Sun-sized Water Vapor Masers in Cepheus A

• Published in: The Astrophysical journal Vol. 856; no. 1; pp. 60 - 68
• Main Authors: Sobolev, A. M., Moran, J. M., Gray, M. D., Alakoz, A., Imai, H., Baan, W. A., Tolmachev, A. M., Samodurov, V. A., Ladeyshchikov, D. A.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.03.2018; IOP Publishing
• Subjects: Astrophysics; Brightness temperature; Cepheus constellation; Computational fluid dynamics; Fluid flow; Interferometers; ISM: individual objects (Cepheus A); ISM: magnetic fields; Kepler laws; Magnetic fields; Masers; Satellites; Solar orbits; stars: formation; Strouhal number; Sun; techniques: interferometric; Vortices; Water vapor
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aab096

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 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 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 as, corresponding to a linear scale of 1.6 × 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 as in angle. This is the smallest angular structure ever observed in a Galactic maser. The brightness temperatures are greater than 2 × 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 as. 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 Kármán vortex street) with a Strouhal number of about 0.3.