pp 1-2 March 2010
pp 3-16 March 2010
In this paper, we consider a statistical method for distance determination of stellar groups. The method depends on the assumption that the members of the group scatter around a mean absolute magnitude in Gaussian distribution. The mean apparent magnitude of the members is then expressed by frequency function, so as to correct for observational incompleteness at the faint end. The problem reduces to the solution of a highly transcendental equation for a given magnitude parameter 𝛼. For the computational developments of the problem, continued fraction by the Top–Down algorithm was developed and applied for the evaluation of the error function erf(𝑧). The distance equation 𝛬(𝑦) = 0 was solved by an iterative method of second order of convergence using homotopy continuation technique. This technique does not need any prior knowledge of the initial guess, a property which avoids the critical situations between divergent and very slow convergent solutions, that may exist in the applications of other iterative methods depending on initial guess.
Finally, we apply the method for the nearby main sequence late type stars assuming that the stars of each group of the same spectral type scatter around a mean absolute magnitude in a Gaussian distribution. The accuracies of the numerical results are satisfactory, in that, the percentage errors between 𝑟 and the mean values are respectively: (2.4%, 1.6%, 0.72%, 0.66%, 3.5%, 2.4%, 2%, 2.5%, 0.9%) for the stars of spectral types: (F5V, F6V, F7V, F8V, F9V, G0V, G2V, G5V, G8V).
pp 17-29 March 2010
Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (𝑅V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (𝑅V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (𝑅V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.
pp 31-41 March 2010
We have made spectral line analysis of CO and its isotopic lines from dark cloud Lynds 183 (L183). Our dataset incorporates 12CO(1––0), 13CO(1––0) and 13CO(2––1) lines using NRAO-12m and 12CO(3––2), 13CO(3––2) lines using CSO-10m telescopes, respectively. Observations suggest steep north-south (direction with respect to the offset position (0, 0)) temperature gradient in the cloud. These are likely to be caused by non-uniform, Inter Stellar Radiation Field (ISRF) illumination due to the shadow of nearby L134 cloud complex. As the emission of radiation depends on local properties like density and kinetic temperature, the present study attempts to deduce the irradiation contrast (and the resulting temperature difference) using 1D Monte Carlo radiative transfer code RATRAN. The model results accord with the observed data and shows a temperature difference of ∼ 7 K mainly within the cloud envelope. This results in a non-uniform intensity distribution of both CO and its species.
pp 43-58 March 2010
W51 is a giant radio complex lying along the tangent to the Sagitarius arm at a distance of about 7 kpc from the Sun, with an extension of about 1° in the sky. It is divided into three components A, B, C where W51A and W51B consist of many compact HII regions while W51C is a supernova remnant. We have made continuum radio observations of these HII regions of the W51 complex at 240, 610, 1060 and 1400 MHz using GMRT with lower resolution (20'' × 15'') at the lowest frequency. The observed spectra of the prominent thermal subcomponents of W51 have been fitted to a free-free emission spectrum and their physical properties like electron temperatures and emission measures have been estimated. The electron temperatures from continuum spectra are found to be lower than the temperatures reported from radio recombination line (RRL) studies of these HII regions indicating the need for a filling factor even at this resolution. Also, the observed brightness at 240 MHz is found to be higher than expected from the best fits suggesting the need for a multicomponent model for the region.
Volume 40 | Issue 5
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