A 0.2 solar mass protostar with a Keplerian disk in the very young L1527
  IRS system
  In their earliest stages, protostars accrete mass from their surrounding
envelopes through circumstellar disks. Until now, the smallest observed
protostar/envelope mass ratio was ~2.1. The protostar L1527 IRS is thought to
be in the earliest stages of star formation. Its envelope contains ~1 solar
mass of material within a ~0.05 pc radius, and earlier observations suggested
the presence of an edge-on disk. Here we report observations of dust continuum
emission and 13CO (J=2-1) line emission from the disk around L1527, from which
we determine a protostellar mass of M = 0.19 +/- 0.04 solar masses and a
protostar/envelope mass ratio of ~0.2. We conclude that most of the luminosity
is generated through the accretion process, with an accretion rate of ~6.6 x
10^-7 solar masses per year. If it has been accreting at that rate through much
of its life, its age is ~300,000 yr, though theory suggests larger accretion
rates earlier, so it may be younger. The presence of a rotationally--supported
disk is confirmed and significantly more mass may be added to its
planet-forming region as well as the protostar itself.
