K. Kuppuswamy

Indian Institute of Astrophysics, Bengalūru, Karnātaka, India

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Publications (10)29.41 Total impact

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    ABSTRACT: A search for variable stars in the globular cluster NGC 288 was carried out using a time-series of CCD images in the $V$ and $I$ filters. The photometry of all stellar sources in the field of view of our images, down to $V\approx19$ mag, was performed using difference image analysis (DIA). For stars of $\approx15$ mag, measurement accuracies of $\approx8$ mmag and $\approx10$ mmag were achieved for $V$ and $I$ respectively. Three independent search strategies were applied to the 5525 light curves but no new variables were found above the threshold limits characteristic of our data set. The use of older data from the literature combined with the present data allowed the refinement of the periods of all known variables. Fourier light curve decomposition was performed for the RRab and the RRc stars to obtain an estimate of ${\rm [Fe/H]}_{ZW}=-1.62\pm0.02$ (statistical) $\pm0.14$ (systematic). A true distance modulus of $14.768\pm0.003$ mag (statistical) $\pm0.042$ mag (systematic), or a distance of $8.99\pm0.01$ kpc (statistical) $\pm0.17$ kpc (systematic) was calculated from the RRab star. The RRc star predicts a discrepant distance about one kiloparsec shorter but it is possibly a Blazhko variable. An independent distance from the P--L relationship for SX Phe stars leads to a distance of $8.9\pm0.3$ kpc. The SX Phe stars V5 and V9 are found to be double mode pulsators.
    Acta Astronomica -Warsaw and Cracow- 01/2014; · 2.68 Impact Factor
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    ABSTRACT: table3.dat file contains the time-series V, R and I photometry for all the RR Lyrae, long-period, SX Phe variables and candidates in NGC 7492. We list standard and instrumental magnitudes and their uncertainties corresponding to the variable star identification, filter, and epoch of mid-exposure. For completeness, we also list the reference flux, difference flux, and photometric scale factor, along with the uncertainties on the reference and difference fluxes. Observations were made from 2004/10/04 to 2012/06/29 with the 2.0m telescope of the Indian Astronomical Observatory (IAO) at Hanle, India, using the Johnson-Kron-Cousins V, R, and I filters. (2 data files).
    VizieR Online Data Catalog. 07/2013;
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    ABSTRACT: We report CCD $V$ and $I$ time-series photometry of the globular cluster NGC 6333 (M9). The technique of difference image analysis has been used, which enables photometric precision better than 0.05 mag for stars brighter than $V \sim 19.0$ mag, even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RRc stars, three eclipsing binaries, seven long-term variables and one field RRab star behind the cluster. A detailed identification chart and equatorial coordinates are given for all the variable stars in the field of our images of the cluster. Our data together with literature $V$-data obtained in 1994 and 1995 allowed us to refine considerably the periods for all RR Lyrae stars. The nature of the new variables is discussed. We argue that variable V12 is a cluster member and an Anomalous Cepheid. Secular period variations, double mode pulsations and/or the Blazhko-like modulations in some RRc variables are addressed. Through the light curve Fourier decomposition of 12 RR Lyrae stars we have calculated a mean metallicity of [Fe/H]$_{\rm ZW}$=$-1.70 \pm 0.01{\rm(statistical)} \pm 0.14{\rm(systematic)}$ or [Fe/H]$_{UVES}=-1.67 \pm 0.01{\rm(statistical)} \pm 0.19{\rm(systematic)}$.Absolute magnitudes, radii and masses are also estimated for the RR Lyrae stars. A detailed search for SX Phe stars in the Blue Straggler region was conducted but none were discovered. If SX Phe exist in the cluster then their amplitudes must be smaller than the detection limit of our photometry. The CMD has been corrected for heavy differential reddening using the detailed extinction map of the cluster of Alonso-Garc\'ia et al. (2012). This has allowed us to set the mean cluster distance from two independent estimates; from the RRab and RRc absolute magnitudes, we find $8.04\pm 0.19$ kpc and $7.88\pm0.30$ kpc respectively.
    Monthly Notices of the Royal Astronomical Society 06/2013; 434(2). · 5.52 Impact Factor
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    ABSTRACT: We have performed a photometric V, R, I CCD time-series analysis with a baseline of ~8 years of the outer-halo globular cluster NGC 7492 with the aim of searching for new variables and using these (and the previously known variables) to determine the physical parameters of interest for the cluster (e.g. metallicity, absolute magnitude of the horizontal branch, distance, etc.). We use difference image analysis (DIA) to extract precise light curves in the relatively crowded star field, especially towards the densely populated central region. Several approaches are used for variability detection that recover the known variables and lead to new discoveries. We determine the physical parameters of the only RR0 star using light curve Fourier decomposition analysis. We find one new long period variable and two SX Phe stars in the blue straggler region. We also present one candidate SX Phe star which requires follow-up observations. Assuming that the SX Phe stars are cluster members and using the period-luminosity relation for these stars, we estimate their distances as ~25.2+-1.8 and 26.8+-1.8 kpc, and identify their possible modes of oscillation. We refine the periods of the two RR Lyrae stars in our field of view. We find that the RR1 star V2 is undergoing a period change and possibly exhibits the Blazhko effect. Fourier decomposition of the light curve of the RR0 star V1 allows us to estimate the metallicity [Fe/H]_ZW-1.68+-0.10 or [Fe/H]_UVES-1.64+-0.13, log-luminosity ~1.76+-0.02, absolute magnitude ~0.38+-0.04 mag, and true distance modulus of ~16.93+-0.04 mag, which is equivalent to a distance of ~24.3+-0.5 kpc. All of these values are consistent with previous estimates in the literature.
    Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor
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    ABSTRACT: Time-series V and I photometry for all the RR Lyrae stars in our field of view. We list standard and instrumental magnitudes and their uncertainties corresponding to the variable star identification, filter, and epoch of mid-exposure. For completeness, we also list the reference flux, difference flux, and photometric scale factor, along with the uncertainties on the reference and difference fluxes. (3 data files).
    VizieR Online Data Catalog. 01/2013;
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    ABSTRACT: V, I and r time-series photometry and difference imaging parameters for all confirmed variables in NGC 1904. (2 data files).
    VizieR Online Data Catalog. 11/2012;
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    ABSTRACT: We present the analysis of 11 nights of V and I time-series observations of the globular cluster NGC 1904 (M 79). Using this we searched for variable stars in this cluster and attempted to refine the periods of known variables, making use of a time baseline spanning almost 8 years. We use our data to derive the metallicity and distance of NGC 1904. We used difference imaging to reduce our data to obtain high-precision light curves of variable stars. We then estimated the cluster parameters by performing a Fourier decomposition of the light curves of RR Lyrae stars for which a good period estimate was possible. We also derive an estimate for the age of the cluster by fitting theoretical isochrones to our colour-magnitude diagram (CMD). Out of 13 stars previously classified as variables, we confirm that 10 are bona fide variables. We cannot detect variability in one other within the precision of our data, while there are two which are saturated in our data frames, but we do not find sufficient evidence in the literature to confirm their variability. We also detect a new RR Lyrae variable, giving a total number of confirmed variable stars in NGC 1904 of 11. Using the Fourier parameters, we find a cluster metallicity [Fe/H]_ZW=-1.63 +- 0.14, or [Fe/H]_UVES=-1.57 \pm 0.18, and a distance of 13.3 +- 0.4 kpc (using RR0 variables) or 12.9 kpc (using the one RR1 variable in our sample for which Fourier decomposition was possible).
    Astronomy and Astrophysics 10/2012; · 5.08 Impact Factor
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    ABSTRACT: The observations employed in the present work were performed using the Johnson V and I filters on 2009 April 17, 18 and 19, on 2010 January 22 and 23, February 21 and 22, March 7 and 8, April 7 and 25, and May 6. A total of 177 epochs in the V filter and 184 in the I filter were obtained. The 2.0-m telescope of the Indian Astronomical Observatory (IAO), Hanle, India, located at 4500 m above sea level, was used. The estimated seeing was ~1arcsec. The detector was a Thompson CCD of 2048x2048pixels with a pixel scale of 0.296arcsec/pix and a field of view of approximately 10.1x10.1arcmin2. (5 data files).
    VizieR Online Data Catalog. 04/2012;
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    ABSTRACT: We report the discovery of amplitude and phase modulations typical of the Blazhko effect in 22 RRc and 9 RRab type RR Lyrae stars in NGC 5024 (M53). This brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab, that represent 66% and 37% of the total population of RRc and RRab stars in the cluster respectively, making NGC 5024 the globular cluster with the largest presently known population of Blazhko RRc stars. We place a lower limit on the overall incidence rate of the Blazhko effect among the RR Lyrae population in this cluster of 52%. New data have allowed us to refine the pulsation periods. The limitations imposed by the time span and sampling of our data prevents reliable estimations of the modulation periods. The amplitudes of the modulations range between 0.02 and 0.39 mag. The RRab and RRc are neatly separated in the CMD, and the RRc Blazhko variables are on averge redder than their stable couterparts; these two facts may support the hypothesis that the HB evolution in this cluster is towards the red and that the Blazhko modulations in the RRc stars are connected with the pulsation mode switch.
    Monthly Notices of the Royal Astronomical Society 11/2011; · 5.52 Impact Factor
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    ABSTRACT: We report CCD V and I time series photometry of the globular cluster NGC 5024 (M53). The technique of difference image analysis has been used which enables photometric precisions better than 10 mmag for stars brighter than V∼ 18.5 mag even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RR1 stars and 13 SX Phe stars. A detailed identification chart is given for all the variable stars in the field of our images of the cluster. Periodicities were calculated for all RR Lyrae and SX Phe stars and a critical comparison is made with previous determinations.Out of the four probable SX Phe variables reported by Dékány & Kovács, the SX Phe nature is confirmed only for V80; V81 is an unlikely case while V82 and V83 remain as dubious cases. Previous misidentifications of three variables are corrected. Astrometric positions with an uncertainty of ∼0.3 arcsec are provided for all variables.The light-curve Fourier decomposition of RR0 and RR1 is discussed, we find a mean metallicity of [Fe/H]=−1.92 ± 0.06 in the scale of Zinn & West from 19 RR0 stars. The true distance moduli 16.36 ± 0.05 and 16.28 ± 0.07 and the corresponding distances 18.7 ± 0.4 and 18.0 ± 0.5 kpc are found from the RR0 and RR1 stars, respectively. These values are in agreement with the theoretical period–luminosity relations for RR Lyrae stars in the I band and with recent luminosity determinations for the RR Lyrae stars in the Large Magellanic Cloud (LMC).The age of 13.25 ± 0.50 Gyr, for NGC 5024, E(B − V) = 0.02 and the above physical parameters of the cluster, as indicated from the RR0 stars, produce a good isochrone fitting to the observed colour–magnitude diagram (CMD).The period–luminosity (PL) relation for SX Phe stars in NGC 5024 in the V and I bands is discussed in the light of the 13 newly found SX Phe stars, and their pulsation mode is identified in most cases.
    Monthly Notices of the Royal Astronomical Society 09/2011; 416(3):2265 - 2284. · 5.52 Impact Factor