Investigating the Nature of Variable Class I and Flat Spectrum Protostars Using 2-4μm Spectroscopy

In this study I present new K and L-band infrared photometry and 2-4μm spectra of ten Class I and flat spectrum stars forming within the Taurus dark cloud complex. Nine sources have H2 v=0-1 S(1) emission, and some show multiple H2 emission features in their K-band spectra. Photospheric absorptions characteristic to low mass stars are detected in five of the targets, and these stars were fit with models to determine spectral type, infrared accretion excess veiling (rK and rL′) and dust temperatures, estimates of visual extinction and characteristics of the 3μm water-ice absorption. On average, the models found high extinction values, infrared accretion excess emission with blackbody temperatures in the 900-1050K range, and 3μm absorption profiles best fit by water frozen onto cold grains rather than thermally processed ice. Five techniques were used to estimate the extinction toward the stellar photospheres; most gave vastly different results. Analysis of emission line ratios suggests that the effect of infrared scattered light toward some protostars should not be neglected. For stars that exhibited Brγ in emission, accretion luminosities were estimated using relations between Lacc and Brγ luminosity. The young stars in this sample were preferentially chosen as variables, but they do not have the accretion dominated luminosities necessary to put them in their main stage of mass-building. The characteristics of the 2-4μm spectra are placed in the context of existing multi-wavelength data, and five of the stars are catagorized as reddened Class IIs or stars in transition between Class I and II, rather than protostars embedded within massive remnant envelopes. Subject headings: stars: formation, stars: pre-main sequence, ISM: dust, extinction, infrared: stars