Further constraints on variation of the fine-structure constant from alkali-doublet QSO absorption lines

Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, α , over cosmological time-scales. We constrain variation in α in 21 Keck/HIRES Si  iv absorption systems using the alkali-doublet (AD) method in which changes in α are related to changes in the doublet spacing. The precision obtained with the AD method has been increased by a factor of 3:     . We also analyse potential systematic errors in this result. Finally, we compare the AD method with the many-multiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method.     

Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r ⁻² density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of     , a dust number density of     at r _in and a dust mass of     . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small     colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles.     

Do LINER 2 galaxies harbour low-luminosity active galactic nuclei?

We use ROSAT HRI spatial data and ASCA spectral measurements for a sample of seven nearby, early-type spiral galaxies, to address the question of whether a low-luminosity active galactic nucleus (LLAGN) is present in galaxies that have a LINER 2 classification. The brightest discrete X-ray source in the ROSAT HRI observations is invariably found to be positionally coincident with the optical galactic nucleus, and in most cases its flux dominates the X-ray emission from the central region of the galaxy. All seven galaxies have X-ray spectra consistent with a two-component, soft thermal plus hard power-law, spectral form. If we exclude the two galaxies with relatively hard X-ray spectra, NGC 3628 and NGC 4594, for which there is supporting evidence for a LLAGN (or alternatively in the case of NGC 3628 a dominant ultraluminous X-ray binary), then the remaining galaxies show surprisingly similar X-ray spectral properties. Specifically the flux ratio F _X(0.5–1)/ F _X(2–5) , which measures the relative strengths of the thermal and non-thermal emission components, shows little scatter about a mean of 0.66, a value very similar to that measured in the classic starburst galaxy NGC 253. As there is no obvious reason why the luminosity of the hard power-law continuum emanating from a putative LLAGN should be very closely correlated with the thermal emission of the surrounding region, this suggests that that the broad-band (0.5–5 keV) X-ray emission from these LINER 2 galaxies may originate in a common set of processes probably associated with the starburst phenomenon. Conversely, it appears that in many LINER 2 galaxies and perhaps the majority, the nuclear X-ray luminosity does not derive directly from the presence of a LLAGN.     

Numerical modelling of permafrost in bedrock in northern Fennoscandia during the Holocene

The occurrence of permafrost in bedrock in northern Fennoscandia and its dependence on past and presently ongoing climatic variations was investigated with one- (1D) and two-dimensional (2D) numerical models by solving the transient heat conduction equation with latent heat effects included. The study area is characterized by discontinuous permafrost occurrences such as palsa mires and local mountain permafrost. The ground temperature changes during the Holocene were constructed using climatic proxy data. This variation was used as a forcing function at the ground surface in the calculations. Several versions of the present ground temperature were applied, resulting in different subsurface freezing–thawing conditions in the past depending on the assumed porosity and geothermal conditions.Our results suggest that in high altitude areas with a cold climate (present mean annual ground temperature between 0°C and −3°C), there may have been considerable variations in permafrost thickness (ranging from 0 to 150 m), as well as periods of no permafrost at all. The higher is the porosity of bedrock filled with ice, the stronger is the retarding effect of permafrost against climatic variations.Two-dimensional models including topographic effects with altitude-dependent ground temperatures and slope orientation and inclination dependent solar radiation were applied to a case of mountain permafrost in Ylläs, western Finnish Lapland, where bedrock permafrost is known to occur in boreholes to a depth of about 60 m. Modelling suggests complicated changes in permafrost thickness with time as well as contrasting situations on southern and northern slopes of the mountain.Extrapolating the climatic warming of the last 200 years to the end of the next century when the anticipated increase in the annual average air temperature is expected to be about 2 K indicates that the permafrost occurrences in bedrock in northern Fennoscandia would be thawing rapidly in low-porosity formations. However, already a porosity of 5% filled with ice would retard the thawing considerably.     

Gamma—Ray Bursts：Afterglows and Central Engines

Gamma-ray bursts (GRBs) are the most intense transient gamma-ray events in the sky; this, together with the strong evidence (the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, makes them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to the total convertion into radiation of the mass energy of more than one solar mass. This is thousand times stronger than the energy of a supernova explosion. Some unconventional energy mechanism and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae at cosmoligical distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on their central engine. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands.     

The orbital resonance amongst the Galilean satellites of Jupiter

The resonance amongst three of the Galilean satellites is described in a way intended to demonstrate the similarities it has to the normal two-satellite resonance. The hypothesis that the resonance was formed by the action of tidal forces is discussed. The problem is too complicated to reach any firm conclusions, but the tidal hypothesis does not seem to be a satisfactory explanation.

     

Escape of asteroids from the Hecuba gap

The dynamics of the 2/1 mean-motion asteroidal resonance with Jupiter is studied by numerical integration of the equations of motion of the Sun-Jupiter-Saturn-asteroid system. The measurement of the fundamental asteroidal frequencies by means of Fourier and wavelet analyses allows us to construct the web of the secular, secondary and Kozai resonances inside the 2/1-resonance boundaries. The structure of the phase space of the 2/1 resonance is discussed with emphasis on the acting depletion mechanisms due to presence of these inner resonances. Special attention is paid to the study of the middle-eccentricity depleted region. The importance of the great inequality of the Jupiter-Saturn system in the acceleration of the diffusion processes in this region is pointed out. The existence of a group of asteroids like (3789) Zhongguo, inside the 2/1 resonance, is also discussed.     

Observations of GRS 1915+105 from the USA Experiment on ARGOS

We present first results from a monitoring campaign of GRS 1915+105 undertaken with the USA X-ray timing experiment on the ARGOS satellite. A variety of behaviour has been observed, ranging from low, steady X-ray emission to rapid quasi-periodic flaring on timescales of approximately 10–120 seconds.     

Capture of particles in hypervelocity impacts in aerogel

Abstract— The capture in aerogel of 106 μm diameter glass beads is investigated for impact speeds of 1 to 7.5 km s^?1. Three different aerogel densities were used, 60,96 and 180 kg m^?3. It was found that the length of the penetration track in the aerogel increases with speed until a maximum is reached. Above the maximum speed the track length decreases. This behaviour is similar to that which has previously been observed for particles impacting polystyrene foams and porous alumina. Whilst track length was not found to be an unambiguous indicator of impact speed, the excavated track volume was found to be a suitable indictor of speed. Further, it was possible to estimate the original particle size by measurements of the track volume and entrance hole size. In addition sub‐100 μm diameter particles composed of various minerals were fired into aerogel and the characterisation of the particles in situ by use of a Raman spectrometer was evaluated. This was found to work well, giving vibrational spectra essentially similar to those of the bulk minerals, thus providing a mineralogical rather than an elemental signature for the captured particles.