• K. Duorah

      Articles written in Journal of Astrophysics and Astronomy

    • A Comparative Study on SN II Progenitors for the Synthesis of Li7 and B11 with the help of Neutrinos

      N. Lahkar S. Kalita H. L. Duorah K. Duorah

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      The synthesis of Li7 and B11 confronts astrophysicists. Type II (SN II) and Type Ic (SN Ic) supernovae are supposed to be the producers of these two elements. In this study we calculate the yields of these two elements for SN II progenitors with 8, 10 and 20 solar masses. The process considered here is the neutral current interaction of heavy flavour neutrinos (𝜈𝜇 or 𝜈𝜏) with He4 nuclei of the helium zone. The low mass progenitors are considered because the helium zone lies much closer to the core and hence experiences large neutrino flux. The starting point of the helium zone depends on detail stellar model. However, the shell radius at which it begins is available for these stars. 20 solar mass is considered for comparison of our production ratio Li/B with that of an earlier work. It is contrasted with the shock heating yields in the hydrogen envelope. The Li/B ratio has been found to be about 0.96. In the three model stars, the Li7 and B11 yields are found to be in the range 6.61×10−6 −2.63×10−6 𝑀Sun and 6.92×10−6 −2.75×10−6 𝑀Sun respectively as we go from 8 to 20 𝑀Sun. Some equivalence is found with shock induced nucleosynthesis model for SN II. The SN II yield is found to be compatible with that of hypernovae produced by C–O core collapse but higher than the yields obtained by neutrino processes in SNIc.

    • Inverse Compton Gamma Rays from Dark Matter Annihilation in the Dwarf Galaxies

      Jayashri Medhi H. L. Duorah A. G. Barua K. Duorah

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      Dwarf spheroidal (dSph) galaxies are thought to be good candidates for dark matter search due to their high mass-to-light (M/L) ratio. One of the most favored dark matter candidates is the lightest neutralino(neutral $\chi$ particle) as predicted in the Minimal Supersymmetric Standard Model (MSSM). In this study, we model the gamma ray emission from dark matter annihilation coming from the nearby dSph galaxies Draco, Segue 1, Ursa Minor and Willman 1, taking into account the contribution from prompt photons and photons produced from inverse Compton scattering off starlight and Cosmic Microwave Background (CMB) photons by the energetic electrons and positrons from dark matter annihilation. We also compute the energy spectra of electrons and positrons from the decay of dark matter annihilation products. Gamma ray spectra and fluxes for both prompt and inverse Compton emission have been calculated for neutralino annihilation over a range of masses and found to be in agreement with the observed data. It has been found that the ultra faint dSph galaxy Segue 1 gives the largest gamma ray flux limits while the lowest gamma ray flux limits has been obtained from Ursa Minor. It is seen that for larger M/L ratio of dwarf galaxies the intensity pattern originating from $e^+e^−-$ pairs scattering off CMB photons is separated by larger amount from that off the starlight photons for the same neutralino mass. As the $e^+e^−-$ energy spectra have an exponential cut off at high energies, this may allow to discriminate some dark matter scenarios from other astrophysical sources. Finally, some more detailed study about the effect of inverse Compton scattering may help constrain the dark matter signature in the dSph galaxies.

    • Abundances of La138 and Ta180 Through ν-Nucleosynthesis in $20 M_\odot$ Type II Supernova Progenitor, Guided by Stellar Models for Seeds

      N. Lahkar S. Kalita H. L. Duorah K. Duorah

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      Yields of nature’s rarest isotopes La^{138} and Ta^{180} are calculated by neutrino processes in the Ne-shell of density $\rho ≈ 10^4 g/cc$ in a type II supernova (SN II) progenitor of mass 20 $M_\odot$. Two extended sets of neutrino temperature $- T_{\nue}$ = 3, 4, 5, 6 MeV and $T_{\nu(\mu/\tau)}$= 4, 6, 8, 10, 12 MeV respectively for charged and neutral current processes are taken. Solar mass fractions of the seeds La139, Ta181, Ba138 and Hf180 are taken for calculation. They are assumed to be produced in some s-processing events of earlier generation massive ‘seed stars’ with average interior density range $\langleρ\rangle \approx 10^3−10^6 g/cc$. The abundances of these two elements are calculated relative to O16 and are found to be sensitive to the neutrino temperature. For neutral current processes with the neutron emission branching ratio, $b_n = 3.81 \times 10^{-4}$ and $b_n = 9.61 \times 10^{−1}$, the relative abundances of La138 lie in the ranges $4.48 \times 10^{−14}−2.94 \times 10^{−13}$ and $1.13 \times 10^{−10} − 7.43 \times 10^{−10}$ respectively. Similarly, the relative abundances of Ta180 lie in the ranges $1.80 \times 10^{−15} − 1.17 \times 10^{−14}$ and 4.53 \times 10^{−12}−2.96 × 10^{−11} respectively for the lower and higher values of the neutron emission branching ratio. For charged current processes, the relative abundances of La138 and Ta180 are found to be in the ranges $1.38 \times 10^{−9} − 7.62 \times 10^{−9}$ and $2.09 \times 10^{-11} − 1.10 \times 10^{−10}$ respectively. Parametrized by density of the ‘seed stars’, the yields are found to be consistent with recent supernova simulation results throughout the range of neutrino temperatures. La138 and Ta180 are found to be efficiently produced in charged current interaction.

    • Contribution of Proton Capture Reactions to the Ascertained Abundance of Fluorine in the Evolved Stars of Globular Cluster M4, M22, 47 Tuc and NGC 6397

      Upakul Mahanta Aruna Goswami H. L. Duorah K. Duorah

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      The origin of the abundance pattern and also the (anti)correlation present among the elements found in stars of globular clusters (GCs) remains unimproved until date. The proton-capture reactions are presently recognised in concert of the necessary candidates for that sort of observed behaviour in the second generation stars. We tend to propose a reaction network of a nuclear cycle namely carbon–nitrogen–oxygen–fluorine (CNOF) at evolved stellar condition since fluorine (19F) is one such element which gets plagued by proton capture reactions. The stellar temperature thought about here ranges from 2×107 to 10×107 K and there has been an accretion occuring, with material density being 102 g/cm3 and 103 g/cm3. Such kind of temperature density conditions are probably going to be prevailing within the H-burning shell of evolved stars. The estimated abundances of 19F are then matched with the info that has been determined for a few some metal-poor giants of GC M4, M22,47 Tuc as well as NGC 6397. As far as the comparison between the observed and calculated abundances is concerned, it is found that the abundance of 19F have shown an excellent agreement with the observed abundances with a correlation coefficent above 0.9, supporting the incidence of that nuclear cycle at the adopted temperature density conditions.

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