Astrophys. J., 856, 60-60 (2018/March-3)
Sun-sized water vapor masers in Cepheus A.
SOBOLEV A.M., MORAN J.M., GRAY M.D., ALAKOZ A., IMAI H., BAAN W.A., TOLMACHEV A.M., SAMODUROV V.A. and LADEYSHCHIKOV D.A.
Abstract (from CDS):
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.
© 2018. The American Astronomical Society. All rights reserved.
ISM: individual objects: Cepheus A - ISM: magnetic fields - masers - stars: formation - techniques: interferometric
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