Long-Term Variations in the Mid-IR Spectra of Dusty Protostellar Disks

As part of a survey of the mid-IR spectral characteristics of stars with circumstellar dust disks (Herbig Ae/Be stars, T Tauri stars, and a few other objects with similar characteristics) we have been monitoring a few select object for variability. In a few cases, such as the isolated Herbig stars HD 31648 (MWC 480) and HD 163296, as well as stars whose nature remains uncertain, such as HD 45677 and HD 50138, lower resolution photometry extend the data set back a quarter of a century. HD 163296 recently underwent a major "flare" in its 3 micron emission, suggesting the possible ejection of its next Herbig-Haro object. HD 50138 continues to exhibit fluctuations on time scales of years, and is found to possess organic emission bands at 7.7 and 8.6 microns. The T Tauri star DG Tau went from exhibiting weak silicate emission to strong silicate absorption. The absorption profile contained the 11.2 micron feature due to crystalline olivine, the first time it has been observed in absorption in an astronomical object.

This work has been supported by NASA's Origins of Solar Systems Program, the University Research Council of the University of Cincinnati, and the Aerospace Corporation's Independent Research and Development Program.