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 microarcseconds ( \mu as ) .
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 8 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 \mu as , corresponding to a linear scale of 1.6 \times 10 ^ { 11 } 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 \pm 35 \mu as in angle .
This is the smallest angular structure ever observed in a Galactic maser .
The brightness temperatures are greater than 2 \times 10 ^ { 14 } 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 \pm 150 \mu as .
This structure is associated with the compact HII 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 Kármán vortex street ) with Strouhal number of about 0.3 .