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Aguirre V.S.,University of Aarhus | Ruchti G.R.,Lund Observatory | Hekker S.,Max Planck Institute for Solar System Research | Hekker S.,University of Amsterdam | And 10 more authors.
Astrophysical Journal Letters | Year: 2014

About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low- and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this Letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li production most likely took place through non-canonical mixing at the RGB tip, possibly during the helium flash. © 2014. The American Astronomical Society. All rights reserved..

Bhattacharyya S.,Bhabha Atomic Research Center | Godambe S.,Bhabha Atomic Research Center | Bhatt N.,Bhabha Atomic Research Center | Mitra A.,Bhabha Atomic Research Center | Choudhury M.,Homi Bhabha Center for Science Education
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

We have recently studied LS 5039, a gamma-ray binary, with the Giant Metrewave Radio Telescope during its periastron and apastron passage. The results presented here show that the spectra are inverted at low frequency and the flux densities do not differ significantly for two different orbital phases. Assuming that the free-free absorption of radio in the stellar wind environment is responsible for the optically thick radio emission, we calculated the free-free absorption optical depth and constrained the height of the radio-emitting region from the orbital plane. The height is found to be around 1.6 au for a spherical stellar wind geometry. This estimate may change if the stellar wind is focused or the radio absorption is due to synchrotron self-absorption. © 2011 The Authors Monthly Notices of the Royal Astronomical Society. © 2011 RAS.

Mukhopadhyay S.,Bhabha Atomic Research Center | Biswas D.C.,Bhabha Atomic Research Center | Tandel S.K.,Center for Excellence in Basic science | Danu L.S.,Bhabha Atomic Research Center | And 8 more authors.
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

A high-spin study of the shape transitional nucleus 188Pt reveals the unusual coexistence of both shape- and K-isomeric states. Reduced B(E2) transition probabilities for decays from these states inferred from the data clearly establish their hindered character. In addition to other excited structures, a rotational band built upon the K isomer is identified, and its configuration has been assigned through an analysis of alignments and branching ratios. The shape evolution with spin in this nucleus has been inferred from both experimental observables and cranking calculations. The yrast positive parity structure appears to evolve from a near-prolate deformed shape through triaxial at intermediate excitation, and eventually to oblate at the highest spins. © 2014 The Authors.

Wu T.T.,Harvard University | Wu T.T.,CERN | Martin A.,CERN | Roy S.M.,Homi Bhabha Center for Science Education | Singh V.,Tata Institute of Fundamental Research
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

Recently, André Martin has proved a rigorous upper bound on the inelastic cross sectionσinel at high energy, which is one-fourth of the known Froissart-Martin-Lukaszuk upper bound on σtot. Here, we obtain an upper bound on σinel in terms of σtot and show that the Martin bound on σinel is improved significantly with this added information. © 2011 American Physical Society.

Hekker S.,University of Amsterdam | Elsworth Y.,University of Birmingham | Basu S.,Yale University | Mazumdar A.,Homi Bhabha Center for Science Education | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

Asteroseismology, i.e. the study of the internal structures of stars via their global oscillations, is a valuable tool to obtain stellar parameters such as mass, radius, surface gravity and mean density. These parameters can be obtained using certain scaling relations which are based on an asymptotic approximation. Usually the observed oscillation parameters are assumed to follow these scaling relations. Recently, it has been questioned whether this is a valid approach, i.e. whether the order of the observed oscillation modes is high enough to be approximated with an asymptotic theory. In this work, we use stellar models to investigate whether the differences between observable oscillation parameters and their asymptotic estimates are indeed significant. We compute the asymptotic values directly from the stellar models and derive the observable values from adiabatic pulsation calculations of the same models. We find that the extent to which the atmosphere is included in the models is a key parameter. Considering a larger extension of the atmosphere beyond the photosphere reduces the difference between the asymptotic and observable values of the large frequency separation. Therefore, we conclude that the currently suggested discrepancies in the scaling relations might have been overestimated. Hence, based on the results presented here we believe that the suggestions of Mosser et al. should not be followed without careful consideration. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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