Photometric survey of binary near-Earth asteroids

Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio Ds/Dp?0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of Ds=0.5-1 km. Systems with Ds/Dp?0.5 are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ∼4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/cm³. Orbital periods show an apparent cut-off at Porb∼11 h; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with Porb>20 h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with Ds/Dp?0.18 among NEAs larger than 0.3 km is 15±4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66⁺¹⁰₋₁₂)%.     

Piloting a network of small telescopes

The Pilot Small Telescope Network (PSTN) is a state‐of the art system of easily replicable and scalable hardware, software, servers, eXtensible Markup Language (XML) protocols, and network middleware connecting and developing a pilot array of robotic telescopes to one another and the user community. The PSTN is a developmental project that will allow growing access to these telescopes, and make available data to faculty, students and others in an environment of collaboration. The underlying goal of the PSTN is to broaden the quantity and quality of astronomical education and research, particularly with a focus on traditionally underserved populations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Suzaku observation of NGC 3516: complex absorption and the broad and narrow Fe K lines

We present preliminary results from a 150 ks Suzaku observation of the Seyfert 1 galaxy NGC 3516. Suzaku 's wide bandpass has enabled us to deconvolve the broadband emitting and absorbing components in this object, breaking model degeneracies inherent in previous, smaller‐bandpass spectra. The primary power‐law continuum is absorbed by an ionized absorber as well as a partial‐covering absorber; the column density of the ionized absorber has increased by a factor of ∼3 since XMM‐Newton observations in 2001. We detect a soft power‐law component which may be scattered emission. We confirm the presence of the broad Fe line, finding a eV equivalent width line that indicates emission extending down to a few Schwarzschild radii. Models which exclude either the broad line or the partial‐covering absorber are rejected. Suzaku 's high effective area and low background near 6 keV also allow us to resolve the narrow Fe K emission line; we find a FWHM velocity width near 4000 km s^–1, commensurate with Broad Line Region velocities. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A modelling study of satellite beacon measurements of protonospheric replenishment

Recent satellite beacon derived measurements of the recovery of protonospheric ionization following periods of increased geomagnetic activity show that the recovery takes longer than is indicated by whistler measurements. Realistic plasmasphere models have been used to determine whether satellite beacon measurements are reliable indicators of this recovery. It is found that the recovery time of the protonospheric content is similar to that of the minimum L-value flux tube intersected by the slant raypaths. Satellite beacon results are therefore useful indicators of protonospheric recovery after a storm provided any unrepresentative diurnal variations are eliminated.     

Optical depth effects on the formation of spectral lines in rotating and expanding spherical atmospheres

We have investigated the effects of increasing optical depths on spectral lines formed in a rotating and expanding spherical shell. We have assumed a shell whose outer radius is 3 times the inner radius, with the radial optical depths equal to 10, 50, 100, 500. We have employed a constant velocity with no velocity gradients in the shell. The shell is assumed to be rotating with velocities varying as 1/_ρ, where_ρ is the perpendicular distance from the axis of rotation, implying the conservation of angular momentum. Two expansion (radial) velocities are treated: (1)V = 0 (static case) and (2)V = 10 mean thermal units. The maximum rotational velocities areV _rot = 0, 5, 10 and 20. In the shell where there are no radial motions, we obtain symmetric lines with emission in the wings forV _rot = 0 and 5 while forV _rot ≥ 10 we obtain symmetric absorption lines. In the case of an expanding shell, we obtain lines with central emission.     

The optical continuum of solar and stellar flares

A further development of the Kostyuk-Pikelner's model is presented. The response of the chromosphere heated by non-thermal electrons of the power-law energy spectrum has been studied on the basis of the numerical solution of the one-dimensional time-dependent equations of gravitational gas dynamics. The ionization and energy loss for the emissions in the Lyman and Balmer lines have been determined separately for the optically thin and thick L-line layers. Due to the initial heating, a higher-pressure region is formed. From this region, disturbances propagate upwards (a shock wave with a velocity of more than 1000 km s^-1) and downwards. A temperature jump propagates downwards, and a shock is formed in front of the thermal wave. During a period of several seconds after the beginning of this process, the temperature jump intensifies the downward shock wave and the large radiative loss gives rise to the high density jump ( ₂/ ₁ 100). The numerical solution has been analyzed in detail for the case heating of the ionized and neutral plasma, and a value of this heating is close to the upper limit of the admissible values. In this case, the condensation located between the temperature jump and the shock wave front, may emit in the observed optical continuum.In their essential features, the gas dynamic processes during the flares in red dwarf atmospheres are the same as those in the solar atmosphere. However, the high atmospheric densities, smaller height scale in red dwarf atmospheres, and greater energy of this processes in stellar flares, give rise, in practice, to the regular generation of optical continuum. The photometric parameters of a source with n 0¹⁵ cm^-3, T 9000 K, and _z 10 km are in a good agreement with observations.     

Short-period intensity fluctuations of integral sunlight

An attempt has been made to detect short-period solar luminosity fluctuations in the vicinity of 5 min, analogous to the observed velocity oscillation. Using silicon photodiodes to monitor integral sunlight, an upper limit for the amplitude of the intensity fluctuations of 3 x 10^-5 rms was found.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

The distribution of infrared sources in the galactic plane

A study of the galactic structure has been made by deriving the brightness distribution of the galactic plane at 2.2 m and 4.2 m near infrared region using infrared objects detected by ground-based sky surveys. The infrared brightness distribution shows distinct peaks at every 8–12° galactic longitude and indicates a periodic structure. The one-to-one positional correlation observed between the periodic structure in 2.2 m infrared brightness and the 2.6 mm CO emission line suggests that the near-infrared sources are strongly associated with dense clouds of molecular hydrogen distributed in the galactic plane.     

Occultation of ε Geminorum by Mars: IV. Oblateness of the martian upper atmosphere

The oblateness of the Martian upper atmosphere was determined from analysis of photoelectric observations of the 8 April 1976 occultation of ε Geminorum by Mars at seven stations. The oblatness is 0.0096 ± 0.0023, consistent with a mean equator-to-pole temperature difference in excess of ～ 50°K, vertically averaged from the surface to the occulation altitude of ～70 km. The astrometric solution provides precise determination of the occultation path relative to the Martian shadow, and absolute vertical alignment of upper atmospheric temperature profiles obtained by inversion of occultation light curves. The observations can be compared directly with models of atmospheric tides computed for the conditions at the suboccultation regions on Mars.     

Electron content modelling: The significance of protonospheric contents

The principal advance of the ATS-6 satellite beacon experiment was the ability to deduce continuously the electron content along the entire slant path from ground-based measurements of the signal group delay. This feature has been exploited in conjunction with the more usual Faraday rotation technique to separate the total electron content into ionospheric and protonospheric components. The physical validity of the deduced quantities is investigated using a mathematical model of the plasmasphere in which integration of the time-dependent continuity and momentum equations for oxygen and hydrogen ions along selected L shells yields the ion concentrations and field-aligned fluxes. The ion concentrations are then integrated along the propagation path to various ground stations from ATS-6 to give computed values for comparison with observations. The mathematical model is used with different sets of atmospheric parameters to investigate the significance of ionospheric and protonospheric contents as derived from beacon data.The calculated electron concentrations are able to reproduce mid-latitude equinoctial electron content observations. The shape parameters τ and F can also be simulated by day, but night-time values do not match the observations well, a greater protonospheric content being required. The calculations show that the quantity N_p, which is readily derived from ATS-6 observations, may be interpreted as the slant H⁺ content above some fixed height in the case of some stations (but not others) if the plasmasphere is reasonably full. The total slant content of H⁺ is approx. twice the value of N_p, though it appears that for the Lancaster raypath a closer relationship exists between N_p and the H⁺ tube content at L = 1.8. In general,N_p is most closely related to the tube content for an L value slightly greater than the minimum L intersected along the raypath.