Stellar Properties of Embedded Protostars

Protostars are precursors to the nearly fully assembled T-Tauri and Herbig Ae/Be type stars undergoing quasi-static contraction towards the zero-age main sequence; they are in the process of acquiring the majority of their stellar mass. Although numerous young stars with spatially extended envelope-like structures appear to fit this description, their high extinction has inhibited observers from directly measuring their stellar and accretion properties and confirming that they are in fact in the main phase of mass accretion (i.e., true protostars). Recently, however, high dispersion spectrographs on large aperture telescopes have allowed observers to begin studying the stellar and accretion properties of a subset of these stars, commonly referred to as Class I stars. In this Chapter, we summarize the newly determined properties of Class I stars and compare them with observations of Class II stars, which are the more optically revealed T Tauri stars, to better understand the relative evolutionary state of the two classes. Class I stars have distributions of spectral types and stellar luminosities that are similar to those of Class II stars, suggesting similar masses and ages. The stellar luminosity and resulting age estimates, however, are especially uncertain given the difficulty in accounting for the large extinctions, scattered light emission and continuum excesses typical of Class I stars. Several candidate Class I brown dwarfs are identified. Class I stars appear to rotate somewhat more rapidly than T Tauri stars, by roughly a factor of 2 in the mean. Likewise, the disk accretion rates inferred from optical excesses and Brγ luminosities are similar to, but larger in the mean by a factor of a few than, the disk accretion rates of T Tauri stars. There is some evidence that the disk accretion rates of Class I stars are more distinct from T Tauri stars within the ρ Ophiuchi star forming region than in others (e.g., Taurus-Auriga), suggesting a possible environmental influence. The determined disk accretion rates are nevertheless 1-2 orders of magnitude less than the mass infall rates predicted by envelope models. In at least a few cases the discrepancy appears to be caused by T Tauri stars being misclassified as Class I stars because of their edge-on disk orientation. In cases where the envelope density and infall velocity have been determined directly and unambiguously, the discrepancy suggests that the stellar mass is not acquired in a steady-state fashion, but instead through brief outbursts of enhanced accretion. If the ages of some Class I stars are in fact as old as T Tauri stars, replenishment may be necessary to sustain the long-lived envelopes, possibly via continued dynamical interactions with cloud material.