500 – 3200 Å observations of the interacting binary stars V356 Sgr and β Lyr

In this paper we present new results from the Voyager ultraviolet spectrometers and the IUE spacecraft on V356 Sgr and β Lyr. The V356 Sgr observations cover, in detail, two eclipses and include one IUE high dispersion SWP image. During both eclipses the total strength of the UV emission lines were found to be invariant. Also, an uneclipsed UV continuum was detected at wavelengths shorter than 1600 Å. The IUE high dispersion SWP spectrum revealed that the emission lines are extremely broad, almost symmetrical lines with weak, slightly blue shifted absorption components. No evidence of carbon is seen in the emission or absorption spectrum of V356 Sgr in eclipse. A model for the origin of the circumstellar matter in this binary system is presented. The Voyager ultraviolet observations of β Lyr show a strong far-UV continuum that is detectable down to 912 Å The far-UV continuum flux level was variable on time scales shorter than the orbital period and displayed no obvious orbital modulation or eclipses. The spectral shape of the far-UV continuum closely resembles that of a UX UMa type cataclysmic variable. On 16 August 1985 an rapid brightening of the far-UV continuum was observed which was also reminiscent of cataclysmic variables. Analysis of the β Lyr data suggest that the central object must be small, with a radius on the order of 1 R_⊙ or less.     

Ultraviolet observations of the X-ray sources in the ETA garinae Nebula

Car is surrounded by a dense nebula ejected during the last 150 years. The Einstein satellite recently detected intense X-ray emission from Car and its nebula indicating the existence of efficient gas heating processes. Ultraviolet observations with IUE confirmed the presence of hot gas in the condensation S of the nebula and in Car. Possible interpretations of the results are discussed.Based on observations by the International Ultraviolet Explorer made at the Villafranca Satellite Tracking Station of the European Space Agency.     

Some results of ionospheric investigations by means of coherent radiowaves emitted by satellites

In this paper we discuss theoretical expressions, determining the difference of Doppler shifts of various coherent radiowave frequencies emitted by a radiator moving in the ionosphere or interplanetary medium. The rotating Doppler effect (Faraday effect) caused by the Doppler shifts ±H of the ordinary and extraordinary waves is also considered. In a three-dimensional inhomogeneous ionosphere, stationary in time (N/t = 0), is determined in the general case, by an equation with three variables. The equation for proper depends only on the local value of the electron concentration N _c around the radiator and on integral values, determining, by means of additional calculations, the angle of refraction or its components, the horizontal gradients of electron concentration N/x and N/y, and in some cases, the integral electron concentration ₀ ^zcN dz. We describe the analysis of the measurements, made with the satellites Cosmos I, II and partially XI, assuming that N/t = N/y = 0, with a two variables equation. The expected errors are considered. The results coincide well for different points (Moscow, The Crimea, Sverdlovsk) and thus agree with the measurements of _H and with height-frequency ionospheric characteristics. The curve giving electron concentration versus height N (z) in the outer ionosphere (above the maximum of F2), shows a new maximum higher than the main maximum of the ionosphere N _MF2 at 120–140 km. At this maximum the value of N (z) is (0.9–0.95) N _MF2. The new data on the large-scale horizontal inhomogeneities of the ionosphere, exceed the previous ones by about a factor 10. By means of the irregular variations of the spectrum W() of the inhomogenous formation is determined. Three unknown constant maxima with values 16 to 18 km, 28 to 32 km and 100 to 120 km are found. The spectrum W () mainly characterizes the local properties of the ionosphere along the orbit of the satellite.     

Gamma-rays from Active Galactic Nuclei: Emission processes and particle acceleration

It is argued that the high-energy X-ray and -ray emission from flaring blazars is beamed radiation from the relativistic jet supporting the relativistic beaming hypothesis and the unified scenario for AGNs. Most probably the high-energy emission results from inverse Compton scattering by relativistic electrons and positrons in the jet of radiation originating external to the jet plus pair annihilation radiation from the jet. Future positive TeV detections of EGRET AGN sources will be decisive to identify the prominent target photon radiation field. Direct -ray production by energetic hadrons is not important for the flaring phase in -ray blazars, but the acceleration of energetic hadrons during the quiescent phase of AGNs is decisive as the source of secondary electrons and positrons through photo-pair and photo-pion production. Injection of ultrahigh energy secondary electrons and positrons into a stochastic quasilinear acceleration scheme during the quiescent AGN phase leads to cooling electron-positron distribution functions with a strong cut-off at low but relativistic energy that under certain local conditions may trigger a plasma instability that gives rise to an explosive event and the flaring -ray phase.     

A New Mechanism of Coronal Heating

The interaction between network magnetic fields and emerging intranetwork fields may lead to magnetic reconnection and microflares, which generate fast shocks with an Alfvén Mach number M _A<2. Protons and less abundant ions in the solar corona are then heated and accelerated by fast shocks. Our study of shock heating shows that (a) the nearly nondeflection of ion motion across the shock ramp leads to a large perpendicular thermal velocity (v _th), which is an increasing function of the mass/charge ratio; (b) the heating by subcritical shocks with 1.1 M_A 1.5 leads to a large temperature anisotropy with T/T 50 for O⁵⁺ ions and a mild anisotropy with T_/T 1.2 for protons; (c) the large perpendicular thermal velocity of He⁺⁺ and O⁵⁺ ions can be converted to the radial outflow velocity (u) in the divergent coronal field lines; and (d) the heating and acceleration by shocks with 1.1 M_A 1.5 can lead to u(O⁵⁺) v _th(O⁵⁺) 460 km s^–1 for O⁵⁺ ions, u(He⁺⁺) v _th(He⁺⁺) 360 km s^–1 for He⁺⁺ ions, and u(H⁺) v _th(H⁺) 240 km s^–1 for protons at r=3–4 R . Our results can explain recent SOHO observations of the heating and acceleration of protons and heavier ions in the solar corona.     

The solar flare plasma: Observation and interpretation

In the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (20 × 10⁶ – 100 × 10⁶ K), low density plasmas associated with solar flare phenomena. In the hard X-ray range ( < 1 ), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of 100 × 10⁶ K.In contrast, the soft X-ray flare spectrum (1 <<30 ) exhibits line emission from hydrogen-like and helium-like ions, e.g. Ne, Mg, Al, Si,... Fe, that indicates electron energies more characteristic of temperatures of 20 × 10⁶ K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s ²2s ² S-1s2s2p ² P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-, and ratios of the higher member lines to the Lyman- line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.This review will discuss the X-ray spectrum of solar flares from 250 keV to 0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.     

The synthesis of helium and CNO isotopes in massive stars

We present helium and CNO isotopic yields for massive mass-losing stars in the initial mass range 15M M _i 50M . We investigate their dependence on assumptions about mass loss rates, internal mixing processes, and metallicity, and specify the contributions from stellar winds and from supernova ejecta.     

Model-independent requirements for the source of positrons in the galactic centre

Model-independent requirements for the positron source in the galactic centre are formulated. From the known physical processes of positron production the most probable seems to be the e ⁺ – e ^– pair production as a result of photon-photon collisions. When certain conditions are satisfied, the efficiency of positron creation due to this mechanism can reach values 10%, which is comparable with the observed ratio of the annihilation line photon luminosity to the continuum one at E > 511 keV. Such a situation can be realized: (i) in a thermal pair-dominated mildly relativistic plasma, and (ii) on the development of a nonthermal electromagnetic cascade, initiated by relativistic particles in the field of ambient X-rays. Future gamma-ray observations at ultrahigh energies can be crucial to the choice of the model.     

The Edison infrared space observatory

For five years, theEdison program has had the goal of developing new designs for infrared space observatories which will break the cost curve by permitting more capable missions at lower cost. Most notably, this has produced a series of models for purely radiative and radiative/mechanical (hybrid) cooling which do not use cryogens and optical designs which are not constrained by the coolant tanks. Purely radiatively-cooled models achieve equilibrium temperatures as low as about 20 K at a distance of 1 AU from the sun. More advancedEdison designs include mechanical cooling systems attached to the telescope assembly which lower the optical system temperature to 5 K or less. Via these designs, near-cryogenic temperatures appear achievable without the limitations of cryogenic cooling. OneEdison model has been proposed to the European Space Agency as the next generation infrared space observatory and is presently under consideration as a candidate ESA Cornerstone mission. The basic design is also the starting point for elements of future infrared space interferometers.     

Hubble Space Telescope spectroscopy of massive hot stars in the Magellanic Clouds and M31

Using the Hubble Space Telescope (HST) and the Faint Object Spectrograph (FOS) high signal to noise spectrograms were obtained for 15 OB stars in the Magellanic Clouds^***, three of which are of spectral type O3. The data cover the spectral region from 1150 A – 2300 A with a resolution of /1 A. One O8.5 supergiant, OB78#231, in M31is also included in this work. These data are a substantial improvement on previous high resolution IUE observations in the Magellanic Clouds (Walborn et al. 1985 and references therein) because of the smaller aperture and the much better signal to noise ratio, while no high resolution UV spectra of O stars in M31 have been obtained before. In this paper we discuss various morphological aspects of the spectra, concerning metallicity and the stellar winds, compared to galactic analogues.