Discovery of linear "building blocks" of water masers shaping linear/arcuate microstructures in Cepheus A

We report three epochs of VLBA water maser observations toward the radio je t Cepheus A HW2. The VLBA data show that some of the masers detected previo usly with the VLA unfold into unexpected and remarkable linear/arcuate "mic rostructures," revealing, in particular three filaments (R1, R2, R3) with l ength sizes similar or equal to3-25 mas (2-18 AU) and unresolved in the per pendicular direction (less than or similar to0.1 AU), an arcuate structure (R4-A) of similar or equal to 20 mas size (15 AU), and a curved chain of ma sers (R5; which we have previously reported) of similar or equal to 100 mas size (similar or equal to 72 AU). Some of these structures unfold into eve n smaller linear "building blocks" (down to scales of 0.4 AU) shaping the " larger" structures. The flattened appearance of these small pieces argues s trongly for a shock nature. Both the morphology and the observed proper mot ions found in these water maser structures have allowed us to identify at l east three different centers of star formation activity in a region of simi lar or equal to 300 mas (200 AU) projected radius, unknown previously, and that could constitute a triple star system. We suggest that R1, R2, and R3, which are located close to the HW2 radio jet but offset by similar or equa l to 200 mas with respect to its axis, could originate at the shocked walls of an inner cavity excavated by the HW2 wind in the circumstellar molecula r gas. The presence of a wide-angle wind associated with the central HW2 ob ject is implied. We also propose that the arcuate structure R4-A correspond s to a bow-shock structure produced by the wind of an undetected protostar (which we predict to be located toward the southeast of R4-A, similar or eq ual to 200 mas south from HW2). Finally, the third energy source of this sm all region could be that one exciting the arc R5, which, as we have previou sly reported, is expanding and related to an unidentified protostar located similar or equal to 600 mas south of HW2. The flux density of the water ma sers of the R5 arc excited by this unknown protostar (similar to 500 Jy) re presents an important fraction (similar to 50%) of the total flux density o f the maser emission in the entire Cepheus A star-forming region, suggestin g that the exciting object could be a high-mass star. Furthermore, we find that the center of the circle, which Dt the R5 arc structure for each epoch , is moving with a velocity of similar to6 km s(-1). This could be caused b y proper motions of the associated protostar because of relative orbital mo tions within the triple star system formed by HW2 and the energy sources of the R4-A and R5 structures.