The Angular Momentum Content and Evolution of Class I and Flat-spectrum Protostars

We report on the angular momentum content of heavily embedded protostars based on our analysis of the projected rotation velocities (v sin is) of 38 Class I/flat spectrum young stellar objects recently presented by Doppmann et al. (2005). After correcting for projection effects, we find that infraredselected Class I/flat spectrum objects rotate significantly more quickly (median equatorial rotation velocity ∼ 38 km/sec) than Classical T Tauri stars (CTTSs; median equatorial rotation velocity ∼ 18 km/sec) in the ρ Ophiuchi and Taurus-Aurigae regions. Projected rotation velocity (v sin i) is weakly correlated with Teff in our sample, but does not seem to correlate with Br γ emission (a common accretion tracer), the amount of excess continuum veiling (rk), or the slope of the SED 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 which transfers angular momentum out of the protostar between the two phases. Assuming Class I/flat spectrum sources possess physical characteristics (M∗,R∗,B∗) typical of pre-main sequence stars, fully disk locked Class I objects should have co-rotation radii within their protostellar disks that match well (within 30%) with the predicted magnetic coupling radii of Shu et al. (1994). The factor of two difference in rotation rates between Class I/flat spectrum and CTTS sources, when interpreted in the context of disk locking models, also imply a factor of 5 or greater difference in mass accretion rates between the two phases. A lower limit of Ṁ ∼ 10 M⊙/year 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. Subject headings: accretion, accretion disks — infrared:stars — stars:formation — stars:low-mass — stars:pre–main-sequence — stars:rotation