The nature and evolutionary status of protostars in clustered environments

We propose to obtain Spitzer IRS spectra of a statistically significant sample of ~50 protostellar sources that we have identified in previous GTO observations of the NGC 2264 and IC 348 clusters. We intend to measure and characterize the mid-infrared spectral features between 5-40 micron of these sources to develop a more refined empirical classification and evolutionary sequence for high and low mass protostellar objects than otherwise possible using broad-band infrared SEDs alone. We intend to employ detailed radiative transfer models to fit the entire SEDs, including the IRS observations, in order to derive constraints for the basic physical properties of the protostars in our sample, including the luminosities of the central sources and the masses, density structures, and infall rates that characterize their envelopes. We intend to investigate how the spectral properties of protostars depend on their mass and star forming environment in order to develop a more complete picture of protostellar evolution. The proposed Spitzer IRS observations will enable us to place the comparison of the individual and collective protostellar characteristics on a firm quantitative footing.