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 jet Cepheus A HW2. The VLBA data show that some of the masers detected previously with the VLA unfold into unexpected and remarkable linear/arcuate "microstructures," revealing, in particular three filaments (R1, R2, R3) with length sizes ≃3-25 mas (2-18 AU) and unresolved in the perpendicular direction (≲0.1 AU), an arcuate structure (R4-A) of ≃20 mas size (15 AU), and a curved chain of masers (R5; which we have previously reported) of ≃100 mas size (≃72 AU). Some of these structures unfold into even smaller linear "building blocks" (down to scales of 0.4 AU) shaping the "larger" structures. The flattened appearance of these small pieces argues strongly for a shock nature. Both the morphology and the observed proper motions found in these water maser structures have allowed us to identify at least three different centers of star formation activity in a region of ≃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 ≃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 molecular gas. The presence of a wide-angle wind associated with the central HW2 object is implied. We also propose that the arcuate structure R4-A corresponds 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, ≃200 mas south from HW2). Finally, the third energy source of this small region could be that one exciting the arc R5, which, as we have previously reported, is expanding and related to an unidentified protostar located ≃600 mas south of HW2. The flux density of the water masers of the R5 arc excited by this unknown protostar (~500 Jy) represents an important fraction (~50%) of the total flux density of the maser emission in the entire Cepheus A star-forming region, suggesting that the exciting object could be a high-mass star. Furthermore, we find that the center of the circle, which fit the R5 arc structure for each epoch, is moving with a velocity of ~6 km s-1. This could be caused by proper motions of the associated protostar because of relative orbital motions within the triple star system formed by HW2 and the energy sources of the R4-A and R5 structures.

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