arXiv subjects

T. Chandrasekhar

Publications and source records attributed to T. Chandrasekhar.

17 recordsLinked to original sources

arXiv2019

Characteristics of the Solar Coronal Line Profiles from Fabry-Perot Interferometric Observations

This article reports the analysis of a set of Fabry-Perot interferograms that were studied to probe the physical parameters of the inner solar corona. The observations were carried out in the coronal green line, Fe XIV 5302.86 Å, during the total solar eclipse of 21 June 2001 which occurred in Lusaka, Zambia. The study was performed in the radial range of 1.1-1.5 Ro and examines the Doppler velocity, halfwidth, centroid and asymmetry and their correlations with each other at various points in the corona. It is found that 59 % of the line profiles are blueshifted, 34 % of them are single components and only 7 % of them are redshifted. The variations of halfwidth and Doppler velocity with respect to coronal height have a large scatter and shows no significant changes. It has been found that the variations of halfwidth with Doppler velocity or centroid follow a parabolic trend with a weak correlation, whereas the relation between halfwidth and the asymmetry is inconclusive. These results may provide more insight to the coronal dynamics and help in understanding the coronal physical problems.

astro-ph.SR↗

arXiv2017

Study of the plutino object (208996) 2003 AZ84 from stellar occultations: size, shape and topographic features

We present results derived from four stellar occultations by the plutino object (208996) 2003~AZ$_{84}$, detected at January 8, 2011 (single-chord event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid solution for 2003~AZ$_{84}$'s shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semi axes $(470 \pm 20) \times (383 \pm 10) \times (245 \pm 8)$~km % axis ratios $b/a= 0.82 \pm 0.05$ and $c/a= 0.52 \pm 0.02$, can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75~h) and the amplitude of its rotation light curve, we derive a density $ρ=0.87 \pm 0.01$~g~cm$^{-3}$ a geometric albedo $p_V= 0.097 \pm 0.009$. A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003~AZ$_{84}$'s limb, that can be interpreted as an abrupt chasm of width $\sim 23$~km and depth $> 8$~km or a smooth depression of width $\sim 80$~km and depth $\sim 13$~km (or an intermediate feature between those two extremes).

astro-ph.EP↗

arXiv2015

Star formation towards the southern Cometary HII region IRAS 17256-3631

IRAS 17256-3631 is a southern Galactic massive star forming region located at a distance of 2 kpc. In this paper, we present a multiwavelength investigation of the embedded cluster, the HII region, as well as the parent cloud. Radio images at 325, 610 and 1372 MHz were obtained using GMRT, India while the near-infrared imaging and spectroscopy were carried out using UKIRT and Mt. Abu Infrared Telescope, India. The near-infrared K-band image reveals the presence of a partially embedded infrared cluster. The spectral features of the brightest star in the cluster, IRS-1, spectroscopically agrees with a late O or early B star and could be the driving source of this region. Filamentary H_2 emission detected towards the outer envelope indicates presence of highly excited gas. The parent cloud is investigated at far-infrared to millimeter wavelengths and eighteen dust clumps have been identified. The spectral energy distributions (SEDs) of these clumps have been fitted as modified blackbodies and the best-fit peak temperatures are found to range from 14-33 K, while the column densities vary from 0.7-8.5x10^22 cm^-2. The radio maps show a cometary morphology for the distribution of ionized gas that is density bounded towards the north-west and ionization bounded towards the south-east. This morphology is better explained with the champagne flow model as compared to the bow shock model. Using observations at near, mid and far-infrared, submillimeter and radio wavelengths, we examine the evolutionary stages of various clumps.

astro-ph.GA↗

arXiv2014

Possible spatial asymmetry in semi-regular variable UZ Arietis

Semi-regular variables (SRVs) though closely related to Mira variables, are a less studied class of AGB stars. While asymmetry in the brightness distribution of many Mira variables is fairly well known, it is detected only in a few SRVs. Asymmetry in the brightness distribution at the level of a few milliarcsecond (mas) can be detected by high angular resolution techniques like lunar occultations (LO), long baseline interferometry, and aperture masking interferometry. Multi-epoch LO observations have the potential to detect a departure of brightness profile from spherical symmetry. Each LO event provides a uniform disk (UD) angular diameter along the position angle of the occultation. Any significant difference in the UD angular diameter values of multi-epoch LO observations signifies a brightness asymmetry. In this paper, we report for the first time three epoch UD angular diameter values of a SRV UZ Arietis using the LO technique at 2.2 $μm$. Optical linear polarization of the source observed by us recently is also reported. The asymmetric brightness distribution of UZ Ari suggested by a small difference in the fitted UD values for the three epochs, is discussed in the context of optical polarization exhibited by the source and the direction of polarization axis in the plane of the sky.

astro-ph.SR↗

arXiv2013

Performance Analysis of Clustering Algorithms for Gene Expression Data

Microarray technology is a process that allows thousands of genes simultaneously monitor to various experimental conditions. It is used to identify the co-expressed genes in specific cells or tissues that are actively used to make proteins, This method is used to analysis the gene expression, an important task in bioinformatics research. Cluster analysis of gene expression data has proved to be a useful tool for identifying co-expressed genes, biologically relevant groupings of genes and samples. In this paper we analysed K-Means with Automatic Generations of Merge Factor for ISODATA- AGMFI, to group the microarray data sets on the basic of ISODATA. AGMFI is to generate initial values for merge and Spilt factor, maximum merge times instead of selecting efficient values as in ISODATA. The initial seeds for each cluster were normally chosen either sequentially or randomly. The quality of the final clusters was found to be influenced by these initial seeds. For the real life problems, the suitable number of clusters cannot be predicted. To overcome the above drawback the current research focused on developing the clustering algorithms without giving the initial number of clusters.

cs.CE↗

arXiv2013

Unsupervised Gene Expression Data using Enhanced Clustering Method

Microarrays are made it possible to simultaneously monitor the expression profiles of thousands of genes under various experimental conditions. Identification of co-expressed genes and coherent patterns is the central goal in microarray or gene expression data analysis and is an important task in bioinformatics research. Feature selection is a process to select features which are more informative. It is one of the important steps in knowledge discovery. The problem is that not all features are important. Some of the features may be redundant, and others may be irrelevant and noisy. In this work the unsupervised Gene selection method and Enhanced Center Initialization Algorithm (ECIA) with K-Means algorithms have been applied for clustering of Gene Expression Data. This proposed clustering algorithm overcomes the drawbacks in terms of specifying the optimal number of clusters and initialization of good cluster centroids. Gene Expression Data show that could identify compact clusters with performs well in terms of the Silhouette Coefficients cluster measure.

cs.CE↗

arXiv2013

Analysis of the solar coronal green line profiles from eclipse observations

Analysis of the solar coronal green line profiles reveals information regarding the physical conditions of the solar corona like temperature, density, Doppler velocity, non-thermal velocity etc. It provides insights to the unresolved problems like the coronal heating and the acceleration of the solar winds. Recent studies have reported excess blueshifts in the coronal line profiles and are interpreted as due to nanoflare heating, type II spicules and nascent solar wind flow. We have analyzed a time series of Fabry-Perot interferograms of the solar corona obtained during the total solar eclipse of 2001 June 21 from Lusaka, Zambia. The spatial behavior of the coronal green line profiles were examined and variations in intensity, linewidth, Doppler velocity and line asymmetry were obtained. Several line profiles showed asymmetry indicating the presence of multicomponents. Such line profiles were fitted with double Gaussian curves. It has been found that 42% of the line profiles were single components, 34% were blueshifted, and 24% were red shifted. The secondary component of a typical line profile with blue asymmetry is found to have a relative intensity about 0.26, Doppler velocity around -30 km/s and a halfwidth about 0.65A.

astro-ph.SR↗

arXiv2013

A Novel Approach for Single Gene Selection Using Clustering and Dimensionality Reduction

We extend the standard rough set-based approach to deal with huge amounts of numeric attributes versus small amount of available objects. Here, a novel approach of clustering along with dimensionality reduction; Hybrid Fuzzy C Means-Quick Reduct (FCMQR) algorithm is proposed for single gene selection. Gene selection is a process to select genes which are more informative. It is one of the important steps in knowledge discovery. The problem is that all genes are not important in gene expression data. Some of the genes may be redundant, and others may be irrelevant and noisy. In this study, the entire dataset is divided in proper grouping of similar genes by applying Fuzzy C Means (FCM) algorithm. A high class discriminated genes has been selected based on their degree of dependence by applying Quick Reduct algorithm based on Rough Set Theory to all the resultant clusters. Average Correlation Value (ACV) is calculated for the high class discriminated genes. The clusters which have the ACV value a s 1 is determined as significant clusters, whose classification accuracy will be equal or high when comparing to the accuracy of the entire dataset. The proposed algorithm is evaluated using WEKA classifiers and compared. Finally, experimental results related to the leukemia cancer data confirm that our approach is quite promising, though it surely requires further research.

cs.CE↗

arXiv2013

Verdict Accuracy of Quick Reduct Algorithm using Clustering and Classification Techniques for Gene Expression Data

In most gene expression data, the number of training samples is very small compared to the large number of genes involved in the experiments. However, among the large amount of genes, only a small fraction is effective for performing a certain task. Furthermore, a small subset of genes is desirable in developing gene expression based diagnostic tools for delivering reliable and understandable results. With the gene selection results, the cost of biological experiment and decision can be greatly reduced by analyzing only the marker genes. An important application of gene expression data in functional genomics is to classify samples according to their gene expression profiles. Feature selection (FS) is a process which attempts to select more informative features. It is one of the important steps in knowledge discovery. Conventional supervised FS methods evaluate various feature subsets using an evaluation function or metric to select only those features which are related to the decision classes of the data under consideration. This paper studies a feature selection method based on rough set theory. Further K-Means, Fuzzy C-Means (FCM) algorithm have implemented for the reduced feature set without considering class labels. Then the obtained results are compared with the original class labels. Back Propagation Network (BPN) has also been used for classification. Then the performance of K-Means, FCM, and BPN are analyzed through the confusion matrix. It is found that the BPN is performing well comparatively.

cs.LG↗

arXiv2012

Effective Clustering Algorithms for Gene Expression Data

cs.CE↗

arXiv2011

Performance Analysis of Enhanced Clustering Algorithm for Gene Expression Data

Microarrays are made it possible to simultaneously monitor the expression profiles of thousands of genes under various experimental conditions. It is used to identify the co-expressed genes in specific cells or tissues that are actively used to make proteins. This method is used to analysis the gene expression, an important task in bioinformatics research. Cluster analysis of gene expression data has proved to be a useful tool for identifying co-expressed genes, biologically relevant groupings of genes and samples. In this paper we applied K-Means with Automatic Generations of Merge Factor for ISODATA- AGMFI. Though AGMFI has been applied for clustering of Gene Expression Data, this proposed Enhanced Automatic Generations of Merge Factor for ISODATA- EAGMFI Algorithms overcome the drawbacks of AGMFI in terms of specifying the optimal number of clusters and initialization of good cluster centroids. Experimental results on Gene Expression Data show that the proposed EAGMFI algorithms could identify compact clusters with perform well in terms of the Silhouette Coefficients cluster measure.

cs.LG↗

arXiv2011

Near Infrared Angular Diameters of a few AGB variables by Lunar Occultations

The uniform disk (UD) angular diameter measurements of two oxygen-rich Mira variables (AW Aur and BS Aur) and three semiregular (SRb) variables (GP Tau, RS Cap, RT Cap), in near Infrared K-band (2.2 micron) by lunar occultation observations are reported. UD angular diameters of the two Miras and one SRV are first time measurements. In addition a method of predicting angular diameters from (V-K) colour is discussed and applied to the five sources. The effect of mass-loss enhancing measured K-band diameters is examined for Miras using (K-[12]) colour excess as an index. In our sample the measured angular diameter of one of the Miras (BS Aur) is found enhanced by nearly 40% compared to its expected value, possibly due to mass loss effects leading to formation of a circumstellar shell.

astro-ph.SR↗

arXiv2011

Analysis of the coronal green line profiles: evidence of excess blueshifts

The coronal green line (Fe XIV 5303 A) profiles were obtained from Fabry-Perot interferometric observations of the solar corona during the total solar eclipse of 21 June 2001 from Lusaka, Zambia. The instrumental width is about 0.2 A and the spectral resolution is about 26000. About 300 line profiles were obtained within a radial range of 1.0-1.5 R\odot and position angle coverage of about 240\circ. The line profiles were fitted with single Gaussian and their intensity, Doppler velocity, and line width have been obtained. Also obtained are the centroids of the line profiles which give a measure of line asymmetry. The histograms of Doppler velocity show excess blueshifts while the centroids reveal a pre-dominant blue wing in the line profiles. It has been found that the centroids and the Doppler velocities are highly correlated. This points to the presence of multiple components in the line profiles with an excess of blueshifted components. We have then obtained the(Blue-Red) wing intensity which clearly reveals the second component, majority of which are blueshifted ones. This confirms that the coronal green line profiles often contain multicomponents with excess blueshifts which also depend on the solar activity. The magnitude of the Doppler velocity of the secondary component is in the range 20-40 km s-1 and they show an increase towards poles. The possible explanations of the multicomponents could be the type II spicules which were recently found to have important to the coronal heating or the nascent solar wind flow, but the cause of the blue asymmetry in the coronal lines above the limb remains unclear.

astro-ph.SR↗

arXiv2008

Milliarcsecond angular resolution of reddened stellar sources in the vicinity of the Galactic Center

For the first time, the lunar occultation technique has been employed on a very large telescope in the near-IR with the aim of achieving systematically milliarcsecond resolution on stellar sources. We have demonstrated the burst mode of the ISAAC instrument, using a fast read-out on a small area of the detector to record many tens of seconds of data at a time on fields of few squared arcsec. We have used the opportunity to record a large number of LO events during a passage of the Moon close to the Galactic Center in March 2006. We have developed a data pipeline for the treatment of LO data, including the automated estimation of the main data analysis parameters using a wavelet-based method, and the preliminary fitting and plotting of all light curves. We recorded 51 LO events over about four hours. Of these, 30 resulted of sufficient quality to enable a detailed fitting. We detected two binaries with subarcsec projected separation and three stars with a marginally resolved angular diameter of about 2 mas. Two more SiO masers, were found to be resolved and in one case we could recover the brightness profile of the extended emission, which is well consistent with an optically thin shell. The remaining unresolved stars were used to characterize the performance of the method. The LO technique at a very large telescope is a powerful and efficient method to achieve angular resolution, sensitivity, and dynamic range that are among the best possible today with any technique. The selection of targets is naturally limited and LOs are fixed-time events, however each observation requires only a few minutes including overheads. As such, LOs are ideally suited to fill small gaps of idle time between standard observations.

astro-ph↗

arXiv2005

Angular diameter measurements of evolved variables by lunar occultations at 2.2 and 3.8 micron

We report the angular diameters of two Mira variables (U Ari and Z Sco), three semiregular (SR) and irregular variables (SW Vir, eta Gem and mu Gem) and a supergiant SR variable (TV Gem) by lunar occultation observations in the near-IR broad K-band (2.2 micron). Lunar occultations of eta Gem and mu Gem were also observed for the first time simultaneously in both the K and L' bands, yielding angular diameters at 2.2 and 3.8 micron. Effective temperatures and linear radii are also derived for all the observed sources and compared with earlier measurements. The mode of pulsation of both Mira and SR sources in our sample is discussed.

astro-ph↗

arXiv2002

Determination of Substellar Mass Function of Young Open Clusters Using 2MASS and GSC Data

We present a statistical method to derive the mass functions of open clusters using sky survey data such as the 2 Micron All Sky Survey and the Guide Star Catalogue. We have used this method to derive the mass functions in the stellar/substellar regime of three young, nearby open clusters, namely IC 348, Sigma-Orionis and Pleiades. The mass function in the low mass range (M < 0.5 solar mass) is appreciably flatter than the stellar Salpeter function for all three open clusters. The contribution of objects below 0.5 solar mass to the total mass of the cluster is ~40% and the contribution of objects below 0.08 solar mass to the total is ~4%.

astro-ph↗

arXiv2002

Substellar Mass Function of Young Open Clusters as Determined through a Statistical Approach Using 2MASS and GSC Data

In this paper we present the mass functions in the substellar regime of three young open clusters, IC 348, $σ$ Orionis and Pleiades, as derived using the data from the 2 Micron All Sky Survey (2MASS) catalogue which has a limiting magnitude of K_s ~ 15, and the latest version of the Guide Star Catalogue (GSC) which has a limiting magnitude of F ~ 21 where F refers to the POSS II IIIa-F passband. Based on recent evolutionary models for low mass stars, we have formulated the selection criteria for stars with masses below 0.5 solar mass. Using a statistical approach to correct for the background contamination, we derive the mass function of objects with masses ranging from 0.5 solar mass down to the substellar domain, well below the Hydrogen Burning Mass Limit. The lowest mass bins in our analysis are 0.025, 0.045 and 0.055 solar masses for IC 348, sigma Orionis and Pleiades, respectively. The resultant slopes of the mass functions are 0.8 +/- 0.2 for IC 348, 1.2 +/- 0.2 for sigma Orionis and 0.5 +/- 0.2 for Pleiades, which are consistent with the previous results. The contribution of objects below 0.5 solar mass to the total mass of the cluster is ~40%, and the contribution of objects below 0.08 solar mass to the total mass is ~4%.

astro-ph↗