Method of growing composites of the type exhibiting the Soret effect

A predetermine amount of segregation is introduced into a molten sample of a composite that exhibits the Soret effect, such amount approximating the amount of segregation resulting from directional solidification of the sample. The molten sample is then directionally solidified starting at the end opposite the end richer in the constituent that would migrate toward the cooler part of a liquid solution of the composite maintained in a temperature gradient. Since solidification commences at the end deficient in such constituent, its migration toward the interface between the solid and liquid during the solidification will compensate for the deficiency, yielding a more homogeneous product

Searches for radio transients

Exploration of the transient Universe is an exciting and fast-emerging area within radio astronomy. Known transient phenomena range in time scales from sub-nanoseconds to years or longer, thus spanning a huge range in time domain and hinting a rich diversity in their underlying physical processes. Transient phenomena are likely locations of explosive or dynamic events and they offer tremendous potential to uncover new physics and astrophysics. A number of upcoming next-generation radio facilities and recent advances in computing and instrumentation have provided a much needed impetus for this field which has remained a relatively uncharted territory for the past several decades. In this paper we focus mainly on the class of phenomena that occur on very short time scales (i.e. from $\sim$ milliseconds to $\sim$ nanoseconds), known as {\it fast transients}, the detections of which involve considerable signal processing and data management challenges, given the high time and frequency resolutions required in their explorations, the role of propagation effects to be considered and a multitude of deleterious effects due to radio frequency interference. We will describe the techniques, strategies and challenges involved in their detections and review the world-wide efforts currently under way, both through scientific discoveries enabled by the ongoing large-scale surveys at Parkes and Arecibo, as well as technical developments involving the exploratory use of multi-element array instruments such as VLBA and GMRT. Such developments will undoubtedly provide valuable inputs as next-generation arrays such as LOFAR and ASKAP are designed and commissioned. With their wider fields of view and higher sensitivities, these instruments, and eventually the SKA, hold great potential to revolutionise this relatively nascent field, thereby opening up exciting new science avenues in astrophysics.Comment: To appear in the special issue of the Bulletin of the Astronomical Society of India on Transients at different wavelengths, eds D.J. Saikia and D.A. Green. 21 pages, 5 figures. http://www.ncra.tifr.res.in/~bas

Rolling contact fatigue life of chromium ion plated 440C bearing steel

Rolling contact fatigue (RCF) test specimens of heat treated 440C bearing steel were chromium ion plated in thicknesses from 0.1 to 8.0 micron and tested in RCF tester using 700 ksi maximum Hertzian stress. Heavy coatings, greater than about 5 micron in thickness, peeled off or spalled readily, whereas thin coatings, less than 3 micron thick, were tenacious and did not come off. Furthermore, significant improvement in RCF life was obtained with thin chromium ion plated test specimens. The average increase in B10 life was 75% compared with unplated 440C. These preliminary results indicate that ion plating is a promising way to improve bearing life

Pulsar Scintillation and the Local Bubble

We present here the results from an extensive scintillation study of twenty pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 caried out using the Ooty Radio Telescope (ORT) at 327 MHz, to investigate the distribution of ionized material in the local interstellar medium. Observations were made during the period January 1993 to August 1995, in which the dynamic scintillation spectra of these pulsars were regularly monitored over 10 - 90 epochs spanning 100 days. Reliable and accurate estimates of strengths of scattering have been deduced from the scintillation parameters averaged out for their long-term fluctuations arising from refractive scintillation (RISS) effects. Our analysis reveals several anomalies in the scattering strength, which suggest tht the distribution of scattering material in the Solar neighborhood is not uniform. We have modelled these anomalous scattering effects in terms of inhomogeneities in the distribution of electron dnsity fluctuations in the local interstellar medium (LISM). Our model suggests the presence of a low density bubble surrounded by a shell of much higher density fluctuations. We are able to put constraints on geometrical and scattering properties of such a structure, and find it to be morphologically similar to the Local Bubble known from other studies.Comment: 35 pages, 12 figure

Long-Term Scintillation Studies of Pulsars: III. Testing Theoretical Models of Refractive Scintillation

Refractive interstellar scintillation (RISS) is thought to be the cause behind a variety of phenomena seen at radio wavelengths in pulsars and compact radio sources. Though there is substantial observational data to support several consequences of it, the quantitative predictions from theories have not been thoroughly tested. In this paper, data from our long-term scintillation study of 18 pulsars are used to test the predictions. The fluctuations of decorrelation bandwidth ($\nu_d$), scintillation time scale ($\tau_d$) and flux density (F) are examined for their cross-correlations and compared with the predictions. The theory predicts a strong correlation between $\nu_d$ and $\tau_d$, and strong anti-correlations between $\nu_d$ and F, and $\tau_d$ and F. For 5 pulsars, we see a reasonable agreement. There is considerable difficulty in reconciling the results for the rest of the pulsars. Our analysis shows the underlying noise sources can sometimes reduce the correlation, but cannot cause an absence of correlation. It is also unlikely that the poor flux correlations arise from a hitherto unrecognized intrinsic flux variations. For PSR B0834+06, which shows anomalous behaviour of persistent drift slopes, positive correlation is found between $\tau_d$ and the drift-corrected $\nu_d$. Many pulsars show an anti-correlation between $\nu_d$ and the drift slope, and this is in accordance with the simple models of RISS. The detections of correlated variations of observables and a reasonable agreement between the predicted and measured correlations for some pulsars confirm RISS as the primary cause of the observed fluctuations. However, the complexity seen with the detailed results suggests the necessity of more comprehensive theoretical treatments for describing refractive fluctuations and their correlations.Comment: 27 pages, 6 Figures, 6 Tables. Accepted for publication in The Astrophysical Journa

An analytical model of prominence dynamics

Solar prominences are magnetic structures incarcerating cool and dense gas in an otherwise hot solar corona. Prominences can be categorized as quiescent and active. Their origin and the presence of cool gas (~$10^4$K) within the hot (~$10^6$K) solar corona remains poorly understood. The structure and dynamics of solar prominences was investigated in a large number of observational and theoretical (both analytical and numerical) studies. In this paper, an analytic model of quiescent solar prominence is developed and used to demonstrate that the prominence velocity increases exponentially, which means that some gas falls downward towards the solar surface, and that Alfven waves are naturally present in the solar prominences. These theoretical predictions are consistent with the current observational data of solar quiescent prominences.Comment: Update Final Journal Print Version along with other Metadat