http://hdl.handle.net/2248/2 Search the Channel search http://prints.iiap.res.in/simple-search http://hdl.handle.net/2248/5652 Title: The unusually large population of Blazhko variables in the globular cluster NGC 5024 (M53)<br/><br/>Authors: Arellano, F. A; Bramich, D. M; Figuera Jaimes, R; Giridhar, S; Kuppuswamy, K<br/><br/>Abstract: We report the discovery of amplitude and phase modulations typical of the Blazhko effect in 22 RRc and nine RRab type RR Lyrae stars in NGC 5024 (M53). This brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab stars, which represent 66 and 37 per cent 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 of 52 per cent on the overall incidence rate of the Blazhko effect among the RR Lyrae population in this cluster. New data have allowed us to refine the pulsation periods. The limitations imposed by the time span and sampling of our data prevent 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 colour–magnitude diagram, and the RRc Blazhko variables are on average redder than their stable counterparts; these two facts may support the hypothesis that the horizontal branch (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.<br/><br/>Description: Open Access Mon, 28 Nov 2011 22:58:59 GMT http://hdl.handle.net/2248/5651 Title: Carbon-enhanced Metal-poor Stars in the Inner and Outer Halo Components of the Milky Way<br/><br/>Authors: Carollo, D; Beers, T. C; Bovy, Jo; Sivarani, T; Norris, J. E; Freeman, K. C; Aoki, W; Lee, Young Sun; Kennedy, C. R<br/><br/>Abstract: Carbon-enhanced metal-poor (CEMP) stars in the halo components of the Milky Way are explored, based on accurate determinations of the carbon-to-iron ([C/Fe]) abundance ratios and kinematic quantities for over 30,000 calibration stars from the Sloan Digital Sky Survey. Using our present criterion that low-metallicity stars exhibiting [C/Fe] ratios ("carbonicity") in excess of [C/Fe] =+0.7 are considered CEMP stars, the global frequency of CEMP stars in the halo system for [Fe/H] <–1.5 is 8%, for [Fe/H] <–2.0 it is 12%, and for [Fe/H] <–2.5 it is 20%. We also confirm a significant increase in the level of carbon enrichment with declining metallicity, growing from lang[C/Fe]rang ~+1.0 at [Fe/H] =–1.5 to lang[C/Fe]rang ~+1.7 at [Fe/H] =–2.7. The nature of the carbonicity distribution function (CarDF) changes dramatically with increasing distance above the Galactic plane, |Z|. For |Z| <5 kpc, relatively few CEMP stars are identified. For distances |Z| >5 kpc, the CarDF exhibits a strong tail toward high values, up to [C/Fe] > +3.0. We also find a clear increase in the CEMP frequency with |Z|. For stars with –2.0 < [Fe/H] <–1.5, the frequency grows from 5% at |Z| ~2 kpc to 10% at |Z| ~10 kpc. For stars with [Fe/H] <–2.0, the frequency grows from 8% at |Z| ~2 kpc to 25% at |Z| ~10 kpc. For stars with –2.0 < [Fe/H] <–1.5, the mean carbonicity is lang[C/Fe]rang ~+1.0 for 0 kpc < |Z| < 10 kpc, with little dependence on |Z|; for [Fe/H] <–2.0, lang[C/Fe]rang ~+1.5, again roughly independent of |Z|. Based on a statistical separation of the halo components in velocity space, we find evidence for a significant contrast in the frequency of CEMP stars between the inner- and outer-halo components—the outer halo possesses roughly twice the fraction of CEMP stars as the inner halo. The carbonicity distribution also differs between the inner-halo and outer-halo components—the inner halo has a greater portion of stars with modest carbon enhancement ([C/Fe] ~+0.5]); the outer halo has a greater portion of stars with large enhancements ([C/Fe] ~+2.0), although considerable overlap still exists. We interpret these results as due to the possible presence of additional astrophysical sources of carbon production associated with outer-halo stars, beyond the asymptotic giant-branch source that may dominate for inner-halo stars, with implications for the progenitors of these populations.<br/><br/>Description: Restricted Access Mon, 09 Jan 2012 22:58:59 GMT http://hdl.handle.net/2248/5650 Title: The Three-dimensional Structure of the Small Magellanic Cloud<br/><br/>Authors: Subramanian, S; Subramaniam, A<br/><br/>Abstract: The three-dimensional structure of the inner Small Magellanic Cloud (SMC) is investigated using the red clump (RC) stars and the RR Lyrae stars (RRLS), which represent the intermediate-age and the old stellar populations of a galaxy. The V- and I-band photometric data from the OGLE III catalog are used for our study. The mean dereddened I 0 magnitude of the RC stars and the RRLS are used to study the relative positions of the different regions in the SMC with respect to the mean SMC distance. This shows that the northeastern part of the SMC is closer to us. The line-of-sight depth (front to back distance) across the SMC is estimated using the dispersion in the I 0 magnitudes of both the RC stars and the RRLS and found to be large (~14 kpc) for both populations. The similarity in their depth distribution suggests that both of these populations occupy a similar volume of the SMC. The surface density distribution and the radial density profile of the RC stars suggest that they are more likely to be distributed in a nearly spheroidal system. The tidal radius estimated for the SMC system is ~7-12 kpc. An elongation along the NE-SW direction is seen in the surface density map of the RC stars. The surface density distribution of the RRLS in the SMC is nearly circular. Based on all of the above results the observed structure of the SMC, in which both the RC stars and RRLS are distributed, is approximated as a triaxial ellipsoid. The parameters of the ellipsoid are obtained using the inertia tensor analysis. We estimated the axes ratio, inclination of the longest axis with the line of sight (i), and the position angle (phi) of the longest axis of the ellipsoid on the sky from the analysis of the RRLS. The analysis of the RC stars with the assumption that they are extended up to a depth of 3.5 times the sigma (width of dereddened I 0 magnitude distribution, corrected for intrinsic spread and observational errors) was also found to give similar axes ratio and orientation angles. The above estimated parameters depend on the data coverage of the SMC. Using the RRLS with equal coverage in all three axes (data within 3° in X-, Y-, and Z-axes), we estimated an axes ratio of 1:1.33:1.61 with i = 2fdg6 and phi = 70fdg2. Our tidal radius estimates and the recent observational studies suggest that the full extent of the SMC in the XY plane is of the order of the front to back distance estimated along the line of sight. These results suggest that the structure of the SMC is spheroidal or slightly ellipsoidal. We propose that the SMC experienced a merger with another dwarf galaxy at ~4-5 Gyr ago, and the merger process was completed in another 2-3 Gyr. This resulted in a spheroidal distribution comprising stars older than 2 Gyr.<br/><br/>Description: Restricted Access Mon, 09 Jan 2012 22:58:59 GMT http://hdl.handle.net/2248/5649 Title: High Angular Resolution Radio Observations of a Coronal Mass Ejection Source Region at Low Frequencies during a Solar Eclipse<br/><br/>Authors: Ramesh, R; Kathiravan, C; Barve, Indrajit V; Rajalingam, M<br/><br/>Abstract: We carried out radio observations of the solar corona in the frequency range 109-50 MHz during the annular eclipse of 2010 January 15 from the Gauribidanur Observatory, located about 100 km north of Bangalore in India. The radio emission in the above frequency range originates typically in the radial distance range ≈1.2-1.5 R ☉ in the "undisturbed" solar atmosphere. Our analysis indicates that (1) the angular size of the smallest observable radio source (associated with a coronal mass ejection in the present case) is ≈1' ± 0farcm3, (2) the source size does not vary with radial distance, (3) the peak brightness temperature of the source corresponding to the above size at a typical frequency like 77 MHz is ≈3 × 109 K, and (4) the coronal magnetic field near the source region is ≈70 mG.<br/><br/>Description: Restricted Access Mon, 09 Jan 2012 22:58:59 GMT