Early Planet Formation in Embedded Disks (eDisk). XVIII. Indication of a possible spiral structure in the dust-continuum emission of the protostellar disk around IRAS 16544−1604 in CB 68

Abstract We performed numerical simulations along with radiative transfer calculations to reproduce an intriguing asymmetric shoulder feature in the dust-continuum emission of the protostellar disk around one of the eDisk targets, the Class 0 protostar IRAS 16544$-$1604 in CB 68. This is our first attempt to bridge the theoretical works of protostellar disk evolution and the eDisk observations. We found that while our hydrodynamic simulations form spiral structures caused by gravitational instability, they become less discernible after the disk is inclined and convolved with the telescope beam. The widths of the spiral structure as obtained by our numerical simulations are ${\sim }$0.1–0.8 times the eDisk beam size of 4.5 au. Our modeling effort implies that the apparent absence of spiral features in the eDisk observations does not necessarily indicate the real absence of internal substructures and gravitational instability. We also found that the asymmetric shoulder structure of the continuum profile along the major axis appears when the disk is massive enough with a Toomre parameter $Q\sim 1$. This mechanism offers a potential explanation for the observed, asymmetric shoulder features in the disks surrounding IRAS 16544$-$1604 and the other eDisk sources.