Sources of maser radio emission in the water vapour spectral line at 1.35 cm are considered, which are situated in star formation regions, in tho vicinity of young stellar objects. The H2O emission of many maser sources in subject to strong variability; maser activity is thereby of a cyclic character with a period of a few years. On the basis of observational data, it is aassumed that the region of maser generation is located in a rotating gas-dust disc around a young stellar object. The author auggests three models explaining the masers´ cyclic non-stationarity.
(1) The maser´s variability is connected with the variable luminosity of the central stellar object, due to non-stationary accretion onto it.
(2) In the central cavity of a large circumstellar disc containing the H2O maser, a smaller disc is located, with its axis tilted to that of the larger disc. The stellar wind jet collimated by the small disc and flowing out from its poles impacts connecutively onto different parts of the large disc´s internal surface. This results in the time- and space-variable maser pump.
(3) If the maser is unsaturated and amplifies the background continuum radiation, then strong maser flares may be connected with radio flares of the centra! object (due, e.g., to its magnetic activity) and the corresponding increase of the maser´s input intensity.
Oboervational tests for the suggested models are discussed.
Naše pracovní skupina vytvořila v letech 2002-2007 globální numerický model magnetosféry, který v sobě obsahuje popis kinetických vlastností iontů v plazmatu, zejména pak popis přenosu energie mezi vlnami a nabitými částicemi v plazmatu. Model je v současné době využíván americkým Národním úřadem pro letectví a kosmonautiku (NASA) k interpretaci pozorování družice MESSENGER, jejímž cílem je výzkum planety Merkur. Pomocí našich simulací jsme se pokusili vysvětlit některé z jevů pozorovaných družicí MESSENGER., Pavel M. Trávníček, Petr Hellinger., and Obsahuje seznam literatury
A conference honored the late Prof. Zdeněk Švestka, one of the great defining personalities of twentieth-century solar physics. His main scientific interest focused on solar flares. During his year at Ondřejov, he became an expert in flare spectra. Later his interests extended into white-light flares, proton flares, and their radio signatures (type II and IV bursts), which were suggestive of shock-acceleration of particles. The conference, organized by the Astronomical Institute, took place on 23-27 June 2014 in Prague. Zdeněk Švestka, born on 30 September 1925 in Prague, passed away on 2 April 2013 in Bunschoten. and Petr Heinzel,Marian Karlický, František Fárník.
The time evolutlon of the metalliclty wlthln the Galaxy has been analysed on the base of published data for star clusters. The resulting age-metalliclty relation seems to split into two independent paths, passing the same age range over 10^10 years. The lower relationship resembles that for the Magellanic Clouds. Such a picture of the chemical evolutlon of the galactic matter is consistent wlth the observed division of the considered sample of open clusters into two groups differing in metallicity and spatial distribution, similarly to the division found for globular clusters. The metal-poor open clusters resemble metal-rich globulars. This can suggest that our galaxy has evolved from spherical metal-poor configuration represented in the considered sample of star clusters by metal-poor globulars, through a thick dlsk phase wlth the middle metallicity up to thin metal-rich disk wlth its representative
metal-rich open clusters. The transition between each of the above phases was more or less abrupt in its characteristic chemical and spatial properties.
Meteor stream membership criteria us ed in evaluating potential associations between individual meteor orbits and the mean orbit of a stream are discussed on the basis of precise photographic orbits of some meteor streams (the Taurids, Geminids and Perseids). Serious shortcomings of the D-criterion are disclosed and suggestions on a more relevant use of the criteria for estimating stream memberships are presented, with attention paid to the distributions of the orbital elements of the streams.
The distribution of open clusters in the Galaxy plane has been studied taking advantage of recent data. It is shown that clusters have a tendency to be localized in Galactic longitudes 120°-300° which coincide with the direction of Magellanic Clouds and Magellanic Stream. Two age peculiarities of the spatial distribution of open clusters are given. The dependence of linear diameters in their distance from the Galaxy centre is discussed. A supposition is made about the necessity to correct parameters of the zero-age main sequence for objects of early spectral classes and of interstellar absorption.