Galactic Parameters from Masers with Trigonometric Parallaxes

Spatial velocities of all currently known 28 masers having trigonometric parallaxes, proper motion and line-of-site velocities are reanalyzed using Bottlinger's equations. These masers are associated with 25 active star-forming regions and are located in the range of galactocentric distances 3<R<14 kpc. To determine the Galactic rotation parameters, we used the first three Taylor expansion terms of angular rotation velocity {\Omega} at the galactocentric distance of the Sun R0=8 kpc. We obtained the following solutions: {\Omega}o=-31.0 +/- 1.2 km/s/kpc, {\Omega}o'=4.46 +/- 0.21 km/s/kpc^2, {\Omega}o"=-0.876 +/- 0.067 km/s/kpc^3, Oort constants: A=17.8 +/- 0.8 km/s/kpc, B=-13.2 +/- 1.5 km/s/kpc and circular velocity of the Solar neighborhood rotation Vo=248 +/- 14 km/s. Fourier analysis of galactocentric radial velocities of masers VR allowed us to estimate the wavelength {\lambda}=2.0 +/- 0.2 kpc and peak velocity f_R=6.5 +/- 2 km/s of periodic perturbations from the density wave and velocity of the perturbations 4 +/- 1 km/s near the location of the Sun. Phase of the Sun in the density wave is estimated as {\chi}o ~ -130^o +/- 10^o. Taking into account perturbations evoked by spiral density wave we obtained the following non-perturbed components of the peculiar Solar velocity with respect to the local standard of rest (LSR) (Uo,Vo,Wo)LSR=(5.5,11,8.5) +/- (2.2,1.7,1.2) km/s.Comment: 8 pages, 1table, 9 figures, accepte

Experience of high resolution VLBI imaging using generalized maximum entropy method

The generalized maximum entropy method (GMEM) is a special modification of the standard maximum entropy method (MEM) which seeks solutions in the space of complex functions. In this work a reduced version of the GMEM intended for reconstructing real images with positive and negative values is used. As compared with the standard MEM, intended for the reconstruction of only non-negative images, the GMEM allows us to obtain higher-quality images with a much lower level of nonlinear distortions caused by errors in the data. Here, we present the results of the GMEM imaging of 36 selected extragalactic radio sources with a resolution of 0.3-0.5 mas on astrometric and geodetic VLBI observations at 8.2 GHz, obtained with a global array in the period from 1994-1996. In VLBI mapping practice this is the first experience of imaging with such a high resolution using maximum entropy technique. A differential maximum entropy method intended for increasing the dynamic range of images is demonstrated on the radio source 0059+581. In the case of unreliable `closure' phases, completely `phaseless' methods of mapping are recommended. Maps of two sources 0615+820 and 0642+214 are obtained using one such method.Comment: Latex, 11 pages with 4 Postscript figures (images of 36 extragalactic radio sources

Phaseless VLBI mapping of compact extragalactic radio sources

The problem of phaseless aperture synthesis is of current interest in phase-unstable VLBI with a small number of elements when either the use of closure phases is not possible (a two-element interferometer) or their quality and number are not enough for acceptable image reconstruction by standard adaptive calibration methods. Therefore, we discuss the problem of unique image reconstruction only from the spectrum magnitude of a source. We suggest an efficient method for phaseless VLBI mapping of compact extragalactic radio sources. This method is based on the reconstruction of the spectrum magnitude for a source on the entire UV plane from the measured visibility magnitude on a limited set of points and the reconstruction of the sought-for image of the source by Fienup's method from the spectrum magnitude reconstructed at the first stage. We present the results of our mapping of the extragalactic radio source 2200 +420 using astrometric and geodetic observations on a global VLBI array. Particular attention is given to studying the capabilities of a two-element interferometer in connection with the putting into operation of a Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure