8-13 Micron Spectroscopy of Young Stars

We present 8-13 μm spectra of 23 young stars acquired with the UKIRT CGS3 spectrometer, including T Tauri, Herbig Ae/Be, and FU Orionis stars. Silicate emission and absorption features can generally be matched with the Trapezium emissivity, by employing simple models to account for optical depth effects. Two Herbig Ae/Be objects, LkHα 208 and LkHα 198, have emission features that peak at 9.3 μm. These spectra can be adequately fitted with the Trapezium emissivity and a variable optical depth model, plus a hydrocarbon component. A different emissivity peaking at shorter wavelength is not ruled out; however, it is not a necessary conclusion from the observed spectra. Two absorption sources appear to require a narrower emissivity profile. The 11.2 μm peak of crystalline olivine observed in comets is not seen in our YSO spectra, to a limit of 5% for emission sources and 15% for absorption profiles. Aromatic hydrocarbon emission is frequently present in our sample of Herbig Ae/Be stars and is also seen in the T Tauri stars SU Aurigae (G2 III) and DK Tau (K7). Five objects were observed at higher spectral resolution (R ~ 200) to search for spectral structure in the 11 μm region; the aromatic hydrocarbon band at 11.2 μm is resolved into two components at 11.22 and 11.06 μm in LkHα 198 and SU Aur.

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