We report on the angular momentum content of heavily embedded protostars based on our analysis of the projected rotation velocities ( v sin i s ) of 38 Class I/flat spectrum young stellar objects recently presented by Doppmann et al .
( 15 ) The data presented herein were obtained at the W.M .
Keck Observatory , which is operated as a scientific partnership among the California Institute of Technology , the University of California and the National Aeronautics and Space Administration .
The Observatory was made possible by the generous financial support of the W.M .
Keck Foundation . .
After correcting for projection effects , we find that infrared-selected Class I/flat spectrum objects rotate significantly more quickly ( median equatorial rotation velocity \sim 38 km/sec ) than Classical T Tauri stars ( CTTSs ; median equatorial rotation velocity \sim 18 km/sec ) in the \rho Ophiuchi and Taurus-Aurigae regions .
Projected rotation velocity ( v sin i ) is weakly correlated with T _ { eff } in our sample , but does not seem to correlate with Br \gamma emission ( a common accretion tracer ) , the amount of excess continuum veiling ( r _ { k } ) , or the slope of the SED between the near and mid IR ( \alpha ) .
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 .
( 64 ) .
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 \sim 10 ^ { -8 } M _ { \odot } /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 .