2005AJ....129.2765C

We report on the angular momentum content of heavily embedded protostars on the basis of our analysis of the projected rotation velocities (vsini) of 38 Class I/flat-spectrum young stellar objects recently presented by Doppmann and others. After correcting for projection effects, we find that infrared-selected Class I/flat-spectrum objects rotate significantly more quickly (median equatorial rotation velocity ∼38 km/s) than classical T Tauri stars (CTTSs; median equatorial rotation velocity ∼18 km/s) in the ρ Ophiuchi and Taurus-Aurigae regions. Projected rotation velocity (vsini) is weakly correlated with T_effin our sample but does not seem to correlate with Brγ emission (a common accretion tracer), the amount of excess continuum veiling (r_k), or the slope of the spectral energy distribution between the near- and mid-IR (α). The detected difference in rotation speeds between Class I/flat-spectrum sources and CTTSs proves difficult to explain without some mechanism that transfers angular momentum out of the protostar between the two phases. Assuming that Class I/flat-spectrum sources possess physical characteristics (M_*, R_*, and B_*) typical of pre-main-sequence stars, fully disk-locked Class I objects should have corotation radii within their protostellar disks that match well (within 30%) with predicted magnetic coupling radii. The factor of 2 difference in rotation rates between Class I/flat-spectrum and CTTS sources when interpreted in the context of disk-locking models also implies a factor of 5 or greater difference in mass accretion rate between the two phases. A lower limit of M{dot}∼10^–8 M_☉/yr for objects transitioning from the Class I/flat-spectrum stage to CTTSs is required to account for the difference in rotation rates of the two classes by angular momentum extraction through a viscous disk via magnetic coupling.