Observations of Cassiopeia A at 20 and 25 MHz with the uran-1 interferometer

The variation in elemental depletions seen in diffuse clouds in the interstellar medium is shown to be consistent with the exposure of grains to weak shocks. Shocks remove a weakly bound layer of amorphous oxide material from refractory cores. Depletions are shown to be the result of the incorporation of elements in refractory cores formed during equilibrium condensation in high temperature sources together with accretion under low (T1000K) temperature conditions. Evidence is presented for the high temperature condensation of CaAlTi grains. In addition to refractory CaAlTi particels, a refractory condensate containing 20% of available Mg, 60% of Si and 90% of Fe is inferred from data on depletions. An amorphous layer with a composition (2MgO) (SiO) condenses on these cores.     

Structure investigations of 3C 196 and 3C 280 in the decameter waveband

Observational results are presented concerning the structure of the quasar 3C 196 and the radio galaxy 3C 280 at 20 and 25 MHz, obtained by the scintillation method with the URAN-1 interferometer. Angular dimensions of the scintillating components and extended regions of the sources have been evaluated. In the case of the quasar 3C 196, the effective angular size of the scintillating component equals 2±1 _. 5 and that of the extended region 18×25.The contribution of the compact component into the total radiation flux is 0.46±0.20. Spectra of the structural formations in 3C 196 have been obtained in the range 20–5000 MHz.For the radio galaxy 3C 280 the effective angular size of the scintillating component equals 1 _. 5±1 _. 2. and that of the extended region exceeds 10 _. The contribution into the total flux due to the scintillating part is 0.35±0.20.The data obtained are analyzed together with the results measured at higher frequencies. It is pointed out that in a wide frequency range, the effect of wave scattering in the interstellar medium does not exceed measurement errors,hence bringing about no increase in the compact component sizes of the sources observed.     

Scintillations of cosmic radio sources in the decametre waveband

The effect of fluctuations of the interplanetary plasma and the ionosphere upon the scintillation spectra of radio sources at decametre waves is considered with due regard for the finite antenna aperture, fluctuation anisotropy, and the direction of their drift in space. It has been shown that scintillation due to interplanetary plasma (IPP), can be reliably separated from the ionospheric scintillation background at decametre wavelengths.For elongations between 90° to 150°, the IPP scintillation power spectrum observed in the 12.6–25 MHz waveband is of a power law form with the index 3.1±0.6, which is in close agreement with the values known for smaller elongations. The solar wind velocity projection orthogonal to the line of sight is estimated for elongations about 110° and has been found to be 300±80 km s^–1. As in the case of smaller elongations, the velocity dispersion is significant.At night, wideband spectra of ionospheric scintillations are observed in the decametre band, with the breaking point at approximately 0.01 Hz in the 12 m band, and narrow-band spectra whose cut-off frequency is below 0.01 Hz. The power spectrum of ionospheric scintillations is of a power-law form with the index 3.4±0.5. In some cases steeper spectra are observed.     

Measurements of radio emission from the compact source in the Crab nebula with the uran-1 interferometer at 16.7, 20 and 25 MHz

This paper presents the results of measurements of the Crab nebula in the decametre range with an interferometer whose baseline is 2.4–3.5×10³ of the wavelength. Visibility function values, which in these observations determine the contribution by the compact source to the total nebula flux, have been measured at frequencies 16.7, 20 and 25 MHz to be 0.64±0.07, 0.43±0.04 and 0.31±0.03, respectively. The spectral index of the spectrum obtained for the compact source in the range 16.7–122 MHz is 2.09±0.04. Flattening of the nebula spectrum without the compact source has been confirmed for the decametre range.