Chemical and Physical Characterization of the Isolated Protostellar Source CB68: FAUST. IV

Abstract: Chemical diversity of low-mass protostellar sources has so far been recognized, and environmental effects are invoked as its origin. In this context, observations of isolated protostellar sources without influences of nearby objects are of particular importance. Here, we report chemical and physical structures of the low-mass Class 0 protostellar source IRAS 16544$-$1604 in the Bok globule CB68, based on 1.3 mm ALMA observations at a spatial resolution of $\sim$70~au that were conducted as part of the large program FAUST. Three interstellar saturated complex organic molecules (iCOMs), CH$3$OH, HCOOCH$_3$, and CH$_3$OCH$_3$, are detected toward the protostar. The rotation temperature and the emitting region size for CH$_3$OH are derived to be $131\pm11$~K and $\sim$10~au, respectively. The detection of iCOMs in close proximity to the protostar indicates that CB68 harbors a hot corino. The kinematic structure of the C$^{18}$O, CH$_3$OH, and OCS lines is explained by an infalling-rotating envelope model, and the protostellar mass and the radius of the centrifugal barrier are estimated to be $0.08-0.30$~$M\odot$ and $< 30$ au, respectively. The small radius of the centrifugal barrier seems to be related to the small emitting region of iCOMs. In addition, we detect emission lines of c-C$_3$H$_2$ and CCH associated with the protostar, revealing a warm carbon chain chemistry (WCCC) on a 1000~au scale. We therefore find that the chemical structure of CB68 is described by a hybrid chemistry. The molecular abundances are discussed in comparison with those in other hot corino sources and reported chemical models.