Does the 3.3 micron polycyclic aromatic hydrocarbon emission feature require ultraviolet excitation?

Tracy L. Smith, Space Science Institute
Geoffrey C. Clayton, Louisiana State University
Lynne Valencic, Universidad Nacional Autónoma de México


Unidentified infrared bands (UIBs) have been observed in virtually every dusty astrophysical environment investigated. The UIB carrier must be abundant and ubiquitous. Strong evidence points to polycyclic aromatic hydrocarbons as likely candidates, but the identification is not complete. Additional diagnostics are needed to further constrain the UIB carrier, such as probing excitation sources ranging from UV-strong to UV-weak to determine the "band gap" of the UIB carrier. Observations and models suggest that the UIBs can be found in sources with weak UV fields. To that end, we present new results of observing the 3.3 μm spectral region in six stars embedded in reflection nebulae and in six Vega-like stars. These objects have effective temperatures ranging from 3500 to 12,000 K. Their environments include dust that should be relatively unprocessed (reflection nebulae) and dust that has most likely undergone significant processing (Vega-like) by the embedded illumination source. Together with data from the literature, we have a sample of 27 sight lines. Our analysis suggests that neither the strength of the UV field impinging on the dust nor the effective temperature of the star is the determining factor in whether the 3.3 μm UIB emission is present in an object. We found three detections of the 3.3 μm emission band, one in a Vega-type object, one in a Herbig Ae/Be object, and one in a reflection nebula, and all with disks. The role of disk geometry is likely to be important in revealing or obscuring the photodis-sociation regions from which the UIB emission arises.