The Spitzer IRS Protostar Survey of the Orion Molecular Cloud Complex
The Orion Molecular Cloud complex is the most active region of star formation within 500 pc of the Sun, where the Spitzer Space Telescope has identified over 400 protostellar candidates in a variety of environments, from dense clusters to isolation. With the Spitzer Infrared Spectrograph (IRS), we have obtained 5-36 micron follow-up spectroscopy of 349 protostellar candidates; these data are the first detailed mid-Infrared spectroscopic survey of a large number of protostars in a single complex at a common distance. The spectra are being used in combination with the Herschel Orion Protostar survey (HOPS) to determine the fundamental parameters of 278 protostars. We concentrate in this presentation on results from analyses of the mid-infrared IRS spectra alone. We first describe how sources can be classified by their mid-IR spectra and show the statistics of source classification (i.e., number of Class 0, Class I, Flat-spectrum sources and contamination). We then present an analysis of the copious silicate and ice absorption features, examining their presence and depth as a function of both evolutionary class and cloud environment. We also present the first unambiguous (and rare) detection of crystalline silicate absorption features from a cold protostellar envelope; we suggest this is material carried by winds and outflows from the hot inner regions to the outer cold envelope.