015942   Hasan S N;Tagare S G;Alladin S M (Department of Astronomy, Osmania University, Hyderabad-500 007) : Scattering in a parabolic encounter of a single star with a circular binary. Bull Astr Soc India 2000, 28(2), 315-20.

Huang and Voltonen (1987) obtained an analytic expression for the enrgy transfer in a parabolic encounter of a single star with a circular binary keeping terms of the first order in a/q where a is the radius of the circular orbit and q is the distance of closest approach of the single star with respect to the centre of mass of the binary. They also obtained numerically the energy transfer and change in eccentricity of the single star for the case of mass of the binary equal to the mass of the single star. Authors extend their analytic treatment to obtain angular momentum transfer and change in eccentricity up to first order a/q and compare the change in eccentricity. It is found that first order analysis is reasonable for Q=q/a <62> 2.5 for direct orbit and Q <62> 1.125 for retrograde orbit.

015941   Gupta V;Gupta P D (Department of Physics and Astrophysics, University of Delhi, Delhi-110 007) : Morphological properties of short duration Gamma ray bursts. Bull Astr Soc India 2000, 28(2), 371-3.

Studies a sample of short duration bursts contained in the 3B BATSE catalogue. Authors fit the time profiles of these GRBs with lognormal functions and study various temporal properties of these objects. In most of the mutipeaked bursts, authors find that the temporal asymmetry of individual pulses evolves with time i.e. subsequent pulses tend to be more symmetric.

015940   Goswami A;Prantzos N (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Chemical evolution of intermediate mass nuclei in the solar neighbourhood and the halo of the galaxy. Bull Astr Soc India 2000, 28(2), 305-8.

Recent observations of Fe group elements in very low metallicity stars show an evolutionary trend very different from that of Fe, implying divergent paths of chemicals evolution for these elements. Investigates the chemical evolution of intermediate mass nuclei in the solar neighbourhood and the halo of the galaxy with a two zone model of Galactic chemical evolution using a comprehensive set of theoretically computed yields covering the entire range from H to Ge. It is shown that with a two zone model the main observational constraints of the solar neighbourhood can be satisfied.

015939   Ghosh S K (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Mid- and far-infrared studies of galactic compact H II regions. Bull Astr Soc India 2000, 28(2), 225-32.

Several Galactic H II region complexes have been observed in the far infrared (FIR) wavebands using the TIFR 1-m balloon borne telescope, and in the mid infrared (MIR) using the Infrared Space Observatory (ISO). Many of these FIR maps show interesting structural details. A self consistent radiative transfer scheme (1-D) has been developed which has been successful in extracting important details like geometric sizes, radial density of distribution, dust composition, etc, from these and other similar measurements. A new scheme (2-D) has been developed in cylindrical geometry to model IRAS 19181 + 1349, which has been resolved into two sources. The model parameters in this scheme have been constrained by the observed spectral energy distribution (SED) and radial profiles at MIR and FIR wavebands. Non-equilibrium processes have also been incorporated in spherical geometry, to explain the PAH emission features. This scheme has successfully explained high resolution spectra (ISO-SWS) of several Galactic compact H II regions.

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015938   Gangadhara R T (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Window-threshold technique for detecting pulse components. Bull Astr Soc India 2000, 28(2), 297-8.

Analyses high-quality single pulse data of PSR B0329+54 at 606 MHz to study the structure of the emission beam. In order to unambiguosly detect weak emission components in the integrated pulse profile, authors have developed a new data analysis technique, which term "window-thresholding". Applying this technique to the data, they detect three new emission components for this pulsar and confirms the presence of a component which was proposed earlier. Hence the analysis indicates that PSR B0329+54 has nine components, which is among the highest of all the known pulsar. The symmetric distribution of pulse components about the pulse centre, defined to be midway between the centres of the outer components, indicates that the emission beam is conal.

015937   Dewangan G C;Singh K P;Bhat P N (NO, Tata Institute of Fundamental Research, Mumbai-400 005) : Multicolour broadband study of a lenticular galaxy : NGC 4753. Bull Astr Soc India 2000, 28(2), 375-6.

Based on VBT observations, reports multicolour broadband study of a lenticular galaxy, NGC 4753 with prominent dust lanes. The extinction curve derived for NGC 4753 is similar to the Galactic extinction curve. The dust mass estimated from optical extinction is 1.51 x 10⁵ M<79> and from the far infrared fluxes observed with Infrared Astronomical Satellite (IRAS) is 3.46 x 10⁵ M_<79>. The dust observed within NGC 4753 can be argued to be internal in origin.

015936   Das M;Jog C J (NO, Raman Research Institute, Bangalore-560 080) : Tidally compressed molecular gas in ultraluminous and early type galaxies. Bull Astr Soc India 2000, 28(2), 389-91.

The effect of the galactic tidal field is usually considered disruptive in the literature. However, in some cases its effect can become compressive. Using the virial theorem, determines the minimum density for a cloud to be stable and gravitationally bound within the tidally compressive region of a galaxy. Applies the results to a sample of early type and ultraluminous galaxy. For early type galaxies with a core-type luminosity profile, the molecular gas is compressed to high densities of at least 10⁴cm^-3 within the inner 50 pc where the field is compressive. For ultraluminous galaxies the treatment predicts molecular gas densities of 10² - 10⁴cm^-3 within the central few 100 pc. These agree well with gas densities observed in the centers of these galaxies.

015935   Das M (NO, Raman Research Institute, Bangalore-560 080) : Kinematics of molecular gas in the nucleus of barred galaxies NGC 253 and M82. Bull Astr Soc India 2000, 28(2), 357-61.

Uses a simple form of a bar potential to model the bar in two nearby starburst galaxies, NGC 253 and M82. Using parameters taken from the observed rotation curves, authors have constructed the position velocity diagram and velocity contour plot for the bar in these galaxies. These plots help understand the observed distribution and velocity field of the gas in these galaxies. It is found that in NGC 253, the nuclear star formation is concentrated mainly along the x2 orbits and the nuclear velocity field shows signs of a past merger event in the galaxy. In M82, authors find that the supernova remnants and HI gas lies along the x1 orbits whereas the ionized gas is mainly along the inner x2 orbits.

015934   Chitre A A (Physical Research Laboratory, , Ahmedabad-380 009) : Photometric studies of starburst galaxies. Bull Astr Soc India 2000, 28(2), 423-8.

Presents the results of detailed morphological analysis of a sample of Markarian starburst galaxies. CCD surface photometry of those galaxies was carried out based on observations made in U BV RI and Hα. The morphology of the sample objects was studied using isophotal contours and colour images. The radial variations of the surface brightness, ellipticity, position angle and the colour profiles were constructed using ellipse fitting techniques. Authors find that the star formation activity is not confined to the central region alone, but it also ocurs at various other locations like the ends of bars, along circum-nuclear rings or even globally in a few cases. The luminosity profiles show an exponential nature in the outer region. Strong isophotal twisting is observed in almost all the S0 galaxies in the sample. In a few cases, this is accompanied by boxiness, indicating a strong interaction or merger. Blue light is more centrally concentrated as compared to red light in early type galaxies while the reverse holds for spirals. The scale lengths in B and R are comparable. Composite models were constructed and ages between 10⁶ and 10⁷ years and burst strength between 10% to 5% were able to explain the observed colours.

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015933   Bhatt B C;Prabhu T P;Anupama G C (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Site characteristics of the Indian astronomical observatory, Hanle. Bull Astr Soc India 2000, 28(2), 441-3.

A brief summary of site characteristics of Indian Astronomical Observatory, Hanle, Ladakh (altitude 4500 m), is presented based on a survey being carried out since the last four years.

015932   Bhat P N et al (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Pachmarhi array of Cerenkov telescopes. Bull Astr Soc India 2000, 28(2), 455-7.

Pachamarhi Array of Cerenkov Telescopes (PACT) consisting of 25 telescopes has been installed at Pachmarhi (longitude : 76° 26' E, latitude : 22° 28' N and altitude : 1075m) recently. The telescopes are deployed in the form of a 5 x 5 matrix spread over an area of 80m x 100m. The array is designed to detect celestial TeV γ-rays by wavefront sampling technique as against the standard imaging technique used at other γ-ray observatories elsewhere in the world. Each telescope consists of 7 parabolic reflectors of 90 cm diameter and and f/d <126>1. Each telescope is independently steerable on separate equatorial mounts operated through a computer based control system capable of orienting them with an angular accuracy better than 0.2°. The pointing is monitored in real time to maintain a source tracking accuracy of better than 0.1°. Discusses the array design, its suitability for wavefront sampling and its pointing capabilities.

015931   Bhat C L (Bhabha Atomic Research Centre, Nuclear Research Laboratory, , Mumbai-400 085) : Gamma ray astronomy : new Indian initiatives. Bull Astr Soc India 2000, 28(2), 235-9.

Major new initiatives under implementation in India in the field of γ-ray astronomy are discussed in the backdrop of the just-concluded exploratory phase in the field.

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015930   Banhatti D G (NO, School of Physics, Madurai Kamaraj University, Madurai-625 021) : Linear sizes and arm asymmetry of intrinsically symmetric doubles. Bull Astr Soc India 2000, 28(2), 367-9.

In an intrinsically symmetric model, the distribution px(x) of the fractional arm differences x of a sample of straight double radio sources reflects the distribution among the doubles of their orientations φ away from the line of sight. In a constant speed expansion model, the two distributions can be simply related, and the resultant distribution p1(⊥) of the projected linear sizes can be derived, to be compared with the observed distribution for the same sample. Such a calculation is illustrated using a sample of extended doubles. The model is perhaps more applicable to compact doubles. However, these are not yet numerous enough to construct usable samples.

015929   Bandyopadhyayay K;Koul D K;Mishra V K;Koul R; Suthar R L;Koul M K;Jayandranath M;Bhat C L (Nuclear Research Laboratory, Bhabha Atomic Research Centre, , Trombay, Mumbai-400 085) : Salient mechanical and optical design features of 25m-diameter light-collector of MACE imaging gamma-ray telescope. Bull Astr Soc India 2000, 28(2), 471-4.

A low-threshold energy, high-sensitivity γ-ray telescope, MACE (for Major Atmospheric Cerenkov Experiment) is proposed to be built in India for explorations of γ-ray sky in 10's keV - 100's GeV photon energy range, hitherto inaccessible from ground. This paper outlines the scientific motivation for this major telescope system and focusses on its mechanical design aspects. Preliminary simulation results, obtained on the expected optical quality of the overall light collector, are also discussed.

015928   Ballabh G M;Alladin S M (Department of Astronomy, Osmania University, Hyderabad-500 007) : Merger time of binary globular clusters. Bull Astr Soc India 2000, 28(2), 261-4.

In a binary globular cluster the tidal forces increase the internal energy of the clusters and may lead to either merger or disruption of a cluster depending upon whether the two clusters are of equal mass or vary much in mass. Since the physical process of merger is the same for both galaxy and cluster pairs, authors have estimated the merger time and disruption time for two globular clusters by using the simple formulae used for merger time of binary spherical galaxies derived by integrating the tidal force over one period of the binary. The results thus obtained are compared with those of n-body simulations and the agreement is found to be fairly good.

015927   Baliyan K S;Joshi U C (Physical Research Laboratory, , Ahmedabad-380 009) : Photoionization cross sections for the Fe XVII ion. Bull Astr Soc India 2000, 28(2), 311-4.

015926   Ashoka B N;Seetha S;Raj E;Chaubey U S;Gupta S K;Joshi S;Martinez P;Kurtz D W;Sagar R;Kasturirangan K (ISRO Satellite Centre, , Bangalore) : Survey for roAp stars in the northern sky from Naini Tal. Bull Astr Soc India 2000, 28(2), 251-4.

Of the 31 known rapidly oscillating Ap (roAp) stars, 28 are in the southern sky and majority of them were discovered at SAAO indicating that a systematic survey also is needed in the norther sky. Therefore, a project has been initiated at the Uttar Pradesh State Observatory in Naini Tal with the objective to find and study new roAp stars in the northern sky. The test observing campaigns were made during 1997 and 1998 from UPSO at Naini Tal. A brief report of these efforts is presented here. The initial results were encouraging and qualified the site, the telescope and the instrument as the right combination needed for the success of the project. Though these preliminary observations did not yield new roAp stars, they did lead to the discovery of delta Scuti pulsations in two chemically peculiar stars, HD 13079 and HD 13038.

015925   Ashok N M;Chandrasekhar T (Astronomy and Astrophysics Division, Physical Research Laboratory, , Ahmedabad-380 009) : Near-infrared spectroscopy of nova Sagittarii 1998. Bull Astr Soc India 2000, 28(2), 265-7.

Near-infrared spectra of Fe-II type nova, Nova Sagittarii 1998 obtained during the period 30 to 70 days after the discovery date are presented. The near infrared spectrum is dominated by emission lines of hydrogen and accompained by lines of CI, OI and NI. The 1.129 μm OI line excited by Lyman flyroscence is prominently seen in our first spectrum obtained in April 1998. In addition the OI line at 1.316 μm is also detected indicating the presence of UV continuum fluroscence.

015924   Apparao K M V (Department of Physics, University of Mumbai, Mumbai) : Radio emission from Cyg X-3. Bull Astr Soc India 2000, 28(2), 275-6.

The acceleration of high energy particles by a shock produced by the passage of the secondary object in the binary system, through the wind of the primary, is considered and is shown to be adequate to account for the quiescent radio emission from Cyg X-3. A mechanism for including large radio flares is outlined.

015923   Apparao K M V (Department of Physics, University of Bombay, Bombay) : Hard x-ray and gamma rays from the Be star LSI + 61° 303. Bull Astr Soc India 2000, 28(2), 271-3.

The hard x-ray flux resulting from inverse Compton scattering of photons from the Be star by the high energy electrons responsible for the radio emission, is calculated and compared with observation.

015922   Anupama G C;Pandey G;Sivarani T (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Optical spectroscopy of SN1998S. Bull Astr Soc India 2000, 28(2), 269-70.

014865   Kishore S (NO, , B-47/II Jia Sarai, New Delhi) : The birth of moon. Proc Indian Sci Congr Ass - Pt IV, Sect-I 1997, (NULL), 88.

013871   Vaidya D B;Desai J N;Gupta R (Gujarat College, , Ahmedabad-380 006) : Porous and fluffy grains in the regions of anomalous extinction. J Astrophys Astr 2000, 21(1-2), 91-9.

It has long been established that the ratio of total to selective extinction is anomalously large (≥ 5) in certain regions of the interstellar medium. In these regions of anomalous extinction the dust grains are likely to be irregular in shape and fluffy in structure. Using discrete dipole approximation (DDA) calculates the extinction for porous and fluffy grains. Apply DDA first to solid spheroidal particles assumed to be made of a certain (large) number of dipoles. Systematically reduces the number of dipoles to model the porous grains. The aggregates of these particles are suggested to form the fluffy grains. The extinction for these particles as a function of grain size, porosity and wavelength. Apply these calculations to interpret the observed extinction data in the regions of star formation (e.g. the Orion complex).

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013870   Samantaray A;Khare P (Physics Department, Utkal University, Bhubaneswar-751 004) : Nature of the background ultraviolet radiation field at high redshifts. J Astrophys Astr 2000, 21(1-2), 19-27.

Tries to determine the flux of the ultraviolet back-ground radiation field from the column density ratios of various ions in several absorption systems observed in the spectra of QSOs. Finds that in most cases the flux is considerably higher than what has been estimated to be contributed by the AGNs. The excess flux could originate locally in hot stars. In a few cases it shows that such galactic flux can only contribute a part of the total required flux. The results suggest that the background gets a significant contribution from an unseen QSO population

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013869   Ram Prakash;Agrawal B M;Singh A P (Department of Physics, Bipin Bihari PG College, Jhansi-284 001) : Theoretical explanation of intensities of storm-time VLF hiss emissions observed at low latitudes. Indian J Radio Space Phys 2001, 30(3), 121-6.

For known plasma parameters and considering quasi-linear electron cylotron resonance instability and incoherent. Cerenkov radiation process, the intensities of intense storm-time VLF hiss emissions, observed in the low latitude ground stations of Japan, are explained. Based on the intensity calculations in low latitude equatorial plane, which are inconsistent with the observed intensities, the local generations by either of the above two mechanisms are ruled out. It is suggested that the emissions are due to mechanism of quasi-linear electron cyclotron resonance instability in the equatorial plane at L ≈ 3.6 by <126>5 keV electrons which drif eastward from higher latitudes and enter the ionosphere at a latitude higher than that of Moshiri (L ≈ 1.6). These then propagate towards the observer in earth-ionosphere wave-guide mode.

013868   Raghunathan A;Subrahmanyan R (Raman Research Institute, C V Raman Avenue, Bangalore-560 080) : Measurement of the cosmic microwave background temperature at 1280 MHz. J Astrophys Astr 2000, 21(1-2), 1-17.

Absolute temperature of the cosmic microwave background (CMB) has been measured at a frequency of 1280 MHz. The observation was made with a modified version of the L-band receiver used in the Giant Metre wavelength Radio Telescope (GMRT): the feed horn was replaced by a corrugated plate and the receiver was placed on the ground, directed at zenith, and shiedled from ground radiation by an aluminium screen with corrugated edges. Novel techniques have been adopted for reducing and cancelling unwanted contributions to the system temperature of the receiver and calibrating the contributions from the feed assembly and receiver. The thermodynamic temperature of the CMB is estimated to be 3.45 ± 0.78 K.

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013867   Ojha D K;Omont A;Ganesh S;Simon G;Schulthesis M (Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Stellar sources in the ISOGAL inner galactic bulge field (l=0°, b=-1°). J Astrophys Astr 2000, 21(1-2), 77-90.

ISOGAL is a surey at 7 and 15 μm with ISOCAM of the inner galactic disk and bulge of our Galaxy. The survy covers 22 deg² in selected areas of the central l = ±30 degree of the inner Galazy. Reports the study of a small ISOGAL field in the inner galactic bulge (l=0°, b=-1°, area=0.033 deg²). Using the multicolor near-infrared data (IJK_s) of DENIS (DEep Near infrared Southern Sky Survey) and mid-infrared ISOGAL data, discusses the nature of the ISOGAL sources. The various color-color and color-magnitude diagrams are discussed in the paper. While most of the detected sources are red giants (RGB tip stars), a few of them show an excess in J-K_s and K_s-[15] colors with respect to the red giant sequence. Most of them are probably AGB stars with large mass-loss rates

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013866   Ojha D K (Tata Institute of Fundamental Research, Dr. Homi Bhabha Road, Mumbai-400 005) : Scale length of the galactic thin disk. J Astrophys Astr 2000, 21(1-2), 53-9.

Presents an analysis of the first 2MASS (The Two Micron All Sky Survey) sampler data as observed at lower Galactic latitude in our Galaxy. These new near-infrared data provide insight into the structure of the thin disk of our Galaxy. The interpretation of star counts and color distributions of stars in the near-infrared with the synthetic stellar population model, gives strong evidence that the Galactic thin disk density scale length, hR, is rather short (2.7±0.1 kpc).

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013865   Narayana Rao D;Singh H R;Bhaskara Rao S V (Department of Physics, S V University, Tirupati-517 502) : Vertical velocity, horizontal divergence and turbulence associated with tropical mesoscale convective system. Indian J Radio Space Phys 2001, 30(2), 91-7.

The MST radars have been used to measure directly the vertical motions above the radar site. Vertical velocity profiles and horizontal divergence/convergence derived from the vertical velocity using the mass continuity equation as well as eddy dissipation rate estimated under strong-to-moderate convention period are presented. Data obtained from the Indian MST radar located at Gadanki (13.5°N, 79.8°E) have been used. The height range of the study is from 4 km to 20 km. The aim is to calculate the horizontal divergence from the vertical velocity and then to find the extent of the enhancement in the vertical velocity and horizontal divergence as well as turbulence distribution during the well developed tropical mesoscale disturbances. Strong enhancement is observed in the vertical velocity and the horizontal divergence/convergence. Maximum turbulence has been observed near the top of the cloud and above. Eddy dissipation rate of the order of 0.007 m²s^-3 has also been observed.

013864   Naik S;Agrawal P C;Paul B;Rao A R;Seetha S; Kasturirangan K (Tata Institute of Fundamental Research, , Homi Bhabha Road, Mumbai-400 005) : X-ray observation of XTE J2012+381 during the 1998 outburst. J Astrophys Astr 2000, 21(1-2), 29-38.

Outburst of X-ray transient source XTE J2012+381 was detected by the RXTE All-Sky Monitor on 1998 May 24th. Following the outburst, X-ray observations of the source were made in the 2-18 keV energy band with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment (IXAE) on-board the Indian Satellite IRS-P3 during 1998 June 2nd-10th. The X-ray flux of the source in the main outburst decreased exponentially during the period of observation. No large amplitude short-term variability in the intensity is detected from the source. The power density spectrum obtained from the timing analysis of the data shows no indication of any quasi-periodic oscillations in 0.002-0.5 Hz band. The hardness ratio i.e. the ratio of counts in 6-18 keV to 2-6 keV band, indicates that the X-ray spectrum is soft with spectral index <62>2. Frin tge sunukaruties of the X-ray properties with those of other black hole transients, author conclude that the X-ray transient XTE J2012+381 is likely to be a black hole.

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013863   Manchanda R K (Tata Institute of Fundamental Research, , Homi Bhabha Road, Mumbai-400 005) : X-ray measurements of black hole x-ray binary source GRS 1915+105 and the evolution of hard x-ray spectrum. J Astrophys Astr 2000, 21(1-2), 39-52.

Reports the spectral measurement of GRS 1915+105 in the hard X-ray energy band of 20-140 keV. The observations were made on March 30th, 1997 during a quiescent phase of the source. The mechanism of emission of hard X-ray photons and the evolution of the spectrum has been discussed by comparing the data with earlier measurements and an axiomatic model for the X-ray source

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013862   Janardhanan K V;Ramakrishna Rao D;Viswanathan K S;Krishna Murthy B V;Shenoy K S V;Mohankumar S V;Kamath K P; Mukundan K K;Sajitha G;Shajahan M;Ayyappan C (Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022) : HF backscatter radar at the magnetic equator : system details and preliminary results. Indian J Radio Space Phys 2001, 30(2), 77-90.

A phase coherent, pulsed, monostatic and moderately high power HF Doppler radar, operating at 18 and 9 MHz has been installed at the magnetic equatorial station, Trivandrum (8.5°N, 77°E; dip 0.5°N), India. It is a powerful tool to study the plasma instability processes responsible for the generation of equatorial electrojet (EEJ) and equatorial spread-F (ESF) irregularities. A novelty in the radar system is the usage of the same phased array antenna at the two harmonically related frequencies of 18 and 9 MHz. The radar employs Doppler beam swinging (DBS) technique with beam orientations of vertical and ±30° off zenith in the E-W plane, providing the unique capability of obtaining both zonal and vertical drifts of the ionospheric irregularities. The radar is also capable of providing zonal wind information in the region of 80-95 km. The new radar has opened up the possibilities of investigating different aspets of EEJ, like the E-W spectral asymmetry, diurnal pattern of the electric field and the effect of small scale plasma turbulence on the large scale EEJ altitude structure. The detailed system description, data processing scheme and some preliminary results obtained using the radar at 18 MHz are presented.

013861   Janardhanan K V;Mukundan K K;Shenoy K S V; Reddy C A (Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum-695 022) : Interlaced phased array antenna for atmospheric radars. Indian J Radio Space Phys 2001, 30(2), 98-105.

Describes the design, construction details and test results of a 4 x 15 phased array Yagi antenna capable of producing three switchable beam orientations in a single plane. The antenna operates at a fixed frequency of 54.95 MHz and is useful for atmospheric studies. The array makes use of the `interlaced phased array (IPA)' concept to reduce the number of feed points and phase shifters, resulting in considerable reduction in cost and complexity of the array antenna. The 4 x 15 array was constructed using four-element Yagi-Uda antennas and was fed using a combination of co-axial and open-wire transmission lines. The performance of the antenna was qualitatively checked using the electrojet as a target. Moreover, the beam orientations and beam widths of the antenna were verified using the sun as a radio source when it transitted over the beams. The performance parameters of the antenna were found to be very close to those computed theoretically.

013860   Ghosh S K;Verma R P (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : A new scheme of radiation transfer in HII regions including transient heating of grains. J Astrophys Astr 2000, 21(1-2), 61-76.

A new scheme of radiation transfer for understanding the infrared spectra of HII regions, has been developed. This scheme considers non-equilibrium processes (e.g. transient heating of the very small grains, VSG; and the polycyclic aromatic hydrocarbon, PAH) also, in addition to the equilibrium thermal emission from normal dust grains (BG). The spherically symmetric interstellar dust cloud is segmented into a large number of `onion skin' shells in order to implement the non-equilibrium processes. The scheme attempts to fit the observed SED originating from the dust component, by exploring the following parameters: geometrical details of the dust cloud; PAH size and abundance; composition of normal grains (BG); radial distribution of all dust (BG, VSG & PAH). The scheme has been applied to a set of five compact H II regions (IRAS 18116 - 1646, 18162 - 2048, 19442 + 2427, 22308 + 5812, and 18434 - 0242) whose spectra are available with adequate spectral resolution. The best fit models and inferences about the parameters for these sources are presented.

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008753   Sagar R;Mohan V;Pandey S B;Pandey A K;Stalin C S;Tirado Castro A J (U P State Observatory, , Manora Peak, Nainital-263 129) : GRB 000301C with peculiar afterglow emission. Bull Astr Soc India 2000, 28(3), 499-513.

CCD magnitudes in Johnson V and Cousins R and I photometric passbands are determined for GRB 000301C afterglow starting <126> 1.5 day after the χ - ray burst. Light curves of the afterglow emissions in U, B, V, R, I, J and K' passbands are obtained by combining the present measurements. Flux decay shows a very uncommon variation relative to other well observed GRBs. Overall, there is a steepening of the optical and near-infrared flux decay caused by a geometric and sideways expanding jet. This is superimposed by a short term variability especially during early time (Δt <8 days). The late time flux decay is the steepest amongst the GRB OTs observed so far with α<126>3. Steepening in the flux decay seems to have started simultaneously around Δt <126> 7.6 day in all passbands. The value of spectral index in the optical-near IR region is <126>-1.0. Redshift determination with z=2.03335 indicates cosmological origin of the GRB having a luminosity distance of 16.6 Gpc. Thus it becomes the second farthest amongst the GRBs with known distances. An direct estimate of the fluence <62> 20 ke V indicates, if isotropic, ≥ 10⁵³ ergs of release of energy. The enormous amount of released energy will be reduced, if the radiation is beamed which is the case for this event. Using a jet break time of 7.6 days, infer a jet opening angle of <126> 0.15 radian. This means the energy released is reduced by a factor of <126> 90 relative to the isotropic value.

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008752   Roukema B F (Inter-University Centre for Astronomy and Astrophysics, , Post Bag 4, Ganeshkhind, Pune) : Topology of the universe. Bull Astr Soc India 2000, 28(3), 483-97.

Hilbert Einstein equations are insufficient to describe the geometry of the Universe, as they only constrain a local geometrical property curvature. A global knowledge of the geometry of space, if possible, would require measurement of the topology of the Universe. Since the subject was discussed in 1900 by Schwarzschild, observational attempts to measure global topology have been rare for most of this century, but have accelerated in the 1990's due to the rapidly increasing amount of observations of non-negligible fractions of the observational sphere. A brief review of basic concepts of cosmic topology and of the rapidly growing gamut of diverse and complementary observational strategies for measuring the topology of the Universe is provided.

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008751   Namboodiri P M S;Cowsik R;Sugimoto D (Indian Institute of Astrophysics, , Bangalore-560 034) : Tidal effects on a satellite galaxy influenced by a massive perturber. Bull Astr Soc India 2000, 28(3), 529-38.

Tidal influence on a satellite galaxy by a massive perturber is studied by numerical simulations. The satellite is modelled as a non-rotating spherical cluster of uniform density. The perturber is a galaxy described as a Plummer model. The satellite undergoes penetrating collisions on hyperbolic, parabolic, ellipic and circular orbits. The disruption of the satellite, in parabolic and elliptic encounters, is estimated in terms of the density ratio ρ/ρR where ρ is the mean density of the satellite and ρR a critical density, the Roche density. In these models the satellite suffers disruption when ρ<ρR and survives the tidal encounter otherwise. The survival of the satellite strongly depends on the distance of closest approach. The direction of the orbital angular momentum after the encounter is aligned with that of the initial orbital angular momentum of the pair. Results also suggests that disruption of the satellite occurs for values of <114?>/ρR <3 in circular orbit encounters. In hyperbolic encounters the satellite galaxy is seen to suffer considerable damage only in the case of near head-on-collision.

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008750   Ghosh S K;Iyengar K V K;Karnik A D;Rengarajan T N;Tandon S N;Verma R P (Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Far-infrared observations of three compact IRAS sources in Sh 247 star forming complex: IRAS 06056+2131, 06058+2138 and 06061+2151. Bull Astr Soc India 2000, 28(3), 515-27.

Three compact IRAS sources-IRAS 06056+2131, 06058+2138 and 06061+2151 in the Sh 247 complex and having IRAS colours typical of compact H II regions have been observed in the far-infrared using the TIFR 1 m balloon-borne telescope. The observations were done simultaneously in two bands; from these intensity maps at 58 μm using the HIRES routine. Using these and other available observations the spectral energy distributions (SED) for the three sources have been obtained. Radiation transfer modelling has been undertaken to fit the observed SED and infer the nature and distribution of dust in the envelope found. The uniform density fits better for two sources while r^-1 distribution is better for the third. The total luminosities of the sources imply spectral types in the range 09 to B0; however, only two of them have been detected in the radio continuum, that too at a level much lower than that expected. All three are presumably optically thick at 2.6 cm. IRAS 06056+2131 shows an extended structure with two cores while IRAS 06061+2151 has probably an additional core. The corresponding structures are also seen in the 25 μm map which has higher resolution. While all the three sources are very young and deeply embedded, the absence of radio continuum and non-fragmentation of the cloud suggest that IRAS 06058+2138 is the youngest of the three.

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008749   Biswas S N (Barasat Satyabharati Vidyapith, , P O Navapalli-743 203) : Some statistical aspects of cometary orbits and their discoveries. Bull Astr Soc India 2000, 28(3), 539-78.

Analysis of the proximity of aphelia distances of SP comet orbits from their respective nearby plantary orbits reveals that the orbits of major planets are closely visited by most of the SP comet aphelia. The set of comets whose aphelia closely visit a particular planetary orbit, are said to constitute a family of comets of the said planet. Classifying these comets, it has been noticed that the Jupiter Family of Comets (JFC) are the largest in number. Also the nodel distances of all the comets have been studied in respect of their distributions. It has been found that 91% of the nodes are within the orbital distance of the planet Neptune at 30 AU away from the Sun. The consequences of the aphelia and nodes of the SP comets being closer to their nearby planetary orbits, have been studied. This study justifies the existence of a number of multiple nuclei comets as well as predicts the possible split of a few more SP cometary nuclei which may finally crash into any one of the perturbing major planetary globes, like the one C/1993 F2 (Shoemaker-Levy 9). Interestingly, it can be inferred from these studies that any group of comets having almost identical orbital elements, may be the outcome of the split suffered by their parent cometary nuclei at a distance far away from the Sun.

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007698   Uniyal V P;Mathur P K (NO, Wildlife Institute of India, Dehra Dun) : Altitudinal distribution of tiger beetles (Cicindelidae : Coleoptera) in great Himalayan national park conservation area, western Himalaya. Indian For 2000, 126(10), 1141-3.

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007697   Mahajan K K;Oyama K I (Radio & Atmospheric Sciences, National Physical Laboratory, New Delhi-110 012) : Ionosphere and upper atmosphere of venus. Proc Indian Natn Sci Acad-Pt A 2000, 66(5), 483-98.

Most of current knowledge of the upper atmosphere and ionosphere of Venus has come from the various aeronomy experiments on the Pioneer Venus Orbiter (PVO) which was inserted into a highly eccentric orbit around Venus on December 4, 1978. The PVO provided ionospheric data till Oct. 7, 1992 after which it entered the dense atmosphere and incinerated. Now know that Venus has an extended atmosphere with CO2 dominating in the lower thermosphere and O and He at higher altitudes. The atmosphere is not very sensitive to solar EUV variability with the daytime exospheric temperature changing only by about 60 K over a solar cycle. The response has been explained due to very strong 15 μm cooling from CO2, collisionally excited by oxygen atoms. This cooling approximately balances the EUV and UV heating. And in the upper ionosphere, major ion is O⁺ above about 200 km, but lower down O2⁺ dominates and forms the main ionospheric peak around 140 km. The ionosphere responds very strongly to changes in solar activity and solar zenith angle. The planet has no intrinsic magnetic field and therefore the solar wind interacts directly with its ionosphere resulting in a sharp density gradient (called the ionopause), above the top of the ionosphere. Inspite of the long Venus night, a substantial nightside ionosphere exists mainly due to transterminator flow of O^{+ from the dayside during solar maximum.}

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007696   Kaushik S C;Shrivastava P K (Department of Physics, Government P G College, Sidhi-486 661) : Study of Forbush decrease event and geomagnetic disturbances during the solar magnetic polarity reversal. Indian J Phys-Pt B 2000, 74(3), 237-9.

Transient decrease in cosmic ray intensity followed by a slow recovery typically lasting for several days, is identified as Forbush decrease event. One such a large Forbush decrease event has been observed from April 8-20, 1990, which occurs during the period of solar magnetic polarity reversal in solar cycle 22. The onset of Forbush decrease took place on April 8-9, 1990 and attained its maxim on 9th April, 1990. After that the long recovery phase started which continued up to 20th April, 1990. It is noteworthy that the three successive events of sudden storm commencements (SSC's) occurred on 9, 12 and 17th April 1990, which cover total time span of Forbush decrease events. Further, a sudden decrease in Dst values in similar pattern of cosmic ray decrease, indicates a significant relation in depression in cosmic rays and Dst values. The geomagnetic index AE has also been observed to increase abruptly on April 9, 1990. An attempt has been made to explain the cosmic ray decrease alongwith the interplanetary/geomagnetic disturbances that occurred during which there was a solar magnetic polarity reversal.

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007695   Hewish A (NO, Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE (UK)) : Three decades of pulsar astronomy. Proc Indian Natn Sci Acad-Pt A 2000, 66(5), 443-50.

There is now strong evidence supporting the rotating neutron star "lighthouse" model of pulsars. With spin rates up to 600 revolutions per second, and surface magnetic fields of 10⁸ T, neutron stars act like powerful dynamoes surrounded by charge-separated magnetospheres. The radiation mechanism is still unknown but is probably related to coherent oscillations within an electron-positron plasma generated by gamma radiation above the magnetic poles. Small variations of observed spin-rate are consistent with the prediction that superfluid neutrons inside neutron stars are contained within a rigid outer crust. Pulsar radiation has been used to probe the interstellar gas and some pulsars act like precise clocks in locations ideally suited to test aspects of general relativity, such as the existence of gravitational waves and geodetic precession. Doppler shifts of pulsar radiation have revealed three planets in orbit about one neutron star, and some pulsar in globular clusters show acceleration due to the cluster dynamics.

007694   Bhattacharyya T;Basu B (Department of Mathematics, Jogamaya Devi College, Calcutta-700 025) : Star formation in galaxies. Indian J Phys-Pt B 2000, 74(4), 243-59.

Reviewing the basic problems associated with the currently understood mechanism of star formation in galaxies. Stars are born out of gas and dust pervading the interstellar medium. Hows this raw material spreads over the intersellar space with spotty distribution and transforms through various physical and dynamical processes into mysterious stars has been discussed. Molecular clouds are the most favourable places for star birth. The mechanism for formation of interstellar clouds along with physics and chemistry of molecular clouds have been included in discussions. The entire process of this repeated fragmentation of the cloud proceeds isothermally unti the density of the individual fragment attains a critical value. The fragmentation is halted when the fragments start becoming opaque to low frequency radiation. This phase is followed by slow adiabatic contraction gradually increasing the opacity and capturing the higher frequency radiation. Opacity helps them to raise their temperature gradually unti they are able to aquire temperature sufficient to generate hydrogen fusion within their centres. They are then transformed into fully developed shinning stars imbedded within the parent gas cloud. It is emphasized that these tiny gas blobs created by fragmentation gather mass either by accreting mass from the surrounding gas cloud or by colliding with other gas blobs in their neighbourhood. An estimate of the minimum and maximum mass of a star in this connection.

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004303   Tikekar R (Department of Mathematics, Sardar Patel University, Vallabh Vidyanagar-388 120) : On spherically symmetric singularity-free model in relativistic cosmology. Pramana J Phys 2000, 55(4), 623-8.

Introduction of time dependence through a scale factor in a non-conformally flat static cosmological model whose spacetime can be embedded in a five demensional flat spacetime is shown to give rise to two spherical models of universe filled with perfect fluid acompannied with radial heat flux without any Big Bang type singularity. The first model describes an ever existing universe which witnesses a transition from state of contraction to that of ever expansion. The second model represents a universe oscillating between two regular states.

004302   Seshadri T R (NO, The Mehra Research Institute of Mathematics and Mathematical Physics, Chhatnag Road, Jhusi, Allahabad-211 019) : Workshop III - cosmology: observations versus theories. Pramana J Phys 2000, 55(4), 599-601.

Topics on which there were presentations in this workshop can broadly be divided into the following categories: Observational aspects of large-scale structures in the universities; phase transitions in the early universe: cosmic microwave background radiation; Observational cosmology.

004301   Sahni V (NO, Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411 007) : Living with lambda. Pramana J Phys 2000, 55(4), 559-73.

Presents a brief overview of recent results pertaining to the cosmological constant `Λ'. Summarize the observational situation focussing on observations of high redshift Type Ia supernovae which suggest Λ<62>O. Observations of small angular anistropies in the cosmic microwave background complement Type Ia supernovae observations and both CMB and Sn can be combined to place strong constraints on the value of Λ. The presence of small Λ-term increase the age of the universe and slows down the formation of large scale structure. Also review recent theoretical attempts to generate a small Λ-term at the current epoch and a model independent approach for determining the cosmic equation of state.

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004300   Ramachandra B S;Vishveshwara C V (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Workshop I - black holes and compact objects : classical aspects. Pramana J Phys 2000, 55(4), 585-90.

004299   Mena F C;Tavakol R (Astronomy Unit, School of Mathematical Sciences, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK) : Density contrast indicators in cosmological dust models. Pramana J Phys 2000, 55(4), 617-21.

Discusses ways of quantifying structuration in relativistic cosmological settings, by employing a family of covariant density constrast indicators. Studies the evolution of these indicators with time in the context of inhomogeneous Szekeres models. Also finds that different observers (having either different spatial locations or different indicators) see different evolutions for the density contrast, which may or may not be monotonically increasing with time. Also found that monotonicity seems to be related to the initial conditions of the model, which may be of potential interest in connection with debates regarding gravitational entropy and the arrow of time.

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004298   Majumdar P (NO, The Institute of Mathematical Sciences, CIT Campus, Chennai-600 113) : Workshop II - black holes and compact objects: quantum aspects. Pramana J Phys 2000, 55(4), 591-8.