ALMA Observations of the Protostar L1527 IRS: Probing Details of the Disk and the Envelope Structures

Abstract

We have recently observed the Class 0/I protostar L1527 IRS using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 1 in 220 GHz dust continuum and C 18 O ( J = 2 − 1 ) line emissions with a ∼2 times higher angular resolution ( ∼ 0 .″ 5 ) and ∼4 times better sensitivity than our ALMA Cycle 0 observations. Continuum emission shows elongation perpendicular to the associated outflow, with a deconvolved size of 0 .″ 53 × 0 .″ 15 . C 18 O emission shows similar elongation, indicating that both emissions trace the disk and the flattened envelope surrounding the protostar. The velocity gradient of the C 18 O emission along the elongation due to rotation of the disk/envelope system is reanalyzed, identifying Keplerian rotation proportional to r − 0.5 more clearly than the Cycle 0 observations. The Keplerian-disk radius and the dynamical stellar mass are kinematically estimated to be ∼74 au and ∼ 0.45 M ☉ , respectively. The continuum visibility is fitted by models without any annulus averaging, revealing that the disk is in hydrostatic equilibrium. The best-fit model also suggests a density jump by a factor of ∼5 between the disk and the envelope, suggesting that disks around protostars can be geometrically distinguishable from the envelope from a viewpoint of density contrast. Importantly, the disk radius geometrically identified with the density jump is consistent with the kinematically estimated radius. Possible origin of the density jump due to the mass accretion from the envelope to the disk is discussed. C 18 O observations can be reproduced by the same geometrical structures derived from the dust observations, with possible C 18 O freeze-out and localized C 18 O desorption.