Photometry of open cluster NGC 1931

PhotoelectricUBV magnitudes and colours have been determined for stars in the field of NGC 1931. The reddening across the cluster varies from 0 _. ^m 33 to 1 _. ^m 20. A distance of 2.16 kiloparsecs has been estimated for the cluster. It is concluded that the age of the cluster lies between the ages of NGC 6231 and NGC 2362 groups.     

The effect of finite electrical conductivity on compressible magnetoconvection

The stability of a polytropic fluid layer in the presence of a uniform vertical magnetic field is studied under the combined influence of thermal and magnetic diffusion. The main objective of the present investigation is to examine the effect of finite electrical conductivity of the medium on the stability of hydromagnetic modes which are believed to be important in the sunspot phenomena. It is shown that the inclusion of finite electrical conductivity has destabilizing effect on convective modes and small-scale convection can occur in the presence of strong magnetic field, provided the magnetic diffusivity is larger than the thermal diffusivity. The magnetic diffusivity, however, has a tendency to stabilize the fast, slow, and Alfvén-modes.     

HI fluctuations at large redshifts: II — the signal expected for the GMRT

For the GMRT, we calculate the expected signal from red-shifted HI emission at two frequency bands centered at 610 and 325 MHz. The study focuses on the visibility-visibility cross-correlations, proposed earlier as the optimal statistical estimator for detecting and analyzing this signal. These correlations directly probe the power spectrum of density fluctuations at the redshift where the radiation originated, and thereby provide a method for studying the large scale structures at large redshifts. We present detailed estimates of the correlations expected between the visibilities measured at different baselines and frequencies. Analytic fitting formulas representing the salient features of the expected signal are also provided. These will be useful in planning observations and deciding an optimal strategy for detecting this signal.     

The occultation of MKE 31 by Neptune on September 12, 1983

The predicted occultation of the star No. 31 (Mink et al., 1981) by Neptune on September 12, 1983 was observed photoelectrically. Four secondary occultations were recorded before the immersion event.     

Relativistic reductions for radiointerferometric observables

The special and general relativistic corrections for the basic radiointerferometric observables —time-delay and fringe frequency — are studied with an accuracy of the order of 5×10^?10 (‘post-Newtonian approximation’).     

Abundance analysis of the cool extreme helium star LSS 3378

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using local thermodynamic equilibrium (LTE) line formation and LTE model atmospheres constructed for EHe stars.
The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s -process nucleosynthesis. The derived abundances of iron peak and α-elements indicate the absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance [log ε(Fe) = 6.1] is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.     

Nanoflares and heating of the solar corona

Coronal heating by nanoflares is presented by using observational, analytical, numerical simulation and statistical results. Numerical simulations show the formation of numerous current sheets if the magnetic field is sheared and bipoles have unequal pole strengths. This fact supports the generation of nanoflares and heating by them. The occurrence frequency of transients such as flares, nano/microflares, on the Sun exhibits a power-law distribution with exponent α varying between 1.4 and 3.3. For nanoflares heating α must be greater than 2. It is likely that the nanoflare heating can be reproduced by dissipating Alfven waves. Only observations from future space missions such as Solar-B, to be launched in 2006, can shed further light on whether Alfven waves or nanoflares, heat the solar corona.     

Strong and Weak Damping of Slow MHD Standing Waves in Hot Coronal Loops

Slow magnetohydrodynamic (MHD) standing wave oscillations in hot coronal loops for both strong (i.e. τ_d/P∼ 1) and weak (i.e. τ_d/P≥ 2) damping are investigated taking account of viscosity, thermal conductivity and optically thin radiation. The individual effect of the dissipative terms is not sufficient to explain the observed damping. However, the combined effect of these dissipative terms is sufficient to explain the observed strong damping, as well as weak damping seen by SUMER. We find that, the ratio of decay time (τ_d) and period (P) of wave, i.e., τ_d/P (which defines the modes of damping, whether it is strong or weak) is density dependent. By varying density from 10⁸ to 10¹⁰ cm⁻³ at a fixed temperature in the temperature range 6 – 10 MK, observed by SUMER, we get two sets of damping: one for which τ _d/P∼ 1 corresponds to strong damping that occurs at lower density and another that occurs at higher density for which τ_d/P ≥ 2 corresponds to weak damping. Contrary to strong-damped oscillations, the effect of optically thin radiation provides some additional dissipation apart from thermal conductivity and viscosity in weak-damped oscillations. We have, therefore, derived a resultant dispersion relation including the effect of optically thin radiation. Solutions of this dispersion relation illustrate how damping time varies with physical parameters of loops in both strong and weak damping cases.     

Possible ring system of Neptune

A re-analysis of the observations of occultation of MKE 31 by Neptune on September 12, 1983 (Pandey et al., 1984) shows that the possible ring system of Neptune extends from 64400 km to 64190 km in Neptune's equatorial plane.