A Microjet: A Protostar’s Cry at Birth

We report the discovery, through Very Long Baseline Array (VLBA) observations of H 2 O masers at 22 GHz, of a newly ejected extraordinarily compact protostellar jet—a microjet—from the class 0 protostar S106 FIR. The sub-AU resolution VLBA observations revealed the presence of a U-shaped distribution of the masers in the western blueshifted cluster. It has a length of 4 AU, has a width of 3 AU, and is located at a distance of 25 AU from the assumed position of the star around the middle point of the two maser clusters. The U-shaped distribution is considered to be a "microbowshock" created by the impact of the microjet on the ambient cloud medium. We succeeded in tracking the proper motion of the microbowshock; its velocity ranges from 25 to 45 km s -1 , estimated from the proper motions and line-of-sight velocities. In addition, this source stands unique among all 40 known class 0 sources in its lack of an extensive molecular outflow, evidenced by our sensitive multitransition CO searches with 10 σ upper limits of ~10 -6 M ☉ km -1 s. We propose that S106 FIR is an archetype of the youngest evolutionary class, a protostar observed just after the creation of the protostellar jet.