2018/02/19 by A. M. Sobolev, J. M. Moran, M. D. Gray +6
Physics and Astronomy · #Angular diameter #Astro and Planetary Science #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Brightness temperature #Flux (metallurgy) #Galaxy #Halo #Maser #Milky Way #Spectral line #Very-long-baseline interferometry #Zeeman effect #astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/aab096
15 pages, 9 figures. Accepted for publication in ApJ, February 16, 2018
arxiv created 2018/02/19 · openalex created_date 2018/03/06 · openalex publication_date 2018/03/20 · arxiv updated 2018/04/11 · openalex updated_date 2026/08/06
Abstract 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 μ 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 μ as, corresponding to a linear scale of 1.6 × 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 ± 35 μ as in angle. This is the smallest angular structure ever observed in a Galactic maser. The brightness temperatures are greater than 2 × 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 ± 150 μ 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.