Volume 20, Issue 1-2
June 1999, pages 1-88
pp 1-22 June 1999
Embedded Young Stellar Objects (YSO) in dense interstellar clouds are treated self-consistently to understand their spectral energy distributions (SED). Radiative transfer calculations in spherical geometry involving the dust as well as the gas component, have been carried out to explain observations covering a wide spectral range encompassing near-infrared to radio continuum wavelengths. Various geometric and physical details of the YSOs are determined from this modelling scheme.
In order to assess the effectiveness of this self-consistent scheme, three young Galactic star forming regions associated with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test cases. They cover a large range of luminosity (≈ 40). The modelling of their SEDs has led to information about various details of these sources, e.g. embedded energy source, cloud structure and size, density distribution, composition and abundance of dust grains etc. In all three cases, the best fit model corresponds to the uniform density distribution. Two types of dust have been considered, viz., Draine & Lee (DL) and the Mezger, Mathis & Panagia (MMP). Models with MMP type dust explain the dust continuum and radio continuum emission from IRAS 18314-0720 and 18355-0532 self-consistently. These models predict much lower intensities for the fine structure lines of ionized heavy elements, than those observed for IRAS 18314-0720 and 18355-0532. This discrepancy has been resolved by invoking clumpiness in the interstellar medium. For IRAS 18316-0602, the model with DL type dust grains is preferred.
pp 23-35 June 1999
Mid-and far-infrared maps of many Galactic star forming regions show multiple peaks in close proximity, implying more than one embedded energy source. With the aim of understanding such interstellar clouds better, the present study models the case of two embedded sources. A radiative transfer scheme has been developed to deal with a uniform density dust cloud in a cylindrical geometry, which includes isotropic scattering in addition to the emission and absorption processes. This scheme has been applied to the Galactic star forming region associated with IRAS 19181 + 1349, which shows observational evidence for two embedded energy sources. Two independent modelling approaches have been adopted, viz., to fit the observed spectral energy distribution (SED) best; or to fit the various radial profiles best, as a function of wavelength. Both the models imply remarkably similar physical parameters.
pp 37-50 June 1999
Most of the known pulsars are sources of highly linearly polarized radiation. Faraday rotation in the intervening medium rotates the plane of the linear polarization as the signals propagate through the medium. The Rotation Measure (RM), which quantifies the amount of such rotation as a function of wavelength, is useful in studying the properties of the medium and in recovering the intrinsic polarization characteristics of the pulsar signal. Conventional methods for polarization measurements use telescopes equipped with dual orthogonally polarized feeds that allow estimation of all 4 Stokes parameters. Some telescopes (such as the Ooty Radio Telescope) that offer high sensitivity for pulsar observations may however be receptive to only a single linear polarization. In such a case, the apparent spectral intensity modulation, resulting from differential Faraday rotation of the linearly polarized signal component within the observing bandwidth, can be exploited to estimate the RM as well as to study the linear polarization properties of the source. In this paper, we present two improved procedures by which these observables can be estimated reliably from the intensity modulation over large bandwidths, particularly at low radio frequencies. We also highlight some other applications where such measurements and procedures would be useful.
pp 51-65 June 1999
We present NeV/MgV and SiVII/MgVII theoretical line intensity ratios as a function of electron densityNe and temperatureTe. These are shown in the form of ratio-ratio diagrams, which should in principle allow bothNe andTe to be deduced for the emitting region of the solar plasma. We apply these diagnostics in the solar atmosphere, and discuss the available observations made from space. In most cases, however, we deduceNe andTe from the computed absolute line intensities in a spherically symmetric model atmosphere of the Sun. Possible future applications of this investigation to spectral data from the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) are briefly discussed.
pp 67-77 June 1999
Cosmological models involving shear and rotation are considered, first in the general relativistic and then in the Newtonian framework with the aim of investigating singularities in them by using numerical and analytical techniques. The dynamics of these rotating models are studied. It is shown that singularities are unavoidable in such models and that the centrifugal force arising due to rotation can never overcome the gravitational and shearing force over a length of time.
pp 79-88 June 1999
We have explored a model for the formation of giant molecular clouds through accretion of smaller clouds; the star disc is assumed to be rigidly rotating in this model. The main results are:
The aggregates consist mainly of the neighbouring clouds whose collision was induced by the velocity dispersion. In this model aggregates take a shorter time to form compared to the model which includes differential rotation, although the aggregates tend to be less massive.
By the same token, the mass of these aggregates does not increase further. The inclusion of differential rotation would have allowed for the possibility of distinct aggregates colliding amongst themselves to form larger aggregates.
Volume 40 | Issue 5
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