LESIA - Observatoire de Paris

Séminaire par 4 chercheurs de l’« Institute of Radio Astronomy, Kharkov, Ukraine »

Thursday 9 December 2010, by A. Konovalenko, V. N. Melnik, M. A. Sidorchuk, S. Stepkin (Institute of Radio Astronomy, Kharkov, Ukraine)

Friday 10 December 2010 à 15h00 , Lieu : Salle de conférence du bât. 17

A. Konovalenko: 50-years of low-frequency radio astronomy in Ukraine
Low-frequency (decameter wavelengths) radio astronomy in Ukraine has a long history. It was initiated at the beginning of 1960’s by Prof. S. Ya. Braude (1911-2003). Now the Institute of Radio Astronomy of NASU (Kharkov, Ukraine) operates the largest existing decameter array UTR-2 and the VLBI system URAN. Many new astrophysical results were obtained with these instruments. Many studies are carried on in international collaboration, especially with the Paris-Meudon Observatory, involving exploitation of the existing radio telescopes as well as development of new generation low-frequency instruments.

V. N. Melnik: Solar radio emission at low frequencies
Recent results obtained on decameter radio emission of the Sun by the radiotelescope UTR-2 are presented. Through the use of modern receiving facilities, new phenomena were observed in the range 10-30 MHz : fast Type III bursts, fine time structure of usual Type III bursts, solar S-bursts, Type IV bursts and their fine « zebra » structure, the third harmonics of Type II bursts, and bursts in absorption at different time scales. New properties of the well-known Type II, III and Iib bursts and of drift pairs were also measured.

M. A. Sidorchuk, et al.: Decametric continuum investigations at UTR-2
We present studies of continuum radiation in the decameter 10-30 MHz range carried out with the largest existing radiotelescope, UTR-2 (effective area 150000 m2), at an angular resolution of 70′–25′ respectively. The survey of the Northern Sky has been the main observing program at UTR-2 for 40 years, combining imaging and discrete sources survey. Maps of the sky have been built for declinations 29°-55° at frequencies 14.7, 16.7, 20 and 25 MHz, with sensitivity ∼3.5-1.1 mK at 1σ level. These maps have the best available resolution and sensitivity at these low frequencies. Their analysis is ongoing. Published catalogs of discrete sources contain information about ∼2300 discrete sources. Systematic observations for declinations ≥ 60° are near completion. Maps of continuum radiation over the whole Northern sky (-15° < δ < +85°) have also been obtained with a coarser resolution, using one section of the UTR-2 radiotelescope (resolution ∼12°×4°) and with the complete URAN-2 radiotelescope at 25 MHz (resolution of 3.5°×7°). These maps adequately represent the large-scale structure of the decametric background radiation of the Galaxy in the Northern hemisphere. We also present investigations of ≥20 extended HII regions, supernova remnants, and clusters of galaxies. Those represent a mere start of the studies of the low frequency radio continuum to come.

S. Stepkin: Low frequency radio recombination lines
Radio spectroscopy at extremely low frequencies provides unique opportunities for studies of the cold low-density partially ionized interstellar medium, which plays a significant role in many astrophysical phenomena (among them, for example, are processes of star formation). Using the radiotelescope UTR-2 we have detected series of radio recombination lines in absorption, arising from bound carbon atoms in the cool tenuous medium located in the Perseus arm in front of the supernova remnant Cassiopeia A, undergoing transitions corresponding to quantum levels up to ∼ 1009 and even larger values of n). Such atoms have a classical diameter of about ∼0.1 mm.