DUST EMISSION FEATURES IN 3 MU-M SPECTRA OF HERBIG AE/BE STARS

Low and medium resolution spectra in the 3 mum region were obtained of 24 Herbig Ae/Be stars in a search for organic features from the dust around young stars. The 3.29 mum emission feature from aromatic hydrocarbons was detected for the first time in three objects, Lk Halpha 25, XY Per, and AS 310. The width of the feature in Lk Halpha 25 and XY Per corresponds to the narrow profile characteristic of material close to other young stellar objects. Two other stars, HD 245185 and HK Ori may have weak features. About 20% of the Herbig Ae/Be stars surveyed to date have firmly detected 3.29 mum features. Among these stars, the feature-to-continuum contrast varies over a wide range; emission equivalent widths vary by a factor approximately 20. The available data indicate that the 3.29 mum feature is more extended around Herbig Ae/Be stars of earlier spectral type, possibly due to dehydrogenization or destruction of the aromatics near these stars. Interpretation of the spectra in the optically thin limit suggests that the total number of aromatics excited by the stars is also greater around the earlier-type objects. The ratio of 3.29 mum feature flux to far-infrared flux, which is a measure of the abundance of aromatics relative to larger grains, shows less variation; however, there may be real abundance differences in some of the stars surveyed. If the small grains around Herbig Ae/Be stars have spectral properties like those seen in reflection nebulas, then they play a significant role in the total energy balance of these objects; they can radiate between 7% and 60% of the far-infrared luminosity. No other Herbig Ae/Be stars observed to date have the strong 3.42 and 3.53 mum features seen in Elias 1 and HD 97048.