EMISSION FROM A YOUNG PROTOSTELLAR OBJECT. I. SIGNATURES OF YOUNG EMBEDDED OUTFLOWS

We examine emission from a young protostellar object (YPO) with three-dimensional ideal magnetohydrodynamic (MHD) simulations and three-dimensional non-local thermodynamic equilibrium line transfer calculations, and show the first results. To calculate the emission field, we employed a snapshot result of an MHD simulation having young bipolar outflows and a dense protostellar disk (a young circumstellar disk) embedded in an infalling envelope. Synthesized line emission of two molecular species (CO and SiO) shows that subthermally excited SiO lines as a high-density tracer can provide a better probe of the complex velocity field of a YPO, compared to fully thermalized CO lines. In a YPO at the earliest stage when the outflows are still embedded in the collapsing envelope, infall, rotation, and outflow motions have similar speeds. We find that the combined velocity field of these components introduces a great complexity in the line emissions through varying optical thickness and emissivity, such as asymmetric double-horn profiles. We show that the rotation of the outflows, one of the features that characterizes an outflow driven by magneto-centrifugal forces, appears clearly in velocity channel maps and intensity-weighted mean velocity (first moment of velocity) maps. The somewhat irregular morphology of the line emission at this youngest stage is dissimilar to a more evolved object such as young Class 0. High angular resolution observation by, e. g., the Atacama Large Millimeter/submillimeter Array telescope can reveal these features. Our results demonstrate a powerful potential of the synthesized emission of the three-dimensional line transfer to probe the velocity field embedded in the envelope, and further analysis will be able to determine the precise velocity field to assess the dynamics in the YPO to gain a better understanding of star formation.