2018ApJ...856...60S

We present the first VLBI observations of a Galactic water maser (in Cepheus A) made with a very long baseline interferometric array involving the RadioAstron Earth-orbiting satellite station as one of its elements. We detected two distinct components at -16.9 and 0.6 km s^–1 with a fringe spacing of 66 µas. In total power, the 0.6 km s^–1 component appears to be a single Gaussian component of strength 580 Jy and width of 0.7 km s^–1. Single-telescope monitoring showed that its lifetime was only eight months. The absence of a Zeeman pattern implies the longitudinal magnetic field component is weaker than 120 mG. The space-Earth cross power spectrum shows two unresolved components smaller than 15 µas, corresponding to a linear scale of 1.6 x 10¹¹ cm, about the diameter of the Sun, for a distance of 700 pc, separated by 0.54 km s^–1 in velocity and by 160 ± 35 µas in angle. This is the smallest angular structure ever observed in a Galactic maser. The brightness temperatures are greater than 2 x 10¹⁴ K, and the line widths are 0.5 km s^–1. Most of the flux (about 87%) is contained in a halo of angular size of 400 ± 150 µas. This structure is associated with the compact H II region HW3diii. We have probably picked up the most prominent peaks in the angular size range of our interferometer. We discuss three dynamical models: (1) Keplerian motion around a central object, (2) two chance overlapping clouds, and (3) vortices caused by flow around an obstacle (i.e., von Karman vortex street) with a Strouhal number of about 0.3.