arXiv subjects

Ranjan Kumar

Publications and source records attributed to Ranjan Kumar.

17 recordsLinked to original sources

arXiv2026

Sector-based morphological analysis of four planetary nebulae (J 320, NGC 2610, A 6672 and PRTM 1) observed with UVIT/ASTROSAT

Four planetary nebulae (PNe), Jonckheere 320, NGC 2610, A 6672, and PRTM 1, have been imaged in the far-ultraviolet (FUV) filter F148W ($\lambda_{\mathrm {eff}} = 1481\, \mathring{A} $) using the Ultraviolet Imaging Telescope (UVIT) onboard ASTROSAT. The FUV images reveal faint extended emission structures surrounding the bright central star. A novel sector-based structural analysis technique was employed to examine the detailed FUV morphologies of PNe. In this method, sector strengths were calculated for each nebula and used to classify their structures. Our results show that Jonckheere 320 exhibits a polypolar morphology, while NGC 2610 and PRTM 1 display spherical structures. In contrast, A 6672 shows irregular or elliptical features in the FUV.

astro-ph.SR↗

arXiv2026

Exploration of UV-bright sources in Globular cluster NGC 3201 using UVIT observations

We present a comprehensive study of the ultraviolet (UV)-bright stellar population in the globular cluster NGC 3201 using the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat. By combining far-UV photometry of the UVIT F148W and F169M filters with Gaia and ground-based telescope optical data, we construct UV$-$optical color-magnitude diagrams (CMDs) to identify UV-bright stellar evolutionary phases. Physical parameters of all detected sources are derived by fitting stellar atmosphere models to the observed multi-wavelength spectral energy distributions (SEDs). The effective temperatures of blue horizontal branch stars (BHBs) span the range 7250$-$11250 K. We identify two extreme horizontal-branch stars (EHBs) with effective temperatures of approximately 25,000 K and 17,000 K; one lies within the cluster core radius ($r_\mathrm{c}$=1.3$'$), while the other is located outside. We also detect two candidate ``gap'' objects in the transitional phase between HB stars and white dwarfs, which may represent hot blue hook (BHk) stars. For the blue straggler stars (BSSs), we determine effective temperatures, radii, and luminosities through SED fittings, and estimate their masses and isochrone-equivalent ages using PARSEC isochrones. Finally, we studied the normalized cumulative radial distribution of the observed BHB stars, BSSs, MS stars, RGB stars, and variable stars along with the specific frequency of BSSs across the cluster. These results provide new insights into the late-stage stellar evolution and dynamical processes in NGC 3201.

astro-ph.GA↗

arXiv2026

Study of Integrated Far-ultraviolet Emissions from Galactic Globular Clusters using AstroSat/UVIT observations

We used observations obtained with the Ultraviolet Imaging Telescope on board the AstroSat satellite to measure the integrated far-ultraviolet (FUV) and optical (V) magnitudes of 30 Galactic globular clusters (GCs). We classified the UV-bright evolved stellar populations of the GCs using FUV$-$V versus FUV color-magnitude diagrams (CMDs) and BaSTI-IAC isochrones and subsequently quantified their contributions to the total integrated FUV emissions. We found that the horizontal branch (HB) and post-HB (post-HB) stars contribute $\sim 40\%-45$\% to the total FUV emission of GCs, while the contribution of blue straggler stars is only $\sim$3\%. The HB stars especially dominate the UV budget of the metal-poor clusters. The observed spread in FUV-optical color in the color-color diagram supports the phenomenon that the UV upturn of early-type galaxies is due to the evolved stars. We studied for the first time the variation of integrated FUV magnitudes and colors with several cluster parameters in the core, intermediate, outer, and tidal regions, such as the fraction of second-generation stars, helium mass fraction, HB morphology, and mass of the GCs. We found that the GCs with a higher second-generation star fraction, helium mass fraction, and cluster mass are brighter in all the regions. The GCs with bluer HB morphologies also have brighter and bluer FUV magnitudes in the core and intermediate regions. Metal-poor GCs show significantly bluer FUV$-$optical colors, consistent with a stronger contribution from hot evolved stars.

astro-ph.GA↗

arXiv2024

A Comprehensive Catalog of UVIT Observations I: Catalog Description and First Release of Source Catalog (UVIT DR1)

We present the first comprehensive source catalog (UVIT DR1) of ultraviolet (UV) photometry in four far-UV (FUV $\sim$1300$-$1800 \AA) and five near-UV (NUV $\sim$2000$-$3000 \AA) filters of the Ultraviolet Imaging Telescope (UVIT) on board {\em AstroSat}. UVIT DR1 includes bright UV sources in 291 fields that UVIT detected during its first two years of pointed observation, encompassing an area of 58 square degrees. We used the {\sc ccdlab} pipeline to reduce the L1 data, source-extractor for source detection, and four photometric procedures to determine the magnitudes of the detected sources. We provided the 3$\sigma$ and 5$\sigma$ detection limits for all the filters of UVIT. We describe the details of observation, source extraction methods, and photometry procedures applied to prepare the catalog. In the final UVIT DR1 catalog, we have point sources, extended sources, clumps from nearby galaxies, There are 239,520 unique sources in the combined UVIT DR1, of which 70,488 sources have FUV magnitudes, and 211,410 have NUV magnitudes. We cross-matched and compared non-crowded sources of UVIT with the {\em Galaxy Evolution Explorer (GALEX)} and {\em Gaia} source catalogs. We provide a clean catalog of the unique sources in various UVIT filters that will help further multi-wavelength scientific analysis of the objects.

astro-ph.GA↗

arXiv2023

Globular Cluster UVIT Legacy Survey (GlobULeS) $-$ II. Evolutionary status of hot stars in M3 and M13

We present a far-ultraviolet (FUV) study of hot stellar populations in the second parameter pair globular clusters (GCs) M3 and M13, as a part of the Globular cluster UVIT Legacy Survey program (GlobULeS). We use observations made with F148W and F169M filters of the Ultraviolet Imaging Telescope (UVIT) onboard {\em AstroSat} along with ground-based data (UBVRI filters), {\em Hubble Space Telescope (HST)} GC catalogue, and {\em Gaia} EDR3 catalogue. Based on the FUV-optical colour-magnitude diagrams, we classify the sources into the horizontal branch (HB) stars, post-HB stars, and hot white dwarfs (WDs) in both the GCs. The comparison of synthetic and observed colours of the observed HB stars suggests that the mass-loss at the red giant branch (RGB) and He spread in both clusters have a simultaneous effect on the different HB distributions detected in M3 and M13, such that, HB stars of M13 require a larger spread in He (${\rm 0.247-0.310}$) than those of M3 (${\rm Y= 0.252-0.266}$). The evolutionary status of HB stars, post-HB stars, and WDs are studied using SED fit parameters and theoretical evolutionary tracks on the H-R diagram. We found that the observed post-HB stars have evolved from zero-age HB (ZAHB) stars of the mass range $0.48-0.55$ \Msun\ in M3 and M13. We detect 24 WD candidates in each cluster having ${\rm \log(L_{bol}/L_\odot)}$ in the range $-0.8$ to $+0.6$ and ${\rm \log(T_{eff}/K)}$ in the range of 4.2 to 5.0. Placing the WDs on the H-R diagram and comparing them with models suggest that M13 has a population of low-mass WDs, probably originating from binary evolution.

astro-ph.SR↗

arXiv2022

Study of UV bright sources in globular cluster NGC 4590 using Ultraviolet Imaging Telescope (UVIT) observations

We have studied ultraviolet (UV) bright sources in the Galactic globular cluster (GGC) NGC 4590 using Ultraviolet Imaging Telescope (UVIT) on-board the \mbox{{\em AstroSat}} satellite. Using UV-optical color-magnitude diagrams (CMDs), we have identified and characterized the sources of different evolutionary stages i.e., blue horizontal branch stars (BHBs), extremely blue horizontal branch stars (EHBs), blue straggler stars (BSs), variable stars, etc. We estimated effective temperature (T$_{\mathrm{eff}}$), gravity ($\log$(g)), luminosity (L$_{bol}$), and hence the radius (R) of these hot stars by fitting spectral energy distribution (SED) with the help of stellar atmosphere models. Two new far-UV (FUV) bright cluster member stars situated near the core of the cluster have been detected; one of them is an EHB star and the other one is either in its post-blue hook evolutionary phase or in white dwarf phase. The evolutionary status of all the hot stars, identified in the cluster, has been investigated by using various evolutionary models. We find the massive and younger BSs are concentrated at the center of the cluster whereas the older and less massive BSs are distributed though out the cluster. The BSs normalized radial distribution seems to be bi-modal with a minimum located at r$_{\mathrm{min}}$ = 4.3 r$_c$. We calculated A$^+$ parameter of the cluster which is obtained using cumulative normalized radial distribution of horizontal branch stars (HBs) and BSs. We measured this value up to half-mass radius of the cluster to be $+ 0.13$, which indicates that NGC 4590 is one of the youngest clusters among dynamically intermediate age GGCs with a dynamical age of $0.423\pm0.096$ Gyr.

astro-ph.GA↗

arXiv2020

Ultraviolet Imaging Telescope (UVIT) observation of the Galactic Globular Cluster NGC 7492

We present detailed photometric observations of the Galactic globular cluster NGC 7492 using the data obtained with two far-ultraviolet (FUV: 1300 - 1800 \AA) and three near-ultraviolet (NUV: 2000 - 3000 \AA) filters of Ultraviolet Imaging Telescope (UVIT) on-board the \textit{AstroSat} satellite. We confirmed the cluster membership of the extracted sources using GAIA data release 2 (Gaia DR2) proper motion data. We have used color-magnitude diagrams (CMDs) using UVIT and GAIA filters to separate out different evolutionary stages of the stars present in the cluster. We have identified a new extreme horizontal branch (EHB) star at the core of the cluster using UV and UV-optical CMDs. The estimated distance-modulus of the cluster is $16.95\pm0.05$ obtained by fitting BaSTI isochrones with cluster parameters, $[Fe/H] = -1.8$ dex and age $= 12.0$ Gyr on the V $-$ I vs V CMD. Interestingly, only the EHB star and blue horizontal branch stars (BHBs) among the UV-bright hot sources are detected in FUV filters of UVIT. We have derived the effective temperature of BHBs using color-temperature relation and spectral energy distributions (SEDs) of multi-band filters, which are in the range from 8,000 K to 10,500 K. We find a variation of He abundance of BHBs by fitting the BaSTI ZAHB. The range in the He abundance of the BHBs corresponding to the best fit isochrones is 0.247 to 0.350. We have estimated various physical parameters of the newly identified EHB star in the cluster using SED fit and post-HB evolutionary tracks. We have studied the radial distribution of all the sources of the cluster detected in UVIT. The sources detected in FUV filters extend beyond the half light radius (1.15$'$) of the cluster, whereas the sources detected in NUV filters extend beyond the tidal radius (9.2$'$) of the cluster.

astro-ph.GA↗

arXiv2020

Study of Galactic Structure Using UVIT/AstroSat Star Counts

The structure of our Galaxy has been studied from ultraviolet (UV) star counts obtained with the Ultra-Violet Imaging Telescope (UVIT) on board the {\em AstroSat} satellite, in Far-UV (FUV) and Near-UV (NUV) bands. The F154W (BaF2) and N263M (NUVB4) filters were used in the FUV and NUV bands, respectively. The point sources are separated from the extra-galactic sources of UVIT observations using infrared (IR) color cut method. The observed UVIT star counts match well with the simulations obtained from the Besan\c{c}on model of stellar population synthesis towards several Galactic directions. We also estimated the scale length and scale height of the thick disc and the scale height of the thin disc using the space density function and the exponential density law for the stars of intermediate Galactic latitudes. The scale length of the thick disc ranges from 3.11 to 5.40 kpc whereas the scale height ranges from 530$\pm$32 pc to 630$\pm$29 pc. The scale height of the thin disc comes out to be in the range of 230$\pm$20 pc to 330$\pm$11 pc.

astro-ph.GA↗

arXiv2020

UVIT study of UV bright stars in the globular cluster NGC 4147

We present far ultraviolet (FUV) observations of globular cluster NGC 4147 using three FUV filters, BaF2 (F154W), Sapphire (F169M), and Silica (F172M) of Ultra-Violet Imaging Telescope (UVIT) on-board the \mbox{{\em AstroSat}} satellite. We confirmed the cluster membership of the UVIT observed sources using proper motions from Gaia data release 2 (GAIA DR2). We identified 37 blue horizontal branch stars (BHBs), one blue straggler star (BSS) and 15 variable stars using UV-optical color magnitude diagrams (CMDs). We find that all the FUV bright BHBs are second generation population stars. Using UV-optical CMDs, we identify two sub-populations, BHB1 and BHB2, among the UV-bright BHBs in the cluster with stars count ratio of 24:13 for BHB1 and BHB2. The effective temperatures (T$_{\mathrm{eff}}$) of BHB1 and BHB2 were derived using color-temperature relation of BaSTI-IAC zero-age horizontal branch (ZAHB). We found that BHB1 stars are more centrally concentrated than BHB2 stars. We also derive physical parameters of the detected FUV bright BSS by fitting younger age BaSTI-IAC isochrones on optical and UV-optical CMDs.

astro-ph.GA↗

arXiv2019

UVIT Observations of UV-Bright Stars in four Galactic Globular Clusters

We have performed photometric analysis of four Galactic globular clusters (GGCs): NGC 4147, NGC 4590, NGC 5053 and NGC 7492 using far-UV and near-UV filters of the Ultraviolet Imaging Telescope (UVIT) on-board AstroSat. With the help of color-magnitude diagrams (CMDs), we have identified 150 blue horizontal branch stars (BHBs), and 40 blue straggler stars (BSS) in the four GGCs. We study the temperature and radial distribution of BHBs and BSS for the four GGCs.

astro-ph.GA↗

arXiv2018

Exploring the Effect of Periastron Advance in Small-Eccentricity Binary Pulsars

Short-orbital period small-eccentricity binary pulsars can, in principle, experience substantial advance of periastron. We explore the possibility of measuring this effect by implementing a timing model, ELL1k, in the popular TEMPO2 pulsar timing package. True secular variations in the Laplace-Lagrange parameters, present in our ELL1k model, can lead to measurable timing residuals while pursuing decade-long timing campaigns using the existing ELL1 timing model of Lange et al. (2001), especially for binaries exhibiting significant periastron advance. We also list the main differences between our approach and various implementations of the ELL1 model present in both TEMPO and TEMPO2 packages. Detailed TEMPO2 simulations suggest the possibility of constraining the apsidal motion constant of pulsar companions in certain observed binary pulsars with minuscule eccentricities such as PSR J1719-1438. Fortunately, the ELL1k timing model does not pose any challenges to the on-going Pulsar Timing Array campaigns that routinely employ the ELL1 timing model. Detailed TEMPO2 simulations suggest the possibility of constraining the apsidal motion constant of pulsar companions in certain observed binary pulsars with minuscule eccentricities such as PSR J1719-1438. Fortunately, the ELL1k timing model does not pose any challenges to the on-going Pulsar Timing Array campaigns that routinely employ the ELL1 timing model.

astro-ph.SR↗

arXiv2017

Negative Thermal expansion of pure and doped Graphene

Graphene and its derivatives distinguish themselves for their large negative thermal expansion even at temperatures as high as 1000K. The linear thermal expansion coefficients (LTEC) of two-dimensional honeycomb structured pure graphene and B/N doped graphene are analyzed using ab initio density functional perturbation theory (DFPT) employed in VASP software under quasiharmonic approximation. One of the essential ingredients required is the phonon frequencies for a set of points in the Brillouin zone and their volume dependence. These were obtained from the dynamical matrix which was calculated using VASP code in interface with phonopy code. In particular, the transverse acoustic modes (ZA) behave drastically differently as compared to planer modes and so also their volume dependence. Using this approach firstly thermal expansion for pure graphene is calculated. The results agree with earlier calculations using similar approach. Thereafter we have studied the effect of boron and nitrogen doping on LTEC. The LTEC of graphene is found to be negative in the entire range of temperature under study (0-1000K) and its value at room temperature (RT) is around -3.26X10-6K-1. The value of LTEC at RT becomes more negative with B/N doping in graphene. In order to get an insight into the cause of negative thermal expansion, we have computed the contribution of individual phonon modes of vibration. We notice that it is principally the ZA modes which are responsible for negative thermal expansion. It has been concluded that transverse mode in 2D hexagonal lattices have an important role to play in many of the thermodynamical properties of 2D structures. We have extended the study to calculate the LTEC of h-BN sheet also.

cond-mat.mes-hall↗

arXiv2016

Thermodynamic properties of pure and doped (B, N) graphene

Ab-initio density functional perturbation theory (DFPT) has been employed to study thermodynamical properties of pure and doped graphene sheet and the results have been compared with available theoretical and experimental data. The concentration of B and N has been varied upto 50% of the carbon atoms in graphene. Phonon frequencies are essential ingredients entering into such a calculation, which have been computed by using the dynamical matrix provided by VASP software in combination with phonopy code in the harmonic approximation. This easily provides us the Helmholtz free energy and leads us to numerical estimates of various thermodynamical properties. The results for specific heat are in good agreement with various theoretical and experimental studies obtained earlier for pure graphene. Interesting new results have been reported for B and N substituted structure. It has been observed that specific heat decreases with the increase in concentration of doping while the entropy increases. Further, large doping concentrations result in unstable sheets resulting in imaginary frequencies in the transverse directions. The instability needs to be compensated by external strains and that has been assumed while carrying out Brilluoin summations. These results will be useful for calculation of thermal conductivity of doped graphene and thus feasibility of using these for device applications. Our preliminary results for thermal expansion have indicated negative thermal expansion behavior of pure graphene at low temperatures which needs investigation on doped graphene as well.

cond-mat.mes-hall↗

arXiv2014

Cloud Computing Simulation Using CloudSim

As we know that Cloud Computing is a new paradigm in IT. It has many advantages and disadvantages. But in future it will spread in the whole world. Many researches are going on for securing the cloud services. Simulation is the act of imitating or pretending. It is a situation in which a particular set of condition is created artificially in order to study that could exit in reality. We need only a simple Operating System with some memory to startup our Computer. All our resources will be available in the cloud.

cs.DC↗

arXiv2013

Load Balancing using Ant Colony in Cloud Computing

Ants are very small insects.They are capable to find food even they are complete blind. The ants lives in their nest and their job is to search food while they get hungry. We are not interested in their living style, such as how they live, how they sleep. But we are interested in how they search for food, and how they find the shortest path. The technique for finding the shortest path are now applying in cloud computing. The Ant Colony approach towards Cloud Computing gives better performance.

cs.DC↗

arXiv2012

Temperature and field dependence of thermally activated flux flow resistance in Bi2Sr2CaCu2O8+{\delta} superconductor

We study the temperature dependence of the resistivity as a function of magnetic field in superconducting transition (Tconset - TcR=0) region for different Bi2Sr2CaCu2O8+{\delta} superconducting samples being synthesized using sol-gel method. The superconducting transition temperature (TcR=0) of the studied samples is increased from 32 K to 82K by simply increasing the final sintering temperature with an improved grains morphology. On the other hand, broadening of transition is increased substantially with decrease in sintering temperature; this is because Tconset is not affected much with grains morphology. Further broadening of the superconducting transition is seen under magnetic field, which is being explained on the basis of thermally activated flux flow (TAFF) below superconducting transition temperature (Tc). TAFF activation energy (U0) is calculated using the resistive broadening of samples in the presence of magnetic field. Temperature dependence of TAFF activation energy revealed linear temperature dependence for all the samples. Further, magnetic field dependence is found to obey power law for all the samples and the negative exponent is increased with increase in sintering temperature or the improved grains morphology for different Bi-2212 samples. We believe that the sintering temperature and the ensuing role of grain morphology is yet a key issue to be addressed in case of cuprate superconductors.

cond-mat.supr-con↗

arXiv2012

Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2

In present study, we report an inter-comparison of various physical and electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is -ve for AlB2. This is consistent our band structure plots. We fitted the experimental specific heat of MgB2 to Debye Einstein model and estimated the value of Debye temperature (theta) and Sommerfeld constant (gamma) for electronic specific heat. Further, from gamma the electronic density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of experimental N (EF) and the one being calculated from DFT, we obtained value of Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below Tc using pi-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2 and hence it is not superconducting. DFT calculations demonstrated that for MgB2 the majority of states come from Sigma and Pi 2p states of boron on the other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as Pi bands are known to be of electron type and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2

cond-mat.supr-con↗