Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio     

Kuntal Misra  Dave Pooley †  Poonam Chandra  D. Bhattacharya  Alak K. Ray  Ram Sagar  Walter H. G. Lewin 《Monthly notices of the Royal Astronomical Society》2007,381(1):280-292

We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as   L _X∝ t ^−0.4  . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of  ∼2 × 10⁻⁶ M_⊙ yr⁻¹  . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of ⁵⁶Ni to be  0.06 ± 0.03 M_⊙  . Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of  (0.98 ± 0.25) × 10⁵¹ erg  . We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of  ∼20 M_⊙  for SN 2004et.  相似文献   

Discovery of unusual pulsations in the cool, evolved Am stars HD 98851 and HD 102480     

Santosh Joshi  V. Girish  Ram Sagar  D. W. Kurtz  P. Martinez  Brijesh Kumar  S. Seetha  B. N. Ashoka  A. Zhou 《Monthly notices of the Royal Astronomical Society》2003,344(2):431-438

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A flattening in the optical light curve of SN 2002ap     

S. B. Pandey  &#;  G. C. Anupama  R. Sagar  D. Bhattacharya  D. K. Sahu  J. C. Pandey 《Monthly notices of the Royal Astronomical Society》2003,340(2):375-380

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On the determination of age and mass functions of stars in young open star clusters from the analysis of their luminosity functions     

A. E. Piskunov  A. N. Belikov  N. V. Kharchenko  R. Sagar  A. Subramaniam 《Monthly notices of the Royal Astronomical Society》2004,349(4):1449-1463

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Optical variability properties of high luminosity AGN classes     

C. S. Stalin  Gopal-Krishna  Ram Sagar  Paul J. Wiita 《Journal of Astrophysics and Astronomy》2004,25(1-2):1-55

We present the results of a comparative study of the intranight optical variability (INOV) characteristics of radio-loud and radioquiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift rangez ≃ 0.2 toz ≃ 2.2. The sample, matched in the optical luminosity—redshift(M ^B—z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobedominated quasars (LDQs), five radio core-dominated quasars (CDQs) and six BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as about 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a given night. Considering the cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 61% for RQQs, LDQs, CDQs, and BLs, respectively. The much lower amplitude and DC of ESfOV shown by RQQs compared to BLs may be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed. This suggests that the physical mechanisms of INOV and long term optical variability (LTOV) do not have a one-to-one relationship and different factors are involved. Also, the absence of a clear negative correlation between the INOV and LTOV characteristics of blazars of our sample points toward an inconspicuous contribution of accretion disk fluctuations to the observed INOV. The INOV duty cycle of the AGNs observed in this program suggests that INOV is associated predominantly with the highly polarized optical emission components. We also report new VLA imaging of two RQQs (1029 + 329 & 1252 + 020) in our sample which has yielded a 5 GHz detection in one of them (1252 + 020;S _5GHZ ≃ 1 mJy).  相似文献   

Bianchi type-III bulk viscous string cosmological model in Brans-Dicke theory of gravitation     

T. Vidya Sagar  C. Purnachandra Rao  R. Bhuvana Vijaya  D. R. K. Reddy 《Astrophysics and Space Science》2014,349(1):479-483

A spatially homogeneous and anisotropic Bianchi type-III space-time is considered in the framework of a scalar-tensor theory of gravitation proposed by Brans and Dicke (Phys. Rev. 124:925, 1961) in the presence of bulk viscous fluid containing one dimensional cosmic strings. We have found a determinate solution of the field equations using the plausible physical conditions (i) a barotropic equation state for the pressure and density, (ii) special law of variation for Hubble’s parameter proposed by Berman (Nuovo Cimento B74:182, 1983), (iii) shear scalar is proportional to scalar expansion and (iv) the trace of the energy tensor of the fluid vanishes. We have also assumed that bulk viscous pressure is proportional to energy density. Some physical and kinematical properties of the model are, also, discussed.  相似文献   

The effect of Main-Sequence stars on age estimates of open clusters     

Ram Sagar 《Astrophysics and Space Science》1985,113(1):171-180

Existing methods for age estimation of open clusters are discussed. Most of the observed open clusters (except extremely young and old) contain 50–90% unevolved Main-Sequence stars. Possible difficulties encountered in estimating the actual age of an unevolved Main-Sequence star are discussed. For a relatively reliable cluster age estimation only a small percentage of cluster members are suitable. The effect of unevolved Main-Sequence stars on open cluster age estimates using the modified method of isochromes is analyzed.On leave of absence from the Department of Physics, Kumaon University, Naini Tal, India.  相似文献   

Radial distribution of the integrated light and photometric colours in open star clusters     

Ram Sagar  Harish C. Bhatt 《Journal of Astrophysics and Astronomy》1989,10(2):173-182

Mass segregation in the form of preferential concentration of more massive stars in the central regions of a number of open star clusters has been known for some time. In this paper, integratedUBV colours in concentric zones have been estimated for 12 nearby open clusters using the observations of individual cluster members. It is found that the clusters showing pronounced mass segregation also show significant radial variations in the integrated colours. However, the effects of stochastic fluctuations around the massive portion of the mass distribution function on the integrated colours should be taken into consideration, if they are present.  相似文献   

Avoiding a COP-Out: Moving Towards Systematic Decision-Making Under the Climate Convention     

Adil Najam  Ambuj Sagar 《Climatic change》1998,39(4):III-IX

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Male-female literacy differential in India--1981     

Sagar P 《Population geography : a journal of the Association of Population Geographers of India》1989,11(1-2):21-39

The author examines inequalities in the literacy rates between males and females in India. Regional variations in literacy are examined by district using maps. "The male-female literacy differential is low in coastal, peripheral and metropolitan areas and is high in inland and mountainous areas. These regional disparities are the product of differences in length of educational background, age at marriage, level of urbanisation, standard of living and proportion of socially conservative and backward sections of society.... The study in spatial dimension provides a strong base for the removal of sex biased discrimination in society."  相似文献