a of a few protostars that show a 3.25 mum absorption band can be well matc hed by laboratory absorption spectra of a mixture of isolated PAHs.