Circumstellar kinematics and the measurement of stellar mass for the protostars TMC1 and TMC1A

We present observations of two protostars in the Taurus molecular cloud, TMC1 and TMC1A, obtained using the Owens Valley Millimeter Array. (CO)-C-13 and (CO)-O-18 J = 1-0 data, and observations at 2.7 mm in the continuum, reveal the presence of molecular gas in circumstellar envelopes out to a radius of 1000 au. The velocity field in these envelopes is well described by Keplerian rotation and shows no signature of infall motions. The dynamical masses of the central objects, derived from the velocity structure of the isotopic CO emission, are 0.2-0.4 M. for TMC1 and 0.35-0.7 M. for TMC1A, depending upon the assumed inclination, with typical uncertainties in the mass measurements at a given inclination of 10 per cent. Our determinations of the stellar masses enable us to place upper limits on the accretion rates in these sources of (M) over dot less than or similar to 4 x 10(-7) M. yr(-1), which is at least an order of magnitude lower than that needed to assemble the observed mass with a constant (M) over dot assuming a typical age of 10(5) yr. We conclude that the accretion rate is not constant in time.