No Keplerian Disk >10 AU around the Protostar B335: Magnetic Braking or Young Age?

Abstract

We have conducted ALMA cycle 2 observations in the 1.3 mm continuum and in the C18O (2-1) and SO (5_6-4_5) lines at a resolution of ~0.3" toward the Class 0 protostar B335. The 1.3 mm continuum, C18O, and SO emission all show central compact components with sizes of ~40-180 AU within more extended components. The C18O component shows signs of infalling and rotational motion. By fitting simple kinematic models to the C18O data, the protostellar mass is estimated to be 0.05 Msun. The specific angular momentum, on a 100 AU scale, is ~4.3E-5 km/s*pc. A similar specific angular momentum, ~3E-5 to 5E-5 km/s*pc, is measured on a 10 AU scale from the velocity gradient observed in the central SO component, and there is no clear sign of an infalling motion in the SO emission. By comparing the infalling and rotational motion, our ALMA results suggest that the observed rotational motion has not yet reached Keplerian velocity neither on a 100 AU nor even on a 10 AU scale. Consequently, the radius of the Keplerian disk in B335 (if present) is expected to be 1-3 AU. The expected disk radius in B335 is one to two orders of magnitude smaller than those of observed Keplerian disks around other Class 0 protostars. Based on the observed infalling and rotational motion from 0.1 pc to inner 100 AU scales, there are two possible scenarios to explain the presence of such a small Keplerian disk in B335: magnetic braking and young age. If our finding is the consequence of magnetic braking, ~50% of the angular momentum of the infalling material within a 1000 AU scale might have been removed, and the magnetic field strength on a 1000 AU scale is estimated to be ~200 uG. If it is young age, the infalling radius in B335 is estimated to be ~2700 AU, corresponding to a collapsing time scale of ~5E4 yr.

Accepted by ApJ

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