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

  • Covey K
  • Greene T
  • Doppmann G
  • et al.
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Abstract

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-1) than classical T Tauri stars (CTTSs; median equatorial rotation velocity ~18 km s-1) in the ρ Ophiuchi and Taurus-Aurigae regions. Projected rotation velocity (vsini) is weakly correlated with Teff in our sample but does not seem to correlate with Brγ emission (a common accretion tracer), the amount of excess continuum veiling (rk), 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˙~10-8 Msolar yr-1 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. 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 NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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Covey, K. R., Greene, T. P., Doppmann, G. W., & Lada, C. J. (2005). The Angular Momentum Content and Evolution of Class I and Flat-Spectrum Protostars. The Astronomical Journal, 129(6), 2765–2776. https://doi.org/10.1086/429736

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