Ring Gap Structure around Class I Protostar WL 17

Abstract

Abstract WL 17 is a Class I object and was considered to have a ring–hole structure. We analyzed the structure around WL 17 to investigate the detailed properties of this object. We used Atacama Large Millimeter/submillimeter Array archival data, which have a higher angular resolution than previous observations. We investigated the WL 17 system with the 1.3 mm dust continuum and 12CO and C18O (J = 2–1) line emissions. The dust continuum emission showed a clear ring structure with inner and outer edges of ∼11 and ∼21 au, respectively. In addition, we detected an inner disk of <5 au radius enclosing the central star within the ring, the first observation of this structure. Thus, WL 17 has a ring–gap structure, not a ring–hole structure. We did not detect any marked emission in either the gap or inner disk, indicating that there is no sign of a planet, circumplanetary disk, or binary companion. We identified the source of both blueshifted and redshifted outflows based on the 12CO emission, which is clearly associated with the disk around WL 17. The outflow mass ejection rate is ∼3.6 × 10−7 M ⊙ yr−1 and the dynamical timescale is as short as ∼104 yr. The C18O emission showed that an inhomogeneous infalling envelope, which can induce episodic mass accretion, is distributed in the region within ∼1000 au from the central protostar. With these new findings, we can constrain the scenarios of planet formation and dust growth in the accretion phase of star formation.