• A. Peraiah

      Articles written in Journal of Astrophysics and Astronomy

    • Comoving frame calculations of spectral lines formed in rapidly expanding media with the partial frequency redistribution function for zero natural line width

      A. Peraiah

      More Details Abstract Fulltext PDF

      Comoving frame calculations have been used to compute the spectral lines formed in rapidly expanding spherical media. We have employed the angle-averaged partial frequency redistribution functionRI with a two-level atom model in non-LTE atom approximation. A linear velocity law increasing with radius has been employed with maximum velocity at Τ=0 being set equal to 30 mean thermal units. It is found that one obtains almost symmetric emission line profiles at large velocities similar to those found in quasars.

    • Lines formed in rotating and expanding atmospheres

      A. Peraiah

      More Details Abstract Fulltext PDF

      Spectral lines formed in a rotating and expanding atmosphere have been computed in the frame of the observer at infinity. Two kinds of velocity laws are employed: (i) a uniform radial velocity of the gas and (ii) velocity increasing with radius (i.e. velocity gradients). The atmosphere has been assumed to be rotating with constant velocity. We have considered maximum radial and rotational velocities to be 10 and 3 mean thermal units respectively in an atmosphere whose geometrical thickness is 10 times the stellar radius. The total radial optical depth at line centre is taken to be about 100. In all cases, Doppler profile and a source function which is varying as 1/r2 have been used.

      Generally, the lines are broadened when rotation is introduced. However, when radial motion is also present, broadening becomes asymmetric and the red emission and blue absorption are enhanced.

    • An iterative simultaneous solution of the equations of statistical equilibrium and radiative transfer in the comoving frame

      A. Peraiah

      More Details Abstract Fulltext PDF

      A direct iteration has been performed to obtain a simultaneous solution of the equations of line transfer in an expanding spherically symmetric atmosphere in the comoving frame with statistical equilibrium for a non-LTE, two-level atom. The solution converges in three or four iterations to an accuracy of 1 per cent of the ratio of the population densities of the two levels. As initial values, the upper level population was set equal to zero or to the LTE densities. The final solution on convergence indicates enhanced population of these levels over the initial values assumed, Large velocity gradients enhance this effect whereas large geometrical sizes of the atmospheres tend to reduce it.

    • Effects of high velocities on photoionization lines

      A. Peraiah G. Raghunath

      More Details Abstract Fulltext PDF

      We have treated formation of spectral lines in a commoving frame where photoionization is predominant over collisional processes. We have assumed that the radiation field for causing photoionization is a function of Planck function. We have also considered the situation in which the continuum contributes to the radiation in the line. In all the models the quantityB/A (ratio of outer to inner radii) is kept equal to 10 and the total optical depth is taken to be 103. The velocity is assumed to be varying according to the lawdV/dτ ∼ < 1/τ whereτ is the optical depth (τ > 0) in the given shell. The velocities at the innermost radius (r =A) are set equal to 0 and at the outermost radius (r =B), the maximum velocities are taken to be 0, 1, 3 and 10 Doppler units. The calculated line profiles are those seen by an observer at infinity.P Cygni-type profiles are observed in the case of a medium with no continuum absorption. For a medium with continuum absorption double peaked asymmetric profiles are noticed when the velocities are small; the two emission peaks merge into a single asymmetric peak for larger velocities.

    • Photon escape probabilities in expanding atmospheres

      A. Peraiah K. E. Rangarajan D. Mohan Rao

      More Details Abstract Fulltext PDF

      A comparison of mean number of scatterings and escape probabilities has been made in isotropic scattering and dipole scattering by using the angle-averaged partial frequency redistribution functionRI. We have solved the equations of radiative transfer and statistical equilibrium simultaneously in a spherically symmetric expanding atmosphere. Two cases of atmospheric extension (i.e.)B/A=3 and 10 (whereB andA are the outer and inner radii of the atmosphere) have been treated.

      We find that the partial frequency redistribution gives a larger mean number of scatterings compared to that given by complete redistribution. Velocities tend to reduce the mean number of scatterings and in crease the mean escape probabilities.

    • Effects of redistribution with dipole scattering on line source functions

      A. Peraiah K. E. Rangarajan

      More Details Abstract Fulltext PDF

      The partial frequency redistribution function for zero natural line width with dipole scattering (RI) has been considered in obtaining the simultaneous solution of the statistical equilibrium and line transfer equations in the comoving frame of the expanding gas. We have considered a non-LTE two level atom in an expanding spherical medium whose outer radii are 3, 10 and 20 times the stellar radius with a total optical depthT ≃ 2 × 103. In all the cases, we have calculated the population ratio of the two levels N2/N1 and compared these results with those obtained by using different expansion velocities and geometrical extensions. Initially, the upper level population (N2) is set equal to zero. The converged simultaneous solution shows that the upper level population is enhanced considerably from the initial value. Variation in velocity gradients seem to have little effect on the ratio N2/N1 when the geometrical thickness of the medium is 3 or 10 times the stellar radius. However, when the thickness is increased to 20 times the central radius, the velocity gradients change the ratio N2/N1 considerably in the region where log T ≤ 2. The effect of variation of geometrical thickness is to reduce the N2/N1 ratio atτ = 0.

    • Optical depth effects on the formation of spectral lines in rotating and expanding spherical atmospheres

      A. Peraiah G. Raghunath K. N. Nagendra

      More Details Abstract Fulltext PDF

      We have investigated the effects of increasing optical depths on spectral lines formed in a rotating and expanding spherical shell. We have assumed a shell whose outer radius is 3 times the inner radius, with the radial optical depths equal to 10, 50, 100, 500. We have employed a constant velocity with no velocity gradients in the shell. The shell is assumed to be rotating with velocities varying as 1/ρ, whereρ is the perpendicular distance from the axis of rotation, implying the conservation of angular momentum. Two expansion (radial) velocities are treated: (1)V = 0 (static case) and (2)V = 10 mean thermal units. The maximum rotational velocities areVrot = 0, 5, 10 and 20. In the shell where there are no radial motions, we obtain symmetric lines with emission in the wings forVrot = 0 and 5 while forVrot ≥ 10 we obtain symmetric absorption lines. In the case of an expanding shell, we obtain lines with central emission.

    • On the Balmer progression in the expanding shell of Pleione

      A. Peraiah

      More Details Abstract Fulltext PDF

      An attempt has been made to explain the Balmer progression in velocities seen in Pleione and other shell stars. Equations of conservation of mass and momentum are simultaneously solved with assumptions which simplify the calculations of the solution. We have considered the radiation pressure due to lines and continuum. It is found that the high-excitation lines are formed where the velocity gradients are high.

    • Reflection effect in close binaries. I. Distribution of radiation from a point source

      A. Peraiah

      More Details Abstract Fulltext PDF

      The radiation field along an irradiated surface of a component in a binary system is calculated. The source of irradiation is assumed to be a point source. This is done primarily to understand easily how the incident radiation will get changed after it is being scattered by the atmosphere. It is noticed that the maximum radiation comes from intermediate points of the atmosphere, the reason being that here we have the combined radiation due to the star and incident radiation from the point source outside the star although both are diluted.

    • Reflection effect in close binaries. II. Distribution of emergent radiation from the irradiated component along the line of sight

      A. Peraiah

      More Details Abstract Fulltext PDF

      We have calculated the effects of irradiation from a point source observed at infinity. Plane-parallel approximation and spherically-symmetric approximations are employed in calculating the self-radiation field for the sake of comparison. It is found that there are considerable changes in the radiation received at infinity between the approximation of plane-parallel stratification and spherical symmetry.

    • Reflection effect in close binaries. III. Distribution of radiation incident from an extended source

      A. Peraiah

      More Details Abstract Fulltext PDF

      We have studied the effects of irradiation from an extended surface of the secondary component on the atmosphere of the primary. We have considered an isothermal and purely scattering medium. The resultant radiation field due to irradiation from an extended surface and self-radiation is different from that due to irradiation from a point source and self-radiation. In the case of the point source the middle layers of the exposed part of the atmosphere show maximum reflection while in the former case the reflection gradually decreases from the centre of the component towards the surface of the outermost layers of the atmosphere. The reflection effect appears to be strongly dependent on the density distribution of the electrons.

    • Reflection effect in close binaries. IV. Limb darkening of the reflected radiation incident from an extended surface of the secondary

      A. Peraiah M. Srinivasa Rao

      More Details Abstract Fulltext PDF

      The law of limb darkening has been calculated when the atmosphere of the primary component is illuminated by the extended surface of the secondary component in a binary system. The specific intensities calculated at infinity show marked changes when the plane-parallel approximation is replaced by the assumption of spherical symmetry. The middle portions of the illuminated surface reflect maximum radiation while the innermost and outermost layers show lesser amount of reflected radiation.

    • Reflection effect in close binaries. V. Effects of reflection on spectral line formation

      A. Peraiah M. Srinivasa Rao

      More Details Abstract Fulltext PDF

      The effects of reflection on the formation of spectral lines is investigated. We have assumed a purely scattering atmosphere and studied how the equivalent widths change due to irradiation from the secondary. Generally, the flux in the lines is increased at all frequency points, the cores of the lines receiving more flux than the wings. Moreover, the proximity of the secondary component changes the equivalent widths considerably. The further away the secondary is from the primary the higher are the equivalent widths.

    • Effects of partial frequency redistribution functions RII, RIII and RV on source functions

      D. Mohan Rao K. E. Rangarajan A. Peraiah

      More Details Abstract Fulltext PDF

      The effects of partial frequency redistribution on the formation of spectral lines have been studied. We considered the angle-averaged RII, RIII andRv types of redistribution with isotropic phase function, Transfer equation with plane-parallel geometry is solved in isothermal atmospheres. For an atmosphere with constant thermal sources, the frequency-dependent source function SL (Rv) lies below SL (RIII) but above SL(RIII) in the line wings.

    • Radiative transfer with Compton scattering in plane parallel geometry

      A. Peraiah

      More Details Abstract Fulltext PDF

      We have solved the equation of radiative transfer with Compton scattering. The specific intensity has been expanded by Taylor series with respect to wavelength and the first three terms have been retained in solving the transfer equation. It is noted that in a medium stratified in plane parallel layers, the multiple Compton scattering redistributes the initial energy over a range of 3 to 5 Compton wavelengths. A good fraction of the incident radiation is transferred across the layer with redistribution in wavelength, the actual value depending on the optical thickness of the medium

    • Equivalent widths of hydrogen Lyman alpha line in an expanding spherical atmosphere

      A. Peraiah M. F. Ingalgi

      More Details Abstract Fulltext PDF

      We have calculated the profiles of hydrogen Lyman a line in an expanding spherical atmosphere containing dust and gas. We have investigated the variation of equivalent widths with velocities of expansion of the atmosphere, together with the amount of dust present in the medium. We have drawn curves of growth for different velocities and dust optical depths

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

© 2017-2019 Indian Academy of Sciences, Bengaluru.