Temperature Density Conditions Research Articles

Validity of Brink-Axel Hypothesis for calculations of allowed stellar weak rates of heavy nuclei

The knowledge of beta-decay transitional probabilities and Gamow-Teller (GT) strength functions from highly excited states of nuclides is of particular importance for applications to astrophysical network calculations of nucleosynthesis in explosive stellar events. These quantities are challenging to achieve from measurements or computations using various nuclear models. Due to unavailability of feasible alternatives, many theoretical studies often rely on the Brink-Axel (BA) hypothesis, that is, the response of strength functions depends merely on the transition energy of the parent nuclear ground state and is independent of the underlying details of the parent state, for the calculation of stellar rates. BA hypothesis has been used in many applications from nuclear structure determination to nucleosynthesis yield in the astrophysical matter. We explore here the the validity of BA hypothesis in the calculation of stellar beta-decay (BD) and electron capture (EC) weak rates of fp- and fpg-shell nuclides for GT transitions. Strength functions have been computed employing the fully microscopic proton-neutron QRPA (quasi-particle random-phase approximation) within a broad density, ρY e = (10-1011) [g cm−3], and temperature, T = (1−30) [GK], grid relevant to the pre-collapse astrophysical environment. Our work provides evidence that the use of the approximation based on the BA hypothesis does not lead to reliable calculations of excited states strength functions under extreme temperature-density conditions characteristic of presupernova and supernova evolution of massive stars. Weak rates obtained by incorporating the BA hypothesis in the calculation of strength functions substantially deviate from the rates based on the state-by-state microscopically calculated strength functions. Deviation in the two calculations becomes significant as early as neon burning phases of massive stars. The deviation in the calculation of BD rates is even more pronounced, reaching up to three orders of magnitude.

First-principles equationof state of CHON resin for inertial confinement fusion applications.

A wide-range (0 to 1044.0 g/cm^{3} and 0 to 10^{9}K) equation-of-state (EOS) table for a CH_{1.72}O_{0.37}N_{0.086} quaternary compound has been constructed based on density-functional theory (DFT) molecular-dynamics (MD) calculations using a combination of Kohn-Sham DFT MD, orbital-free DFT MD, and numerical extrapolation. The first-principles EOS data are compared with predictions of simple models, including the fully ionized ideal gas and the Fermi-degenerate electron gas models, to chart their temperature-density conditions of applicability. The shock Hugoniot, thermodynamic properties, and bulk sound velocities are predicted based on the EOS table and compared to those of C-H compounds. The Hugoniot results show the maximum compression ratio of the C-H-O-N resin is larger than that of CH polystyrene due to the existence of oxygen and nitrogen; while the other properties are similar between CHON and CH. Radiation hydrodynamic simulations have been performed using the table for inertial confinement fusion targets with a CHON ablator and compared with a similar design with CH. The simulations show CHON outperforms CH as the ablator for laser-direct-drive target designs.

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

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 $$(^{19}\hbox {F})$$ is one such element which gets plagued by proton capture reactions. The stellar temperature thought about here ranges from $$2\times 10^{7}$$ to $$10\times 10^{7}$$ K and there has been an accretion occuring, with material density being $$10^{2}\hbox { g/cm}^{3}$$ and $$10^{3}\hbox { g/cm}^{3}$$ . Such kind of temperature density conditions are probably going to be prevailing within the H-burning shell of evolved stars. The estimated abundances of $$^{19}\hbox {F}$$ are then matched with the info that has been determined for a few some metal-poor giants of GC $$\hbox {M}4,\hbox { M}22,\, 47$$ 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 $$^{19}\hbox {F}$$ 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.

ON POTASSIUM AND OTHER ABUNDANCE ANOMALIES OF RED GIANTS IN NGC 2419

ABSTRACT Globular clusters are of paramount importance for testing theories of stellar evolution and early galaxy formation. Strong evidence for multiple populations of stars in globular clusters derives from observed abundance anomalies. A puzzling example is the recently detected Mg–K anticorrelation in NGC 2419. We perform Monte Carlo nuclear reaction network calculations to constrain the temperature–density conditions that gave rise to the elemental abundances observed in this elusive cluster. We find a correlation between stellar temperature and density values that provide a satisfactory match between simulated and observed abundances in NGC 2419 for all relevant elements (Mg, Si, K, Ca, Sc, Ti, and V). Except at the highest densities (ρ ≳ 108 g cm−3), the acceptable conditions range from ≈100 MK at ≈108 g cm−3 to ≈200 MK at ≈10−4 g cm−3. This result accounts for uncertainties in nuclear reaction rates and variations in the assumed initial composition. We review hydrogen-burning sites and find that low-mass stars, asymptotic giant branch (AGB) stars, massive stars, or supermassive stars cannot account for the observed abundance anomalies in NGC 2419. Super-AGB stars could be viable candidates for the polluter stars if stellar model parameters can be fine-tuned to produce higher temperatures. Novae, involving either CO or ONe white dwarfs, could be interesting polluter candidates, but a current lack of low-metallicity nova models precludes firmer conclusions. We also discuss whether additional constraints for the first-generation polluters can be obtained by future measurements of oxygen, or by evolving models of second-generation low-mass stars with a non-canonical initial composition.

Semiempirical wide-range conductivity model with exploding wire verification.

Based on well-established physical relationships, a semiempirical set of equations dictating the electrical conductivity of dense, strongly coupled, partially ionized copper is presented. With the empirical coefficients, the model is tuned to experimental conductivity data obtained from exploding wire experiments [A. W. DeSilva and J. D. Katsouros, Phys. Rev. E 57, 5945 (1998)]. The result is a wide-range conductivity model, with demonstrated accuracy from room temperature-density conditions to 0.01 g/cm(3) and 30 kK. Using magnetohydrodynamic simulation the ability to utilize the conductivity model for predictive simulations is demonstrated. A complete electrical conductivity dataset for copper has been made available to the public.

P-process nucleosynthesis in postshock supernova envelope environments

view Abstract Citations (65) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS p-process nucleosynthesis in postshock supernova envelope environments. Audouze, J. ; Truran, J. W. Abstract Nuclear transformations giving rise to the production of the p-process (bypassed) elements are examined within the framework established by explosive nucleosynthesis studies. The guiding assumption is that thermonuclear processes occurring in hydrogen-rich matter in supernova envelopes following the passage of a shock wave are responsible for the formation of these nuclei. It is demonstrated that there exists a restrictive range of temperature-density conditions for which the enhanced production of most of the p-process nuclei occurs in relative concentrations consistent with solar system abundances. The implications of these results for galactic nucleosynthesis are discussed.- Publication: The Astrophysical Journal Pub Date: November 1975 DOI: 10.1086/153965 Bibcode: 1975ApJ...202..204A Keywords: Nuclear Fusion; Shock Waves; Stellar Envelopes; Stellar Evolution; Supernovae; Density Distribution; Galactic Evolution; Heavy Nuclei; Nuclear Reactions; Solar System; Stellar Temperature; Astrophysics full text sources ADS |

Interstellar HDCO - Upper limit and significance as an indicator of interstellar molecule reactions

Upper limits to the strength of the HDCO (1$sub 11$-1$sub 10$) transition at 5346 MHz are obtained from observations of Cas A, Cloud 4, NGC 2264, DR 21, Orion B, and Sgr B2. The observations are interpreted to yield an upper limit to the abundance ratio HDCO/H$sub 2$CO that is most likely)about 1/ 170. We show that likely pure gas-phase processes (those employing CH$sub 3$+O and CH$sub 3$$sup +$+O) which have been proposed to form interstellar formaldehyde must also produce an enhanced HDCO/H$sub 2$CO ratio. Our observations indicate that the required enhancement probably does not occur. Formaldehyde is observed widely throughout the Galaxy and apparently requires a more complicated formation process than the simplest molecules. If gas reactions to produce formaldehyde cannot be found, it seems unlikely that the more complex interstellar molecules are produced purely by gas-phase reactions under the low temperature-density conditions where they are observed. (AIP)