Simulation of adaptive optics for the Vacuum Tower Telescope

A simulation model of the adaptive optics of the German Vacuum Tower Telescope (VTT), Observatorio del Teide, Tenerife, is presented. The model uses modules from the integrated model of the Euro50 extremely large telescope, and includes submodels of a Shack-Hartmann wavefront sensor, a de-formable mirror, a tip-tilt mirror, high-voltage amplifier low-pass filters, a reconstructor and a controller. We investigate the impact on the closed loop bandwidth of changes in controller configuration and certain system parameters, such as low pass filter bandwidth and camera integration and readout time. Control strategies were tested on simple models before implementation on the full VTT model. Using the models, different control strategies are compared.     

An Early Investigation Of The Striated Tail Of Comet Hale-Bopp (C/1995 O1)

The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring outbursts with a periodicity of 11^h21^m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion. The parent particles were subjected to a radiation pressure acceleration of β_p ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU. This revised version was published online in July 2006 with corrections to the Cover Date.     

Coronal Holes and Flare Index: A correlation study

The correlation between the presence of coronal holes and flare indices has been investigated for the period from 1976 to 1995. The analysis shows that in the cases of 227 Carrington rotations (CRs) backward time lags yield the highest correlation between the coronal holes and flare indices. The maximum correlations were found at time lags of 222 and 142 CRs for polar and equatorial coronal holes, respectively. The period of study covers the past two solar cycles (21 and 22). Correlation analysis of both solar cycles has also been studied individually. The correlation analysis reveals that there is in general a forward shift in the maximum correlation for polar coronal holes, but it cannot be recommended to use polar coronal hole numbers for forecasting the next solar cycle.     

Spatially resolved spectrophotometry of comet P/Stephan-Oterma

Observations of Comet P/Stephan-Oterma were made with an Intensified Dissector Scanner spectrograph on the McDonald Observatory 2.7-m telescope during the period from July 1980 to February 1981. These spectra cover a range of heliocentric distances from 2.3 AU preperihelion to 1.8 AU postperihelion. A small aperture was used to map the spatial distributions of the gases in the coma. Column densities of the observed cometary emissions (CN, C₃, CH, and C₂) were calculated and it is shown that Stephan-Oterma appeared nearly spherically symmetric. These date are used by Cochran (1985, Icarus62, 82–92) to constrain chemical models of Stephan-Oterma.     

A warped m= 2 water maser disc in V778 Cyg?

The silicate carbon star V778 Cyg is a source of 22-GHz water maser emission which was recently resolved by MERLIN. Observations revealed an elongated     -like structure along which the velocities of the maser features show a linear dependence on the impact parameter. This is consistent with a doubly warped   m = 2  disc observed edge-on. Water masers and silicate dust emission (detected by the Infrared Astronomical Satellite and Infrared Space Observatory ) have a common origin in O-rich material and are likely to be co-located in the disc. We propose a detailed self-consistent model of a masing gas–dust disc around a companion to the carbon star in a binary system, which allows us to estimate the companion mass of  1.7 ± 0.1 M_⊙  , the disc radius of  40 ± 3  au and the distance between companions of ∼80 au. Using a dust–gas coupling model for water masing, we calculate the maser power self-consistently, accounting for both the gas and the dust energy balances. Comparing the simulation results with the observational data, we deduce the main physical parameters of the masing disc, such as the gas and dust temperatures and their densities. We also present an analysis of the stability of the disc.     

The gravitational field of topographic-isostatic masses and the hypothesis of mass condensation II-the topographic-isostatic geoid

In Part I we focussed on a convergent representation of the gravitational potential generated bytopographic masses on top of the equipotential surface atMean Sea Level, thegeoid, and by those masses which compensate topography. Topographic masses have also been condensated, namely represented by a single layer. Part II extends the computation of the gravitational field of topographic-isostatic masses by a detailed analysis of itsforce field in terms ofvector-spherical harmonic functions. In addition, the discontinuous mass-condensated topographic gravitational force vector (head force) is given. Once we identify theMoho discontinuity asone interface of isostatically compensated topographical masses, we have computed the topographic potential and the gravitational potential which is generated by isostatically compensated masses atMean Sea Level, the geoid, and illustrated by various figures of geoidal undulations. In comparison to a data oriented global geoid computation ofJ. Engels (1991) the conclusion can be made that the assumption of aconstant crustal mass density, the basic condition for isostatic modeling, does not apply. Insteaddensity variations in the crust, e.g. betweenoceanic and continental crust densities, have to be introduced in order to match the global real geoid and its topographic-isostatic model. The performed analysis documents that thestandard isostatic models based upon aconstant crustal density areunreal.     

A comparison of mud- and sand-dominated meanders in a downstream coarsening reach of the mixed bedrock-alluvial Klip River, eastern Free State, South Africa

Along a 28 km reach of the Klip River, eastern Free State, South Africa, mud- and sand-dominated meanders have developed in close proximity within a floodplain wetland up to 1.5 km wide, providing an unusual opportunity to compare their characteristics under similar hydrological conditions. Throughout the reach, the channel bed is grounded on sandstone/shale bedrock although the banks are alluvial, and most river activity occurs during summer high flows. The reach can be divided into three geomorphological zones: Zone 1 (0–11 km), a muddy proximal part with a single meandering channel (w/d < 10) and near-permanent standing water in oxbows and backswamps; Zone 2 (11–17.5 km), a transitional mud-to-sand part with one main channel (w/d  20–30), a number of sinuous palaeochannels and oxbows, and only limited standing water; and Zone 3 (17.5–28 km), a sandy distal part with a single meandering channel (w/d  15–30), scroll bars and oxbows, and little standing water. Each zone also has a distinctive sedimentology: Zone 1 is characterised by an  3–4 m thick succession of basal sand and minor granules overlain by dominantly muddy sediment deposited primarily by oblique accretion in meander bends; Zone 2 is characterised by < 4 m of interbedded sand and mud deposited primarily by lateral point-bar accretion, although a history of avulsions also attests to the importance of abandoned-channel accretion; and Zone 3 is characterised by < 3 m of dominantly sand deposited primarily by lateral point-bar accretion. This unusual downstream sediment coarsening trend, and the associated changes in channel and floodplain character, are independent of sediment inputs from tributaries, and result from a downstream increase in bankfull unit stream power from < 3.5 W m^− 2 (Zone 1) to  4–10 W m^− 2 (Zone 3). Mud is deposited primarily in low-energy Zone 1 but is conveyed in suspension more effectively through higher energy Zones 2 and 3, only forming drapes over sandy lateral accretion deposits during waning flood stages. The downstream increase in unit stream power is controlled in part by a slight downstream increase in floodplain gradient that may be related to a subtle variation in the erosional resistance of the bedrock underlying the channel bed. These findings add to previous work on meandering rivers by demonstrating that mud-dominated meanders can occur in long-term erosional settings where the channel bed is grounded on bedrock, and that downstream fining trends may be reversed locally.     

The spacetime gravitational field of a deformable body

The high-resolution analysis of orbit perturbations of terrestrial artificial satellites has documented that the eigengravitation of a massive body like the Earth changes in time, namely with periodic and aperiodic constituents. For the space-time variation of the gravitational field the action of internal and external volume as well as surface forces on a deformable massive body are responsible. Free of any assumption on the symmetry of the constitution of the deformable body we review the incremental spatial (“Eulerian”) and material (“Lagrangean”) gravitational field equations, in particular the source terms (two constituents: the divergence of the displacement field as well as the projection of the displacement field onto the gradient of the reference mass density function) and the `jump conditions' at the boundary surface of the body as well as at internal interfaces both in linear approximation. A spherical harmonic expansion in terms of multipoles of the incremental Eulerian gravitational potential is presented. Three types of spherical multipoles are identified, namely the dilatation multipoles, the transport displacement multipoles and those multipoles which are generated by mass condensation onto the boundary reference surface or internal interfaces. The degree-one term has been identified as non-zero, thus as a “dipole moment” being responsible for the varying position of the deformable body's mass centre. Finally, for those deformable bodies which enjoy a spherically symmetric constitution, emphasis is on the functional relation between Green functions, namely between Fourier-/ Laplace-transformed volume versus surface Love-Shida functions (h(r),l(r) versus h ^′(r),l ^′(r)) and Love functions k(r) versus k ^′(r). The functional relation is numerically tested for an active tidal force/potential and an active loading force/potential, proving an excellent agreement with experimental results. Received: December 1995 / Accepted: 1 February 1997     

Public participation in monitoring programmes as a tool for lakeshore monitoring: the example of Lake Pyhäjärvi, Karelia, Eastern Finland

Lake Pyhäjärvi, on the border between Finland and Russia in Karelia, is a very valuable clear-water lake of the Lobelia type. It belongs to the European Union's Natura 2000 programme in Finland, and has been included in regional and national monitoring programmes since the 1960s. The main monitoring station is situated near the outlet of the lake. Deterioration of its water quality was suspected already in the 1980s because of decreasing Secchi depths (transparency) and increasing chlorophyll a.The occurrence of algal blooms on the lakeshores is monitored weekly during each summer at one site on Lake Pyhäjärvi (site 1). This is a part of nationwide intensive algae monitoring programme organised by the environmental authorities together with voluntary observers at some 270 lake sites in Finland since 1998. Since 1997, Secchi depth observations have been carried out by volunteers biweekly or monthly at 17 sites on the lake. In the vicinity of one of these transparency observation sites (station 100), intensive monitoring of algae has been carried out. At this lakeshore monitoring site 69 algal observations were made, ten of which recorded algal blooms during the study period 1998–2002. The observed algal blooms were caused by algae of the Anabaena species, mainly by Anabaena lemmermannii. At Lake Pyhäjärvi the number of algal bloom observations received from the public have decreased from the 1990s to the 2000s. The range of Secchi disc transparency was 5.0–8.4 m with a mean value of 6.2 m at station 100 and 4.3–7.7 m (mean 6.1 m) at the main monitoring station 2 during the open water periods in 1998–2002. During this study period, the maximum values at site 100 seem to have increased slightly, which might indicate some improvement in the water quality due to decreased point source loading.We conclude that the intensive algal monitoring results of 5 years at the lakeshore site and the transparency results — both compiled by trained volunteers — reflect an improvement in the state of Lake Pyhäjärvi in Karelia. This conclusion is in accordance with the long-term water quality and short-core studies of sedimentary diatoms in Lake Pyhäjärvi. We suggest that the intensive algal observations and transparency measurements are both suitable methods for the monitoring of lakeshores and lakes, and that both are suitable for voluntary monitoring. We found public participation a good tool for monitoring lakes and lakeshores.