Discovery of Linear “Building Blocks” of Water Masers Shaping Linear/Arcuate Microstructures in Cepheus A

Abstract

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,ÏÏ revealing, in particular three Ðlaments (R1, R2, R3) with length sizes^3È25 mas (2È18 AU) and unresolved in the perpendicular direction AU), an ([0.1 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 ÏÏ (down to scales of 0.4 AU) shaping the "" larger ÏÏ structures. The Ñattened 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 di †erent 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 o †set 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 unidentiÐed protostar located^600 mas south of HW2. The Ñux density of the water masers of the R5 arc excited by this unknown protostar (D500 Jy) represents an important fraction (D50%) of the total Ñux 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 Ðnd that the center of the circle, which Ðt the R5 arc structure for each epoch, is moving with a velocity of D6 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.

A Pulsed, Precessing Jet in Cepheus A

“…HW3b is the third source in Cep A to contain masers (Torrelles et al 1998(Torrelles et al , 2001a. The submillimeter source SMA4 (Brogan et al 2007) is located on the axis of this chain close to HW3b.…”

“…• (Torrelles et al 1998(Torrelles et al , 2001a) that has the spectral index and morphology of a thermal radio jet that has the same orientation as the suspected large-scale outflow from HW3c.…”

Statistical Analysis of Water Masers in Star-Forming Regions: Cepheus a and W75 N

“…Statistical properties of the spatial distribution of water masers Using VLBA data of three epochs of water maser observations toward the SFRs Cepheus A and W75 N (Torrelles et al 2001b(Torrelles et al , 2003, we compute two-point spatial correlation functions of maser spots in the following subregions: R5 in Cepheus A, and VLA 1 and VLA 2 in W75 N. In each of these subregions, a few hundred maser spots were detected with an S/N ≥ 10 in each one of the three epochs (see Table 1). The uncertainties in the values of these functions are mainly due to position uncertainty of maser spots, which is more significant toward small separations (represented by the error bar sizes shown in Figures 1-3).…”

“…In fact, in W75 N VLA 2, masers delineate an incomplete elliptical ring on the plane of the sky; while in Cep A R5, masers delineate part of a circle on the plane of the sky (a circle in projection is produced only by a limbbrightened spherical shell). In addition, the masers in W75 N VLA 2 present a higher Doppler-velocity dispersion than maser spots in Cep A R5 (25 km s −1 vs. 2 km s −1 ; Torrelles et al 2001bTorrelles et al , 2003). For the reasons described above, we have considered a ring with an arbitrary orientation with respect to the observer, applying a similar statistical analysis to a model that simulates organized motions of masers.…”

“…This source is associated with a cluster of water masers detected by Torrelles et al (1996) using the Very Large Array (VLA). Through VLBA multi-epoch water maser observations toward the radio jet HW2, Torrelles et al (2001b) detected ≃1000 water maser spots in each of the three observed epochs, forming remarkable linear/arcuate structures with sizes of the order of tens of AU. These structures are grouped in five subregions (named as R1 to R5) within ∼0.…”