Disk Structure around the Class i Protostar L1489 IRS Revealed by ALMA: A Warped-disk System

We have observed the Class I protostar L1489 IRS with the Atacama Millimeter/submillimeter Array (ALMA) in Band 6. The C¹⁸O J = 2-1 line emission shows flattened and non-axisymmetric structures in the same direction as its velocity gradient due to rotation. We discovered that the C¹⁸O emission shows dips at a radius of ∼200-300 au while the 1.3 mm continuum emission extends smoothly up to r ∼ 400 au. At the radius of the C¹⁸O dips, the rotational axis of the outer portion appears to be tilted by ∼15° from that of the inner component. Both the inner and outer components with respect to the C¹⁸O dips exhibit the r ^-0.5 Keplerian rotation profiles until r ∼ 600 au. These results not only indicate that a Keplerian disk extends up to ∼600 au but also that the disk is warped. We constructed a three-dimensional warped-disk model rotating at the Keplerian velocity, and demonstrated that the warped-disk model reproduces main observed features in the velocity channel maps and the PV diagrams. Such a warped-disk system can form by mass accretion from a misaligned envelope. We also discuss a possible disk evolution scenario based on comparisons of disk radii and masses between Class I and Class II sources.