scispace - formally typeset
Search or ask a question
Author

Ram Sagar

Bio: Ram Sagar is an academic researcher from Aryabhatta Research Institute of Observational Sciences. The author has contributed to research in topics: Open cluster & Star cluster. The author has an hindex of 32, co-authored 65 publications receiving 3288 citations. Previous affiliations of Ram Sagar include Ariès & University of Bonn.
Topics: Open cluster, Star cluster, Stars, Blazar, Light curve


Papers
More filters
Journal ArticleDOI
TL;DR: In this article, the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high-altitude station, Manora Peak in central Himalayas, during a comprehensive aerosol field campaign in December 2004.
Abstract: [1] Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high-altitude station, Manora Peak in central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of central Himalayas and having a monthly mean AOD (at 500 nm) of 0.059 ± 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15–40 μg m−3 (mean value 27.1 ± 8.3 μg m−3). Interestingly, aerosol BC had a mean concentration of 1.36 ± 0.99 μg m−3 and contributed ∼5.0 ± 1.3% to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 ± 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as −4.2 W m−2 at the surface, +0.7 W m−2 at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m−2. Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was ∼88 W m−2, which is attributed to the high BC mass fraction.

194 citations

Journal ArticleDOI
TL;DR: In this paper, multi-year measurements of near surface aerosol black carbon (BC) mass concentration, made from a high altitude station at Manora Peak (29.4° N, 79.5° E, 1958 mmsl) in the Central Himalayas, using a 7-channel Aethalometer for 38 months from November 2004 to December 2007, are examined.

154 citations

Journal ArticleDOI
01 Apr 1999-Nature
Abstract: Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

150 citations

Journal ArticleDOI
TL;DR: In this article, the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high altitude station, Manora Peak in Central Himalayas, during a comprehensive aerosol field campaign in December 2004.
Abstract: Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high altitude station, Manora Peak in Central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of Central Himalayas, and having a monthly mean AOD (at 500 nm) of 0.059 $\pm$ 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15 - 40 micro g m^(-3) (mean value 27.1 $\pm$ 8.3 micro g m^(-3)). Interestingly, aerosol BC had a mean concentration of 1.36 $\pm$ 0.99 micro g m^(-3), contributed to ~5.0 $\pm$ 1.3 % to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 $\pm$ 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as 4.2 W m^(-2) at the surface, +0.7 W m^(-2) at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m^(-2). Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was $\sim$88 W m^(-2), which is attributed to the high BC mass fraction.

142 citations

Journal ArticleDOI
TL;DR: In this article, the relative proper motions and membership probabilities of stars in the open cluster M 67 (C0847+120, NGC 2682) were determined using the Wide Field Imager (WFIM) at the 2.2 m MPG/ESO telescope.
Abstract: The solar-age open cluster M 67 (C0847+120, NGC 2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M 67 at fainter magnitudes and their properties are not well-established. Aims. Using proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34 × 33 arcmin 2 field centered on the old open cluster M 67. Methods. The two-epoch archival observations separated by 4 years and acquired with the Wide-Field Imager at the 2.2 m MPG/ESO telescope were reduced with our new astrometric techniques, as described in the first paper in this series. The same observations served to derive calibrated BVI photometry in M 67. Radial velocities were measured using the archival and new spectroscopic data obtained at the VLT. Results. We have determined relative proper motions and membership probabilities for ~2400 stars. The precision of proper motions for optimally exposed stars is 1.9 mas yr -1 , gradually degrading down to ~5 mas yr -1 at V=20. Our relatively precise proper motions at V>16 were first obtained in this magnitude range for M 67. Radial velocities were measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. Conclusions. For the M 67 area, we provide positions, calibrated BVI photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing reliable membership determination. We have produced a catalog that is electronically available to the astronomical community.

140 citations


Cited by
More filters
Journal ArticleDOI
Pavel Kroupa1
04 Jan 2002-Science
TL;DR: Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF, which appears to hold for populations including present-day star formation in small molecular clouds.
Abstract: The distribution of stellar masses that form in one star formation event in a given volume of space is called the initial mass function (IMF). The IMF has been estimated from low-mass brown dwarfs to very massive stars. Combining IMF estimates for different populations in which the stars can be observed individually unveils an extraordinary uniformity of the IMF. This general insight appears to hold for populations including present-day star formation in small molecular clouds, rich and dense massive star-clusters forming in giant clouds, through to ancient and metal-poor exotic stellar populations that may be dominated by dark matter. This apparent universality of the IMF is a challenge for star formation theory, because elementary considerations suggest that the IMF ought to systematically vary with star-forming conditions.

1,733 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that most long-duration soft-spectrum gamma-ray bursts are accompanied by massive stellar explosions (GRB-SNe) and that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow.
Abstract: Observations show that at least some gamma-ray bursts (GRBs) happen simultaneously with core-collapse supernovae (SNe), thus linking by a common thread nature's two grandest explosions. We review here the growing evidence for and theoretical implications of this association, and conclude that most long-duration soft-spectrum GRBs are accompanied by massive stellar explosions (GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to be greater than those of ordinary SNe, but evidence exists, even in a limited sample, for considerable diversity. The existing sample also suggests that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow. Neither all SNe, nor even all SNe of Type Ibc produce GRBs. The degree of differential rotation in the collapsing iron core of massive stars when they die may be what makes the difference.

1,389 citations

Journal ArticleDOI
TL;DR: In this article, the interplay between these observations and theoretical models of the prompt gamma-ray burst and its afterglow is reviewed, and a model of the burst's origin and mechanism is proposed.
Abstract: Gamma-ray bursts are the most luminous explosions in the Universe, and their origin and mechanism are the focus of intense research and debate. More than three decades after their discovery, and after pioneering breakthroughs from space and ground experiments, their study is entering a new phase with the recently launched Swift satellite. The interplay between these observations and theoretical models of the prompt gamma-ray burst and its afterglow is reviewed.

1,294 citations

Journal ArticleDOI
TL;DR: In this article, a new set of isochrones, called the Y^2 Isochrones, were constructed for the scaled solar mixture, which represent an update of the Revised Yale IsoChrones (RYI), using improved opacities and equations of state.
Abstract: We have constructed a new set of isochrones, called the Y^2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of alpha-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main sequence birthline instead of from the zero-age main sequence, and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the alpha-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages. Isochrones are available from this http URL

921 citations

Journal ArticleDOI
TL;DR: The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called ''machos'') could be a major component of the dark matter halo of the Milky Way galaxy.
Abstract: The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called ''machos'') could be a major component of the dark matter halo of the Milky Way galaxy. To this end, EROS-2 monitored millions of stars in the Magellanic clouds for microlensing events caused by such objects. (abridged)

908 citations