arXiv subjects

Jameel-Un Nabi

Publications and source records attributed to Jameel-Un Nabi.

At least 91 records · Page 5Linked to original sources

arXiv2014

Neutrino and anti-neutrino energy loss rates due to iron isotopes suitable for core-collapse simulations

Accurate estimate of neutrino energy loss rates are needed for the study of the late stages of the stellar evolution, in particular for cooling of neutron stars and white dwarfs. The energy spectra of neutrinos and antineutrinos arriving at the Earth can also provide useful information on the primary neutrino fluxes as well as neutrino mixing scenario (it is to be noted that these supernova neutrinos are emitted after the supernova explosion which is a much later stage of stellar evolution than that considered in this paper). Recently an improved microscopic calculation of weak-interaction mediated rates for iron isotopes was introduced using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. Here I present for the first time the fine-grid calculation of the neutrino and anti-neutrino energy loss rates due to $^{54,55,56}$Fe in stellar matter. In the core of massive stars isotopes of iron, $^{54,55,56}$Fe, are considered to be key players in decreasing the electron-to-baryon ratio ($Y_{e}$) mainly via electron capture on these nuclide. Core-collapse simulators may find this calculation suitable for interpolation purposes and for necessary incorporation in the stellar evolution codes. The calculated cooling rates are also compared with previous calculations.

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arXiv2014

Stellar $β^{\pm}$ decay rates of iron isotopes and its implications in astrophysics

$β$-decay and positron decay are believed to play a consequential role during the late phases of stellar evolution of a massive star culminating in a supernova explosion. Recently the microscopic calculation of weak-interaction mediated rates on key isotopes of iron was introduced using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory with improved model parameters. Here I discuss in detail the improved calculation of $β^{\pm}$ decay rates for iron isotopes ($^{54,55,56}$Fe) in stellar environment. The pn-QRPA theory allows a microscopic "state-by-state" calculation of stellar rates as explained later in text. Excited state Gamow-Teller distributions are much different from ground state and a microscopic calculation of decay rates from these excited states greatly increases the reliability of the total decay rate calculation specially during the late stages of stellar evolution. The reported decay rates are also compared with earlier calculations. The positron decay rates are in reasonable agreement with the large-scale shell model calculation. The main finding of this work includes that the stellar $β$-decay rates of $^{54,55,56}$Fe are around 3 -- 5 orders of magnitude smaller than previously assumed and hence irrelevant for the determination of the evolution of $Y_{e}$ during the presupernova phase of massive stars. The current work discourages the inclusion of $^{55,56}$Fe in the list of key stellar $β$-decay nuclei as suggested by former simulation results.

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arXiv2014

Nickel isotopes in stellar matter

Isotopes of nickel play a key role during the silicon burning phase up to the presupernova phase of massive stars. Electron capture rates on these nickel isotopes are also important during the phase of core contraction. I present here the microscopic calculation of ground and excited states Gamow-Teller (GT) strength distributions for key nickel isotopes. The calculation is performed within the frame-work of pn-QRPA model. A judicious choice of model parameters, specially of the Gamow-Teller strength parameters and the deformation parameter, resulted in a much improved calculation of GT strength functions. The excited state GT distributions are much different from the corresponding ground-state distributions resulting in a failure of the Brink's hypothesis. The electron capture and positron decay rates on nickel isotopes are also calculated within the framework of pn-QRPA model relevant to the presupernova evolution of massive stars. The electron capture rates on odd-A isotopes of nickel are shown to have dominant contributions from parent excited states during as early as silicon burning phases. Comparison is being made with the large scale shell model calculation. During the silicon burning phases of massive stars the electron capture rates on $^{57,59}$Ni are around an order of magnitude bigger than shell model rates and can bear consequences for core-collapse simulators.

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arXiv2014

Electron and Positron Capture Rates on $\bf{^{55}}$Co in Stellar Matter

Cobalt-55 is not only present in abundance in presupernova phase but is also advocated to play a decisive role in the core collapse of massive stars. The spectroscopy of electron capture and emitted neutrinos yields useful information on the physical conditions and stellar core composition. B(GT) values to low-lying states are calculated microscopically using the pn-QRPA theory. Our rates are enhanced compared to the shell model rates. The enhancement is attributed partly to the liberty of selecting a huge model space, allowing consideration of many more parent excited states in our rate calculation. Unlike previous calculations, the so-called Brink's hypothesis is not assumed leading to a more realistic estimate of the rates. The electron and positron capture rates are calculated over a wide temperature and density grid.

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arXiv2014

Detailed microscopic calculation of stellar electron and positron capture rates on $^{24}$Mg for O+Ne+Mg core simulations

Few white dwarfs, located in binary systems, may acquire sufficiently high mass accretion rates resulting in the burning of carbon and oxygen under nondegenerate conditions forming a O+Ne+Mg core. These O+Ne+Mg cores are gravitationally less bound than more massive progenitor stars and can release more energy due to the nuclear burning. They are also amongst the probable candidates for low entropy r-process sites. Recent observations of subluminous Type II-P supernovae (e.g., 2005cs, 2003gd, 1999br, 1997D) were able to rekindle the interest in 8 -- 10 M$_{\odot}$ which develop O+Ne+Mg cores. Microscopic calculations of capture rates on $^{24}$Mg, which may contribute significantly to the collapse of O+Ne+Mg cores, using shell model and proton-neutron quasiparticle random phase approximation (pn-QRPA) theory, were performed earlier and comparisons made. Simulators, however, may require these capture rates on a fine scale. For the first time a detailed microscopic calculation of the electron and positron capture rates on $^{24}$Mg on an extensive temperature-density scale is presented here. This type of scale is more appropriate for interpolation purposes and of greater utility for simulation codes. The calculations are done using the pn-QRPA theory using a separable interaction. The deformation parameter, believed to be a key parameter in QRPA calculations, is adopted from experimental data to further increase the reliability of the QRPA results. The resulting calculated rates are up to a factor of 14 or more enhanced as compared to shell model rates and may lead to some interesting scenario for core collapse simulators.

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arXiv2014

Expanded calculation of weak-interaction mediated neutrino cooling rates due to $^{56}$Ni in stellar matter

Accurate estimate of the neutrino cooling rates is required in order to study the various stages of stellar evolution of massive stars. Neutrino losses from proto-neutron stars play a crucial role in deciding whether these stars would be crushed into black holes or explode as supernovae. Both pure leptonic and weak-interaction processes contribute to the neutrino energy losses in stellar matter. At low temperatures and densities, characteristic of the early phase of presupernova evolution, cooling through neutrinos produced via the weak-interaction is important. Proton-neutron quasi-particle random phase approximation (pn-QRPA) theory has recently being used for calculation of stellar weak-interaction rates of $fp$-shell nuclide with success. The lepton-to-baryon ratio ($Y_{e}$) during early phases of stellar evolution of massive stars changes substantially alone due to electron captures on $^{56}$Ni. The stellar matter is transparent to the neutrinos produced during the presupernova evolution of massive stars. These neutrinos escape the site and assist the stellar core in maintaining a lower entropy. Here I present the expanded calculation of weak-interaction mediated neutrino and antineutrino cooling rates due to $^{56}$Ni in stellar matter using the pn-QRPA theory. This detailed scale is appropriate for interpolation purposes and of greater utility for simulation codes. The calculated rates are compared against earlier calculations. During the relevant temperature and density regions of stellar matter the reported rates show little differences with the shell model rates and might contribute in fine-tuning of the lepton-to-baryon ratio during the presupernova phases of stellar evolution of massive stars.

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arXiv2014

Unique first-forbidden $β$-decay rates for neutron-rich nickel isotopes in stellar environment

In astrophysical environments, allowed Gamow-Teller (GT) transitions are important, particularly for $β$-decay rates in presupernova evolution of massive stars, since they contribute to the fine-tuning of the lepton-to-baryon content of the stellar matter prior to and during the collapse of a heavy star. In environments where GT transitions are unfavored, first-forbidden transitions become important especially in medium heavy and heavy nuclei. Particularly in case of neutron-rich nuclei, first-forbidden transitions are favored primarily due to the phase-space amplification for these transitions. In this work the total $β$-decay half-lives and the unique first-forbidden(U1F) $β$-decay rates for a number of neutron-rich nickel isotopes, $^{72-78}$Ni, are calculated using the proton-neutron quasi-particle random phase approximation (pn-QRPA) theory in stellar environment for the first time. For the calculation of the $β$-decay half-lives both allowed and unique first-forbidden transitions were considered. Comparison of the total half-lives is made with measurements and other theoretical calculations where it was found that the pn-QRPA results are in better agreement with experiments and at the same time are suggestive of inclusion of rank 0 and rank 1 operators in first-forbidden rates for still better results.

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arXiv2013

Three-body calculation of the rate of reaction $p+p+e \rightarrow d+ν_e$ in the Sun

Using expansion of the three-body wave function of the $pep$ system in the initial state on hyperharmonic functions, the rate of the $p+p+e^{-} \rightarrow d+ν_e$ reaction in the Sun is calculated. The results of calculation of the flux at 1 AU are compared with the results of a measurement made by the Borexino collaboration and Bahcall \textit{et al.} theoretical predictions.

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arXiv2013

Comparison of Gamow-Teller strengths in the random phase approximation

The Gamow-Teller response is astrophysically important for a number of nuclides, particularly around iron. The random phase approximation (RPA) is an efficient way to generate strength distributions. In order to better understand both theoretical systematics and uncertainties, we compare the Gamow-Teller strength distributions for a suite of nuclides and for a suite of interactions, including semi-realistic interactions in the $1p$-$0f$ space with the RPA and a separable multi-shell interaction in the quasi-particle RPA. We also compare with experimental results for GT$_-$ on ${54}$Fe.

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arXiv2012

Quantum Gravity effect on the Quark-Gluon Plasma

The Generalized Uncertainty Principle (GUP), which has been predicted by various theories of quantum gravity near the Planck scale is implemented on deriving the thermodynamics of ideal Quark-Gluon Plasma (QGP) consisting of two massless quark flavors at the hadron-QGP phase equilibrium and at a vanishing chemical potential. The effective degrees of freedom and MIT bag pressure are utilized to distinguish between the hadronic and partonic phases. We find that GUP makes a non-negligible contribution to all thermodynamic quantities, especially at high temperatures. The asymptotic behavior of corresponding QGP thermodynamic quantities characterized by the Stephan-Boltzmann limit would be approached, when the GUP approach is taken into consideration.

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arXiv2012

Ground and excited states Gamow-Teller strength distributions of iron isotopes and associated capture rates for core-collapse simulations

This paper reports on the microscopic calculation of ground and excited states Gamow-Teller (GT) strength distributions, both in the electron capture and electron decay direction, for $^{54,55,56}$Fe. The associated electron and positron capture rates for these isotopes of iron are also calculated in stellar matter. These calculations were recently introduced and this paper is a follow-up which discusses in detail the GT strength distributions and stellar capture rates of key iron isotopes. The calculations are performed within the framework of the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. The pn-QRPA theory allows a microscopic \textit{state-by-state} calculation of GT strength functions and stellar capture rates which greatly increases the reliability of the results. For the first time experimental deformation of nuclei are taken into account. In the core of massive stars isotopes of iron, $^{54,55,56}$Fe, are considered to be key players in decreasing the electron-to-baryon ratio ($Y_{e}$) mainly via electron capture on these nuclide. The structure of the presupernova star is altered both by the changes in $Y_{e}$ and the entropy of the core material. Results are encouraging and are compared against measurements (where possible) and other calculations. The calculated electron capture rates are in overall good agreement with the shell model results. During the presupernova evolution of massive stars, from oxygen shell burning stages till around end of convective core silicon burning, the calculated electron capture rates on $^{54}$Fe are around three times bigger than the corresponding shell model rates. The calculated positron capture rates, however, are suppressed by two to five orders of magnitude.

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arXiv2012

Gamow-Teller strength distribution in proton-rich nucleus $^{57}$Zn and its implications in astrophysics

Gamow-Teller (GT) transitions play a preeminent role in the collapse of stellar core in the stages leading to a Type-II supernova. The B(GT) strength distributions for ground and excited states of $^{57}$Zn are calculated in the domain of proton-neutron Quasiparticle Random Phase Approximation (pn-QRPA) theory. No experimental insertions were made (as usually made in other pn-QRPA calculations of B(GT) strength function) to check the performance of the model for proton-rich nuclei. The calculated ground-state B(GT) strength distribution is in good agreement with measurements and shows differences with the earlier reported shell model calculation. The pn-QRPA model reproduced the measured low-lying strength for $^{57}$Zn better in comparison to the KB3G interaction used in the large-scale shell model calculation. The stellar weak rates are sensitive to the location and structure of these low-lying states in daughter $^{57}$Cu. The primary mechanism for producing such nuclei is the rp-process and is believed to be important in the dynamics of the collapsing supermassive stars. Small changes in the binding and excitation energies can lead to significant modifications of the predictions for the synthesis of proton rich isotopes. The $β^{+}$-decay and electron capture (EC) rates on $^{57}$Zn are compared to the seminal work of Fuller, Fowler and Newman (FFN). The pn-QRPA calculated $β^{+}$-decay rates are generally in good agreement with the FFN calculation. However at high stellar temperatures the calculated $β^{+}$-decay rates are almost half of FFN rates. On the other hand, for rp-process conditions, the calculated electron capture ($β^{+}$-decay) rates are bigger than FFN rates by more than a factor 2 (1.5) and may have interesting astrophysical consequences.

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arXiv2012

rp-Process weak-interaction mediated rates of waiting-point nuclei

Electron capture and positron decay rates are calculated for neutron-deficient Kr and Sr waiting point nuclei in stellar matter. The calculation is performed within the framework of pn-QRPA model for rp-process conditions. Fine tuning of particle-particle, particle-hole interaction parameters and a proper choice of the deformation parameter resulted in an accurate reproduction of the measured half-lives. The same model parameters were used to calculate stellar rates. Inclusion of measured Gamow-Teller strength distributions finally led to a reliable calculation of weak rates that reproduced the measured half-lives well under limiting conditions. For the rp-process conditions, electron capture and positron decay rates on $^{72}$Kr and $^{76}$Sr are of comparable magnitude whereas electron capture rates on $^{78}$Sr and $^{74}$Kr are 1--2 orders of magnitude bigger than the corresponding positron decay rates. The pn-QRPA calculated electron capture rates on $^{74}$Kr are bigger than previously calculated. The present calculation strongly suggests that, under rp-process conditions, electron capture rates form an integral part of weak-interaction mediated rates and should not be neglected in nuclear reaction network calculations as done previously.

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arXiv2012

Neutrino cooling rates due to $^{54,55,56}$Fe for presupernova evolution of massive stars

Accurate estimate of neutrino energy loss rates are needed for the study of the late stages of the stellar evolution, in particular for cooling of neutron stars and white dwarfs. Proton-neutron quasi-particle random phase approximation (pn-QRPA) theory has recently being used for a microscopic calculation of stellar weak interaction rates of iron isotopes with success. Here I present the detailed calculation of neutrino and antineutrino cooling rates due to key iron isotopes in stellar matter using the pn-QRPA theory. The rates are calculated on a fine grid of temperature-density scale suitable for core-collapse simulators. The calculated rates are compared against earlier calculations. The neutrino cooling rates due to isotopes of iron are in overall good agreement with the rates calculated using the large-scale shell model. During the presupernova evolution of massive stars, from oxygen shell burning till around end of convective core silicon burning phases, the calculated neutrino cooling rates due to $^{54}$Fe are three to four times larger than the corresponding shell model rates. The Brink's hypothesis used in previous calculations can at times lead to erroneous results. The Brink's hypothesis assumes that the Gamow-Teller strength distributions for all excited states are the same. It is, however, shown by the present calculation that both the centroid and total strength for excited states differ appreciably from the ground state distribution. These changes in the strength distributions of thermally populated excited states can alter the total weak interaction rates rather significantly. The calculated antineutrino cooling rates, due to positron capture and $β$-decay of iron isotopes, are orders of magnitude smaller than the corresponding neutrino cooling rates and can safely be neglected specially at low temperatures and high stellar densities.

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arXiv2011

Coulomb breakup of $^6\rm{Li}$ into $α\,+\,\rm{d}$ in the field of ion $^{208}\rm{Pb}$

The triple differential cross section of $^{208}\rm{Pb}(^6\rm{Li};α,\rm{d})^{208}\rm{Pb}$ elastic Coulomb breakup is calculated using the semiclassical method. We fit the parameters of the Woods-Saxon potential using the experimental $α-\rm{d}$ phase shifts for different states to describe the relative motion of $α$-particle and deuteron. In order to check the validity of the two particle approach for $α-d$ system we apply a potential model to describe the $^2\rm{H}(α,γ)^6\rm{Li}$ radiative capture. Our results for the Coulomb breakup of $^6\rm{Li}$ show large value of the forward-backward asymmetry of the $α$-particle and deuteron emission around zero energy in the $^6\rm{Li}$ center-of-mass (c.m.) system. Comparison of the results of our calculation with experimental data gives evidence for the dominance of the Coulomb dissociation mechanism at the $α-d$ relative energy larger than $E_{α\,d}>300\,\rm{keV}$ and sufficiently big contribution of nuclear distortion at $E_{α\,d}$ near zero energy, but essentially smaller than the value reported in Ref.[F. Hammache et al. Phys.Rev. C82, 065802 (2010)].

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arXiv2011

Fine-Grid Calculations for Stellar Electron and Positron Capture Rates on Fe-Isotopes

The acquisition of precise and reliable nuclear data is a prerequisite to success for stellar evolution and nucleosynthesis studies. Core-collapse simulators find it challenging to generate an explosion from the collapse of the core of massive stars. It is believed that a better understanding of the microphysics of core-collapse can lead to successful results. The weak interaction processes are able to trigger the collapse and control the lepton-to-baryon ratio ($Y_{e}$) of the core material. It is suggested that the temporal variation of $Y_{e}$ within the core of a massive star has a pivotal role to play in the stellar evolution and a fine-tuning of this parameter at various stages of presupernova evolution is the key to generate an explosion. During the presupernova evolution of massive stars, isotopes of iron, mainly $^{54,55,56}$Fe, are considered to be key players in controlling $Y_{e}$ ratio via electron capture on these nuclide. Recently an improved microscopic calculation of weak interaction mediated rates for iron isotopes was introduced using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. The pn-QRPA theory allows a microscopic \textit{state-by-state} calculation of stellar capture rates which greatly increases the reliability of calculated rates. The results were suggestive of some fine-tuning of the $Y_{e}$ ratio during various phases of stellar evolution. Here we present for the first time the fine-grid calculation of the electron and positron capture rates on $^{54,55,56}$Fe. Core-collapse simulators may find this calculation suitable for interpolation purposes and for necessary incorporation in the stellar evolution codes.

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arXiv2011

Neutrino and antineutrino energy loss rates in massive stars due to isotopes of titanium

Weak interaction rates on titanium isotopes are important during the late phases of evolution of massive stars. A search was made for key titanium isotopes from available literature and a microscopic calculation of weak rates of these nuclei were performed using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. Earlier the author presented the stellar electron capture rates on titanium isotopes. In this paper I present the neutrino and antineutrino energy loss rates due to capture and decay rates on isotopes of titanium in stellar environment. Accurate estimate of neutrino energy loss rates are needed for the study of the late stages of the stellar evolution, in particular for cooling of neutron stars and white dwarfs. The results are also compared against previous calculations. At high stellar temperatures the calculated neutrino and antineutrino energy loss rates are bigger by more than two orders of magnitude as compared to the large scale shell model results and favor stellar cores with lower entropies. This study can prove useful for core-collapse simulators.

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arXiv2011

$β$-decay of key titanium isotopes in stellar environment

Amongst iron regime nuclei, $β$-decay rates on titanium isotopes are considered to be important during the late phases of evolution of massive stars. The key $β$-decay isotopes during presupernova evolution were searched from available literature and a microscopic calculation of the decay rates were performed using the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. As per earlier simulation results electron capture and $β$-decay on certain isotopes of titanium are considered to be important for the presupernova evolution of massive stars. Earlier the stellar electron capture rates and neutrino energy loss rates due to relevant titanium isotopes were presented. In this paper we finally present the $β$-decay rates of key titanium isotopes in stellar environment. The results are also compared against previous calculations. The pn-QRPA $β$-decay rates are bigger at high stellar temperatures and smaller at high stellar densities compared to the large scale shell model results. This study can prove useful for the core-collapse simulators.

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