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S. Ravichandran K. Preethi M. Safonova Jayant Murthy 《Astrophysics and Space Science》2013,344(2):361-364
The Ultraviolet Imaging Telescope (UVIT) is scheduled to be launched as a part of the ASTROSAT satellite. As part of the mission planning for the instrument we have studied the efficacy of UVIT observations for interstellar extinction measurements. We find that in the best case scenario, the UVIT can measure the reddening to an accuracy of about 0.02 magnitudes, which combined with the derived distances to the stars, will enable us to model the three-dimensional distribution of extinction in our Galaxy. The knowledge of the distribution of the ISM will then be used to study distant objects, affected by it. This work points the way to further refining the UVIT mission plan to best satisfy different science studies. 相似文献
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Shyamal K. Banerjee Jayant V. Narlikar 《Monthly notices of the Royal Astronomical Society》1999,307(1):73-78
The data on angular sizes and redshifts of ultracompact radio sources used by Jackson & Dodgson in a recent paper have been applied to the various theoretical models in the framework of the quasi-steady-state cosmology proposed by Hoyle, Burbidge & Narlikar. It is found that although acceptable fits to the data are available for the flat models, those with a negative curvature of spatial sections provide a better fit. These latter models require low densities of matter and as such do not demand too high a proportion of dark matter. A comparison is made with the work of Jackson & Dodgson and theoretical implications of the test are discussed. 相似文献
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Jayant Murthy 《Astrophysics and Space Science》2009,320(1-3):21-26
The diffuse ultraviolet radiation field is seen over the entire sky and is a tracer of both the hot (in the form of lines such as CIV and OVI) and cold component (in the form of interstellar dust or molecular hydrogen) of the interstellar medium. Observations over the last 40 years have been continually pushing the boundaries of the available instrumentation but are now bearing fruit in a much better definition of the radiation and an understanding of its constituents, both foreground, such as airglow or zodiacal light, and cosmic. I present a review of the current state of observations of the far and near ultraviolet background. 相似文献
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Peter Jenniskens Ed Tedesco Jayant Murthy Christophe O. Laux Stephen Price 《Meteoritics & planetary science》2002,37(8):1071-1078
Abstract— We used the ultraviolet to visible spectrometers onboard the midcourse space experiment to obtain the first ultraviolet spectral measurements of a bright meteor during the 1997 Leonid shower. The meteor was most likely a Leonid with a brightness of about‐2 magnitude at 100 km altitude. In the region between 251 and 310 nm, the two strongest emission lines are from neutral and ionized magnesium. Ionized Ca lines, indicative of a hot T ? 10 000 K plasma, are not detected. The Mg and Mg+ line intensity ratio alone does not yield the ionization temperature, which can be determined only by assuming the electron density. A typical air plasma temperature of T = 4400 K would imply a very high electron density: ne = 2.2 times 1018 m‐3, but at chondritic abundances of Fe/Mg and Si/Mg ? 1. For a more reasonable local‐thermodynamic‐equilibrium (LTE) air plasma electron density, the Mg and Mg+ line ratio implies a less than chondritic Fe/Mg = 0.06 abundance ratio and a cool non‐LTE T = 2830 K ionization temperature for the ablation vapor plasma. The present observations do not permit a choice between these alternatives. The new data provide also the first spectral confirmation of the presence of molecular OH and NO emission in meteor spectra. 相似文献
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Jayant Murthy 《Journal of Astrophysics and Astronomy》2002,23(1-2):23-26
We have examined 426Voyager fields distributed across the sky for O VI (γγ 1032/1038 å) emission from the Galactic diffuse interstellar medium. No such emission was detected in any of our observed fields. Our most constraining limit was a 90% confidence upper limit of 2600 photons cm?2 sr?1 s?1 on the doublet emission in the direction (l, b) = (117.3, 50.6). Combining this with an absorption line measurement in nearly the same direction allows us to place an upper limit of 0.01 cm?3 on the electron density of the hot gas in this direction. We have placed 90% confidence upper limits of less than or equal to 10,000 photons cm?2 sr?1 s?1 on the O VI emission in 16 of our 426 observations. 相似文献
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Margarita Safonova Joice Mathew Rekhesh Mohan A. G. Sreejith Jayant Murthy Noah Brosch Norbert Kappelmann Arpit Sharma Rahul Narayan 《Astrophysics and Space Science》2014,353(2):329-346
Space astronomy in the last 40 years has largely been done from spacecraft in low Earth orbit (LEO) for which the technology is proven and delivery mechanisms are readily available. However, new opportunities are arising with the surge in commercial aerospace missions. We describe here one such possibility: deploying a small instrument on the Moon. This can be accomplished by flying onboard the Indian entry to the Google Lunar X PRIZE competition, Team Indus mission, which is expected to deliver a nearly 30 kgs of payloads to the Moon, with a rover as its primary payload. We propose to mount a wide-field far-UV (130–180 nm) imaging telescope as a payload on the Team Indus lander. Our baseline operation is a fixed zenith pointing but with the option of a mechanism to allow observations of different attitudes. Pointing towards intermediate ecliptic latitude (50° or above) ensures that the Sun is at least 40° off the line of sight at all times. In this position, the telescope can cover higher galactic latitudes as well as parts of Galactic plane. The scientific objectives of such a prospective are delineated and discussed. 相似文献
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A Gaussian correlation function characterizes smoothly heterogeneous media, while real heterogeneities in the Earth are often non-Gaussian in nature. Using the Born approximation, mean square amplitudes of the scattered waves have been derived for an elastic media characterized by the Von Karman correlation function. Heterogeneities with different power laws can be defined by the Von Karman correlation function. The sensitivity of fore- and backscattering to heterogeneities with different scales and properties (that is velocity and impedance) is discussed in this paper. The analytical expression for total scattered energy for the incident P waves is also derived for a random medium having the Von Karman correlation function. We find that at high frequencies, the scattered power of converted waves is a function of frequency. In the case of codawave excitation by local earthquakes, which must be handled by the full elastic-wave theory, we can define any type of inhomogeneity by the Von Karman correlation function. It also supports the idea that the lithosphere might have multiple-scale inhomogeneities. 相似文献
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Jayant Nath Tripathi 《Journal of Seismology》2006,10(1):119-130
Kutch region of Gujrat is one of the most seismic prone regions of India. Recently, it has been rocked by a large earthquake (M
w
= 7.7) on January 26, 2001. The probabilities of occurrence of large earthquake (M≥6.0 and M≥5.0) in a specified interval of time for different elapsed times have been estimated on the basis of observed time-intervals between the large earthquakes (M≥6.0 and M≥5.0) using three probabilistic models, namely, Weibull, Gamma and Lognormal. The earthquakes of magnitude ≥5.0 covering about 180 years have been used for this analysis. However, the method of maximum likelihood estimation (MLE) has been applied for computation of earthquake hazard parameters. The mean interval of occurrence of earthquakes and standard deviation are estimated as 20.18 and 8.40 years for M≥5.0 and 36.32 and 12.49 years, for M≥6.0, respectively, for this region. For the earthquakes M≥5.0, the estimated cumulative probability reaches 0.8 after about 27 years for Lognormal and Gamma models and about 28 years for Weibull model while it reaches 0.9 after about 32 years for all the models. However, for the earthquakes M≥6.0, the estimated cumulative probability reaches 0.8 after about 47 years for all the models while it reaches 0.9 after about 53, 54 and 55 years for Weibull, Gamma and Lognormal model, respectively. The conditional probability also reaches about 0.8 to 0.9 for the time period of 28 to 40 years and 50 to 60 years for M≥5.0 and M≥6.0, respectively, for all the models. The probability of occurrence of an earthquake is very high between 28 to 42 years for the magnitudes ≥5.0 and between 47 to 55 years for the magnitudes ≥6.0, respectively, past from the last earthquake (2001). 相似文献