Radio-continuum study of the nearby sculptor group galaxies. Part 3: NGC 7793 at λ=12.2, 6 and 3 cm

We re-examine a series of archived centimetre radio-continuum observations (λ=16, 6 and 3 cm) focusing on NGC?7793 using the Australia Telescope Compact Array. These new images are both very sensitive (σ=0.011 mJy/beam) and feature reasonably high angular resolution (down to <3″). Using these images, a total of 76 discrete radio sources are identified, of which 57 have been classified. We also studied the radio component of the micro-quasar NGC7793-S26 which shows two distinct regions of somewhat steep spectral index (α) between ?0.3 and ?0.7.     

Mineral and chemical composition of the Jezersko meteorite—A new chondrite from Slovenia

The Jezersko meteorite is a newly confirmed stony meteorite found in 1992 in the Karavanke mountains, Slovenia. The meteorite is moderately weathered (W2), indicating short terrestrial residence time. Chondrules in partially recrystallized matrix are clearly discernible but often fragmented and have mean diameter of 0.73 mm. The meteorite consists of homogeneous olivine (Fa_19.4) and low‐Ca pyroxenes (Fs_16.7Wo_1.2), of which 34% are monoclinic, and minor plagioclase (Ab₈₃An₁₁Or₆) and Ca‐pyroxene (Fs₆Wo_45.8). Troilite, kamacite, zoned taenite, tetrataenite, chromite, and metallic copper comprise about 16.5 vol% of the meteorite. Phosphates are represented by merrillite and minor chlorapatite. Undulatory extinction in some olivine grains and other shock indicators suggests weak shock metamorphism between stages S2 and S3. The bulk chemical composition generally corresponds to the mean H chondrite composition. Low siderophile element contents indicate the oxidized character of the Jezersko parent body. The temperatures recorded by two‐pyroxene, olivine‐chromite, and olivine‐orthopyroxene geothermometers are 854 °C, 737–787 °C, and 750 °C, respectively. Mg concentration profiles across orthopyroxenes and clinopyroxenes indicate relatively fast cooling at temperatures above 700 °C. A low cooling rate of 10 °C Myr^?1 was obtained from metallographic data. Considering physical, chemical, and mineralogical properties, meteorite Jezersko was classified as an H4 S2(3) ordinary chondrite.     

Dendrogeomorphic chronologies of landslides: Dating of true slide movements?

Dendrogeomorphic chronologies of landslide movements are frequently used to investigate past landslide activity. Slide areas are often affected by other slope movements (e.g. creep) simultaneously. Trees growing on landslides record all types of ground movements, which potentially creates significant noise in tree ring based chronologies of landslide movements. The effect of creep movements on dendrogeomorphic landslide chronologies was evaluated in a block‐type landslide in the south‐western foreland of the Orlické hory Mountains. In total, 272 trees (Picea abies and Fagus sylvatica) were sampled (1088 increment cores) on the sub‐horizontal surfaces of rotated slide blocks, which were presumably created only by slide movements, and on the steep internal scarps separating landslide blocks, which were presumably created and affected by a combination of slide and creep movements. Ground movements were dated based on growth disturbances identified in an analysis of eccentric tree growth. The trees growing on the internal landslide scarps separating the rotated blocks exhibited a significantly higher number and frequency of growth disturbances than those growing on the sub‐horizontal block surfaces. All eight dated block surface movements were also identified on the internal scarps. Creep‐based events represented as many as 70% of the dated movement events on the internal scarps. Varying the I_t thresholds did not filter out more than 40% of the noise without significantly reducing the number of true dated slide events. A significant difference was observed between the ability of P. abies and F. sylvatica to record ground movements by eccentric growth. Probably due to its shallower roots (and weaker anchoring of the tree to landslide blocks), P. abies appears to be more sensitive to surficial ground movement, which potentially increases the proportion of dated creep events (noise). Thus, the careful selection of sampled tree species with different physiologies should be considered during dendrogeomorphic field sampling. Copyright © 2017 John Wiley & Sons, Ltd.     

Temporal trends of bulk precipitation and stream water chemistry (1977–1997) in a small forested area,Krusné hory,northern Bohemia,Czech Republic

The Krusné hory (Erzgebirge or Ore Mountains) has been heavily affected by high atmospheric pollutant deposition caused by fossil fuel combustion in an adjacent Tertiary coal basin. Long‐term routine sampling of bulk precipitation (1977–1996) and stream water (1977–1998) in a forested area on the south‐eastern slope of the mountains were used to evaluate trends and patterns in solute concentration and flux with respect to controlling processes. From 1977 to 1996, the annual volume‐weighted Ca²⁺ and SOconcentrations decreased in bulk precipitation. However, after 1989, when a pronounced and continuous decrease occurred in coal production, annual volume‐weighted concentrations decreased for most solutes, except H⁺. The concentration decreases were marked, with 1996 levels at or below 50% of those in 1989. The lack of a trend in H⁺ is attributed to similar decreases in both acid anions and neutralizing base cations. Stream water concentrations of most solutes, i.e. H⁺, Ca²⁺, Mg²⁺, SONOwere highest at the onset of sampling in 1977, decreased markedly from 1977 to 1983 and decreased more gradually from 1983 to 1998. The spruce forest die‐back and removal reduced dry deposition of these solutes by reducing the filtering action, which was provided by the forest canopy. A notable decrease in stream water Ca²⁺ concentrations occurred after 1995 and may be due to the depletion of Ca²⁺, which was provided by catchment liming in 1986, 1988 and 1989. Solute flux trends in bulk atmospheric deposition and stream water generally were not significant and the lack of trend is attributed to the large interannual variability in precipitation quantity and runoff, respectively. All solutes except Na⁺ varied seasonally. The average seasonal concentrations varied between the solutes, but for most solutes were highest in winter and spring and lowest in summer, correlating with the seasonal trend in runoff. For Ca²⁺, Mg²⁺ and SOthe concentration minimum occurs in September and the maximum occurs in February or March, correlating with the seasonal baseflow. These solutes are primarily controlled by the contribution of soil water and groundwater to stream flow. During snowmelt, the meltwater generally causes concentrations to decrease as soil water and groundwater are diluted. For NO₃ , average minimum concentrations occur in August at the end of the growing season concurrent with the lowest stream flow, and the maximum occurs in February and March with high stream flow during snowmelt. Seasonal stream water NOconcentration variations are large compared with the long‐term decrease. Copyright © 1999 John Wiley & Sons, Ltd.     

Landslide-dammed lake sediment volume calculation using waterborne ERT and SONAR profiling

Lake sediment volume calculation is a challenging task, namely in cases when detailed drilling is complicated, expensive, or impossible, information on the pre-sedimentation surface unavailable, and record of siltation rate non-existent or too short. This study shows how waterborne, non-invasive geophysical survey, such as electrical resistivity tomography (ERT) can be very effective in acquiring the missing data, namely when combined with sound navigation ranging (SONAR) water depth measurements and supported by information from auxiliary sources. However, ERT surveying in water environment requires specific approaches, as we illustrate on the case of the Mladotice lake study. The lake was created after a landslide in May 1872, and since its formation, the depth has gradually decreased due to sedimentation. We have reconstructed the original surface, calculated the sediment volume, and compiled information on sedimentation to estimate its remaining life span. To achieve this, we measured nine waterborne ERT profiles across the lake. To reach the necessary depth, all ERT profiles were extended on land and crossed the lake using custom-built flotation pads. ERT profiling was combined with SONAR depth measurements, historical bathymetric surveys, borehole core analysis, sediment flux measurements, volumetric calculations, and water conductivity probing. The study has achieved three main results. First, practical applicability and advantages of stationary waterborne ERT profiling in combination with bathymetric sounding were demonstrated. Second, the original lake volume and accumulated sediment was calculated. We estimate that the volume of lake sediment is 187 000 m³, two-thirds of the original lake volume (over 275 000 m³). Finally, based on three volumetric data sets from 1972, 2003, and 2017, and recent monitoring of the sediment inflow, we propose scenarios of lake filling and its future development. Most interestingly, the sedimentation rate has decreased significantly in the last 20 years, suggesting that the lake may survive much longer than hitherto expected. © 2020 John Wiley & Sons, Ltd.     

Purely coronal flare-like variations

A detailed study of the quasi-periodical post-flare variations on November 6, 1980 in X-rays, UV lines, microwaves, and metric waves confirms that these variations were predominantly thermal phenomena and occurred solely in the corona. Only the short-lived impulsive components that preceded all or most of the individual variations were of non-thermal character and penetrated down to the transition layer. The chromosphere (in Hα) did not participate in any part of these events, in contrast to a flare that appeared at the same place a few hours later. However, the X-ray emission of these variations was so strong that the transition layer and the chromosphere definitely should have been enhanced through heat conduction along the magnetic field lines. The expected heat flux at the top of the chromosphere coming from some of these coronal brightenings was 60–80% of the flux expected in the flare at 17:26 which gave rise to a 2B flare in Hα (Figure 8). Therefore, we suggest that the variations were produced in a coronal plasmoid with closed field lines completely detached from the lower atmospheric layers (Figure 9b). We also give reasons why such a detached plasmoid can be expected to be formded in the very late phase (some 4–5 hr after the onset) of a major two-ribbon flare.     

Spectral analysis of highly inhomogeneous chromospheric flares

Analysis of a hydrogen flare spectrum is carried out, assuming that the flare radiation is diluted due to a highly inhomogeneous space distribution of the flare elements in the chromosphere. It is shown that one obtains the correct physical parameters in the flare elements, irrespective of the extent of dilution, if all the elements are optically thin in the spectral regions considered. However, if this is not true for all the elements, the parameters deduced are in error, which increases with the extent of dilution. The worst affected parameter is the linear thickness of the flaring region which is found to be too low by a factor which may be as high as 10. Errors also enter into electron and atom densities which are found to be too high and into the column density of hydrogen atoms which is underestimated. On the other hand, the deduced source function, and in consequence of it the electron temperature as well, remain unaffected, if the analysis is properly carried out. All the errors may increase strikingly if the analysis is performed for optically thick regions. Several kinds of observation are mentioned which indicate that dilution of radiation may occur in flares. However, reasonable estimates of the flare parameters are still possible provided that one avoids spectral regions with 1.Senior Visiting Scientist to ESTEC, 1971–1972.