Venus cloud properties: Infrared opacity and mass mixing ratio

By using the Mariner 5 temperature profile and a homogeneous cloud model, and assuming that CO₂ and cloud particles are the only opacity sources, the wavelength dependence of the Venus cloud opacity is infrared from the infrared spectrum of the planet between 450 and 1250 cm^?1. Justification for applying the homogeneous cloud model is found in the fact that numerous polarization and infrared data are mutually consistent within the framework of such a model; on the other hand, dense cloud models are not satisfactory.Volume extinction coefficients varying from 0.5 × 10^?5 to 1.5 × 10^?5 cm^?1, depending on the wavelength, are determined at the tropopause level of 6110 km. By using all available data, a cloud mass mixing ratio of approximately 5 × 10^?6 and a particle concentration of about 900 particles cm^?3 at this level are also inferred. The derived cloud opacity compares favorably with that expected for a haze of droplets of a 75% aqueous solution of sulfuric acid.     

Evolutionary and pulsational properties of white dwarf stars

White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. Since the coolest white dwarfs are very old objects, the present population of white dwarfs contains a wealth of information on the evolution of stars from birth to death, and on the star formation rate throughout the history of our Galaxy. Thus, the study of white dwarfs has potential applications in different fields of astrophysics. In particular, white dwarfs can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, such as our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow these stars to be used as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. Last but not least, since many white dwarf stars undergo pulsational instabilities, the study of their properties constitutes a powerful tool for applications beyond stellar astrophysics. In particular, white dwarfs can be used to constrain fundamental properties of elementary particles such as axions and neutrinos and to study problems related to the variation of fundamental constants. These potential applications of white dwarfs have led to renewed interest in the calculation of very detailed evolutionary and pulsational models for these stars. In this work, we review the essentials of the physics of white dwarf stars. We enumerate the reasons that make these stars excellent chronometers, and we describe why white dwarfs provide tools for a wide variety of applications. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with a unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows one to measure stellar masses with unprecedented precision and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology.     

Effect of low-frequency instability on hall conductivity in plasma

In the present experiment which is the continuation previous our work we study the effect of low-frequency drift wave instability on Hall conductivity in plasma. Using an external oscillation we can affect on the drift wave amplitude (mainly around resonance) and the variation on Hall conductivity is observed. The effect is probably attributed to electron trapping by the wave potential. Good agreement between experimental and calculated values of azimuthal drift currents on the resonance and away from resonance, lead us to believe that the proposed explanation by electron trapping is correct.     

Effect of magnetic field on a shock-induced thermal instability

The effect of a magnetic field on a thermal instability has been studied in a radiatively cooling region behind an interstellar shock of moderate propagation velocity ( 10 km s^–1). It is shown that the presence of a magnetic field of a few microgauss is very effective in preventing the thermal instability from building-up density concentrations. In the absence of the magnetic field, the shock-induced thermal instability will amplify a pre-shock density inhomogeneity by more than an order of magnitude. However, in the field's presence, the amplified density contrast is shown to be only a factor 2. Implications for the trace of a sweeping broom in the Pleiades nebula are discussed.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

The pulsational stability of intermediate- and low-luminosity red giants in globular clusters

Some theoretical calculations of linear non-adiabatic pulsations of intermediate- and low- luminosity red giants in globular clusters have been carried out using a time-dependent theory of nonlocal stellar convection. As shown by the results, for all models with temperatures higher than 5400 K the modes up to the fourth overtone are pulsationally stable. With the increase of stellar luminosity, the low-order overtones also become pulsationally unstable. For red giants of intermediate and low luminosities, the pulsational stability is exceedingly low and is close to neutral stability. Therefore, they will be either non-variables or short-period variables (P < 2 days) with extremely small amplitudes.     

RR Lyrae pulsational temperature scales: on the consistency between different empirical relations

In this paper, by assuming the equilibrium temperatures of RRab Lyrae variables defined by Carney, Storm & Jones as correct we show that temperatures derived from ( B − V ) colour (mean colour over the pulsational cycle calculated on the magnitude scale) transformations by Bessel, Castelli & Plez are consistent with the Carney et al. equilibrium temperatures within a probable error of δ  log  T _e =±0.003 . As a consequence, it is shown that the pulsational temperature scale temperature–period–blue amplitude [ T _eff= f ( P , A _B )] relation provided by De Santis, who studied the ( B − V ) colour of about 70 stars of Lub's sample, is a suitable relation, being reddening- and metallicity-free, to calculate equilibrium temperatures for RRab variables. This relation is independent of variable mass and luminosity within a large range of period-shift from the mean period–amplitude relation valid for Lub's sample of variables. On the contrary, it is also shown that a temperature–amplitude–metallicity relation is strictly dependent on the period–amplitude relation of the sample used for calibrating it: we prove that this means it is dependent on both the mass and luminosity variations of variables.     

RR Lyrae variables in M5 as a test of pulsational theory

We present B and V CCD photometry for variables in the cluster central region, adding new data for 32 variables and giving suitable light curves, mean magnitudes and corrected colours for 17 RR Lyrae variables. Adding the data given in this paper to similar data that have already appeared in the literature, we discuss a sample of 42 variables, as given by 22 RR_ab and 20 RR_c, in the light of recent predictions from pulsational theories. We find that the observational evidence concerning M5 pulsators appears in marginal disagreement with predictions concerning the colour of the first overtone blue edge (FOBE), whereas a clear disagreement appears between the zero-age horizontal branch (ZAHB) luminosities predicted through evolutionary and pulsational theories.     

Effect of the Coriolis force and slowly varying flow on the Kelvin-Helmholtz instability

The effect of the Coriolis force on the Kelvin-Helmholtz instability has been investigated, in which basic velocities of fluid are varying slowly with lateral coordinatey. The problem is solved by J.W.K.B. approximation method. Parabolic and other types of profiles for wide long channel flow have also been studied in detail.     

Effect of suspended particles on the thermal convection instability in hydromagnetics

The effect of suspended particles in a finitely conducting gas on the thermal convection instability is studied. The critical Rayleigh number at which instability sets in is reduced by the presence of suspended particles. The effect of vertical magnetic field is stabilizing. We also study the effect of conducting particles suspended in a non-conducting gas. It is found that the stabilizing effect of the magnetic field is reduced by the electrically conducting suspended particles.     

Effect of general loss-cone distribution function on the shear-driven electrostatic ion-cyclotron instability

The shear-driven electrostatic ion-cyclotron instability (EICI) is studied using the loss-cone distribution function by particle aspect analysis. The effect of the loss-cone distribution on the dispersion relation and growth rate of weak shear-driven EICI is studied. The whole plasma is considered to consist of resonant and non-resonant particles. The wave is assumed to propagate obliquely to the static magnetic field. It is found that the frequency of the EICI is Doppler shifted due to the transverse inhomogeneous flow in the direction of the magnetic field. It is also found that for anisotropic plasma the critical velocity shear needed to excite EICI depends upon the loss-cone distribution index (J). With the increasing values the loss-cone distribution indices (J), the critical value of normalized velocity shear needed to generate EICI in anisotropic plasma, decreases and is of the order of the weak shear. The loss-cone distribution acts as a source of free energy and generates the weak shear-driven EICI at longer perpendicular perturbations. It also lowers the transverse and parallel energy of the resonant ions. The study may explain the frequently observed EICI in the auroral acceleration region.     

Probing model-independent cosmic opacity and its spatial properties

Cosmic opacity and its spatial distribution have been constrained with a model independent method. The average opacity of the universe is not zero, but can be zero in the 1σ error range. The bestfit value of the spatial distribution of cosmic opacity is not a constant as the redshift varies, though a homogeneous and transparent universe is favored in the 2σ error range.     

Effect of Coulomb collisions on time variations of the solar neutrino flux

We consider the processes that might suppress the time variations in the solar neutrino flux produced by the radial motion of the Earth through the neutrino interference pattern. We calculate these time variations and the extent to which they are suppressed by Coulomb collisions of the neutrino-emitting nuclei. This is done for both the 0.862-MeV ⁷Be neutrino line and the continuous neutrino spectrum, assuming a Gaussian energy response function of the neutrino detector. We find that the collisional decoherence averages out the time variations for neutrino masses A simple and clear physical picture of the time-dependent solar neutrino problem is presented and qualitative coherence criteria are discussed.     

The opacity of grains in protoplanetary atmospheres

We have computed the size distribution of silicate grains in the outer radiative region of the envelope of a protoplanet evolving according to the scenario of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62-85]. Our computation includes grain growth due to Brownian motion and overtake of smaller grains by larger ones. We also include the input of new grains due to the breakup of planetesimals in the atmosphere. We follow the procedure of Podolak [Podolak, M., 2003. Icarus 165, 428-437], but have speeded it up significantly. This allows us to test the sensitivity of the code to various parameters. We have also made a more careful estimate of the resulting grain opacity. We find that the grain opacity is of the order of throughout most of the outer radiative zone as Hubickyj et al. [Hubickyj, O., Bodenheimer, P., Lissauer, J.J., 2005. Icarus 179, 415-431] assumed for their low opacity case, but near the outer edge of the envelope, the opacity can increase to . We discuss the effect of this on the evolution of the models.     

Effect of quantum corrections on the Jeans instability of self-gravitating viscoelastic dusty fluid

In this paper we investigate the effects of quantum correction on the Jeans instability of self-gravitating viscoelastic dusty electron-ion quantum fluids. The massive self-gravitating dust grains are assumed to be strongly coupled and non-degenerate having both viscous and elastic behavior while the inertialess electrons and ions are considered as weakly coupled and Fermi degenerate. The hydrodynamic model is modified and a linear dispersion relation is derived employing the plane wave solutions on the linearized perturbation equations for the considered system. It is observed that the dispersion properties are affected due to the presence of viscoelastic effects and quantum statistical corrections. The modified condition of Jeans instability and expression of critical Jeans wavenumber are obtained. Numerically it is shown that viscoelastic effects, dust plasma frequency and quantum statistical effects all have stabilizing influence on the growth rate of gravitationally Jeans mode. The growth rates are also compared in kinetic and hydrodynamic limits and it is found that decay in the growth of unstable Jeans mode is larger under the kinetic limits than the hydrodynamic limits. The results are discussed for the understanding of formation of dense degenerate dwarf star through gravitational collapsing which is assumed to be strongly coupled dusty quantum fluid where the strongly coupled dust provides inertia and Fermi degenerate electron and ions provide quantum statistical effects.