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

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Abstract
In this study I present new K- and L'-band infrared photometry and 2-4 μm spectra of 10 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 of low-mass stars are detected in five of the targets, and these stars are fit with models to determine spectral type, infrared accretion excess veiling (rK and r) and dust temperatures, estimates of visual extinction, and characteristics of the 3 μm water ice absorption. On average, the models find high extinction values, infrared accretion excess emission with blackbody temperatures in the 900-1050 K range, and 3 μm absorption profiles best fit by water frozen onto cold grains rather than thermally processed ice. Five techniques are used to estimate the extinction toward the stellar photospheres; most give 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 exhibit Brγ in emission, accretion luminosities are estimated using relations between Lacc and Brγ luminosity. The young stars in this sample are 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 2-4 μm spectra are placed in the context of existing multiwavelength data, and five of the stars are categorized as reddened Class II stars or stars in transition between Class I and Class II, rather than protostars embedded within massive remnant envelopes.
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