arXiv subjects

Grant J. Mathews

Publications and source records attributed to Grant J. Mathews.

At least 91 records · Page 5Linked to original sources

arXiv2004

Early Star Formation and Chemical Evolution in Proto-Galactic Clouds

We present numerical simulations to describe the evolution of pre-Galactic clouds in a model which is motivated by cold dark matter simulations of hierarchical galaxy formation. We adopt a SN-induced star-formation mechanism within a model that follows the evolution of chemical enrichment and energy input to the clouds by Type II and Type Ia supernovae. We utilize metallicity-dependent yields for all elements at all times, and include effects of finite stellar lifetimes. We derive the metallicity distribution functions for stars in the clouds, their age-metallicity relation, and relative elemental abundances for a number of alpha- and Fe-group elements. The stability of these clouds against destruction is discussed, and results are compared for different initial mass functions. We find that the dispersion of the metallicity distribution function observed in the outer halo is naturally reproduced by contributions from many clouds with different initial conditions. The predicted relative abundances of some alpha- and Fe-group elements show reasonable agreement with the observed values down to metallicities below [Fe/H] ~ -4 when the iron yields are reduced relative to stellar models. The observed scatter is also reproduced for most elements including the observed bifurcation in [alpha/Fe] for stars with low [Fe/H]. However, the predicted dispersion may be too large for some elements unless a limited range of progenitor masses contributing to the abundances of these elements is assumed. The results suggest that the low-mass end of SNeII was probably absent at the very lowest metallicities, and that the upper mass limit for the first stars that contributed to nucleosynthesis may be < 40 M_solar.

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arXiv2004

The Fine-structure Constant as a Probe of Chemical Evolution and AGB Nucleosynthesis in Damped Lyman-alpha Systems

Evidence from a large sample of quasar absorption-line spectra in damped Lyman-alpha systems has suggested a possible time variation of the fine structure constant alpha. The most statistically significant portion of this sample involves the comparison of Mg and Fe wavelength shifts using the many-multiplet (MM) method. However, the sensitivity of this method to the abundance of heavy isotopes, especially Mg, is enough to imitate an apparent variation in alpha in the redshift range 0.5 < z < 1.8. We implement recent yields of intermediate mass (IM) stars into a chemical evolution model and show that the ensuing isotope distribution of Mg can account for the observed variation in alpha provided the early IMF was particularly rich in intermediate mass stars (or the heavy Mg isotope yields from AGB stars are even higher than in present-day models). As such, these observations of quasar absorption spectra can be used to probe the nucleosynthetic history of low-metallicity damped Lyman-alpha systems in the redshift range 0.5 < z < 1.8. This analysis, in conjunction with other abundance measurements of low-metallicity systems, reinforces the mounting evidence that star formation at low metallicities may have been strongly influenced by a population of IM stars. Such IM stars have a significant influence on other abundances, particularly nitrogen. We constrain our models with independent measurements of N, Si, and Fe in damped Lyman-alpha systems as well as C/O in low-metallicity stars. In this way, we obtain consistent model parameters for this chemical-evolution interpretation of the MM method results.

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arXiv2003

The Chemical Evolution of Mg Isotopes vs. the Time Variation of the Fine Structure Constant

The many-multiplet method applied to high redshift quasar absorption spectra has indicated a possible time variation of the fine structure constant. Alternatively, a constant value of alpha is consistent with the observational analysis if a non-solar isotopic ratio of Mg24,25,26 occurs at high redshift. In particular, a higher abundance of the heavier isotopes Mg25,26 are required to explain the observed multiplet splitting. We show that the synthesis of Mg25,26 at the base of the convective envelope in low-metallicity asymptotic giant branch stars, combined with a simple model of galactic chemical evolution, can produce the required isotopic ratios and is supported by recent observations of high abundances of the neutron-rich Mg isotopes in metal-poor globular-cluster stars. We conclude that the present data based on high redshift quasar absorption spectra may be providing interesting information on the nucleosynthetic history of such systems, rather than a time variation of fundamental constants.

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arXiv2003

Europium Isotope Ratios in s-Process-Element-Enhanced, Metal-Poor Stars: A New Probe of the 151Sm Branching

We report on the first measurement of the Eu isotope fractions (151Eu and 153Eu) in s-process-element-enhanced, metal-poor stars. We use these ratios to investigate the 151Sm branching of s-process nucleosynthesis. The measurement was made by detailed study of Eu II lines that are significantly affected by hyperfine splitting and isotope shifts in spectra of the carbon-rich, very metal-poor stars LP625-44 and CS31062-050, observed with the Subaru Telescope High Dispersion Spectrograph. The 151Eu fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived for LP625-44 and CS31062-050 are 0.60 and 0.55, respectively, with uncertainties of about +/- 0.05. These values are higher than found in solar-system material, but agree well with the predictions of recent s-process models. We derive new constraints on the temperature and neutron density during the s-process based on calculations of pulsed s-process models for the 151Eu fraction.

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arXiv2003

r-Process Abundance Universality and Actinide Cosmochronology

We review recent observational and theoretical results concerning the presence of actinide nuclei on the surfaces of old halo stars and their use as an age determinant. We present model calculations which show that the observed universality of abundances for 56 < Z < 75 elements in these stars does not imply a unique astrophysical site for the r-process. Neither does it imply a universality of abundances of nuclei outside of this range. In particular, we show that a variety of astrophysical $r$-process models can be constructed which reproduce the same observed universal r-process curve for 56 < Z < 75 nuclei, yet have vastly different abundances for Z >= 75 and possibly Z <= 56 as well. This introduces an uncertainty into the use of the Th/Eu chronometer as a means to estimate the ages of the metal deficient stars. We do find, however, that the U/Th ratio is a robust chronometer. This is because the initial production ratio of U to Th is almost independent of the astrophysical nucleosynthesis environment. The largest remaining uncertainties in the U/Th initial production ratio are due to the input nuclear physics models.

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arXiv2002

s-Process Nucleosynthesis in Low-Metallicity Stars

We have made a parametric study of s-process nucleosynthesis in the metal poor ([Fe/H]=-2.7) stars LP625-44 and LP706-7. We find that a high neutron exposure and a small overlap factor are necessary to fit the abundance pattern observed in these two metal-deficient stars, particularly the abundance ratios, Pb/Ba $\approx 1$ and Ba/Sr $\approx 10$. We have also constructed stellar models to better understand how such s-process conditions could arise. We have calculated a 2$M_\odot$ model star with metallicity [Fe/H]=-2.7 from the ZAMS up to AGB phase. We find that for such low-metallicity stars the He convective shell reaches the bottom of the overlying H-rich envelope at the second thermal pulse. Protons are then carried into the hotter He burning layers and $^{13}$C is formed as protons mix into the He shell. Subsequently, material in the H-flash driven convective zone experiences a high neutron exposure due to the $^{13}$C($α$, n) reaction. This results in a new neutron-capture s-process paradigm in which the abundances are characterized by only one neutron exposure. We suggest that this new s-process site may be a significant contributor to the s-process abundances in low-metallicity ([Fe/H] $\le -2.5$) stars.

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arXiv2002

Model Independent Primordial Power Spectrum from Maxima, Boomerang, and DASI Data

A model-independent determination of the primordial power spectrum of matter density fluctuations could uniquely probe physics of the very early universe, and provide powerful constraints on inflationary models. We parametrize the primordial power spectrum $A_s^2(k)$ as an arbitrary function, and deduce its binned amplitude from the cosmic microwave background radiation anisotropy (CMB) measurements of Maxima, Boomerang, and DASI. We find that for a flat universe with $A_s^2(k)=1$ (scale-invariant) for scales $k<0.001 $h/Mpc, the primordial power spectrum is marginally consistent with a scale-invariant Harrison-Zeldovich spectrum. However, we deduce a rise in power compared to a scale-invariant power spectrum for $0.001 h/{Mpc} \la k \la 0.01 h/{Mpc}$. Our results are consistent with large-scale structure data, and seem to suggest that the current observational data allow for the possibility of unusual physics in the very early universe.

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arXiv2001

Prospects for Constraining Cosmology with the Extragalactic Cosmic Microwave Background Temperature

Observers have demonstrated that it is now feasible to measure the cosmic microwave background (CMB) temperature at high redshifts. We explore the possible constraints on cosmology which might ultimately be derived from such measurements. Besides providing a consistency check on standard and alternative cosmologies, possibilities include: constraints on the inhomogeneity and anisotropy of the universe at intermediate redshift $z ^<_\sim 10$; an independent probe of peculiar motions with respect to the Hubble flow; and constraining the epoch of reionization. We argue that the best possibility is as a probe of peculiar motions. We show, however, that the current measurement uncertainty ($ΔT= \pm 0.002 $K) in the local present absolute CMB temperature imposes intrinsic limits on the use of such CMB temperature measurements as a cosmological probe. At best, anisotropies at intermediate redshift could only be constrained at a level of $^>_\sim 0.1%$ and peculiar motions could only be determined to an uncertainty of $^>_\sim 311$ km s$^{-1}$. If the high $z$ CMB temperature can only be measured with a precision comparable to the uncertainty of the local interstellar CMB temperature, then peculiar motions could be determined to an uncertainty of $1101 (1+z)^{-1} [ΔT_{CMB}(z)/0.01 K] km s^{-1}$.

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arXiv2001

Neutron Capture Elements in s-Process-Rich, Very Metal-Poor Stars

We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP625-44 and LP706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z=38) to Pb (Z=82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure tau is estimated to be about 0.7mb^-1, significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed abundance pattern can be explained by a few recurrent neutron exposures, and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r~0.1).

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arXiv2001

Gamma-Ray Bursts via the Neutrino Emission from Heated Neutron Stars

A model is proposed for gamma-ray bursts based upon a neutrino burst of about 10^52 ergs lasting a few seconds above a heated collapsing neutron star. This type of thermal neutrino burst is suggested by relativistic hydrodynamic studies of the compression, heating, and collapse of close binary neutron stars as they approach their last stable orbit, but may arise from other sources as well. We present a hydrodynamic simulation of the formation and evolution of the pair plasma associated with such a neutrino burst. This pair plasma leads to the production of ~10^51 - 10^52 ergs in gamma-rays with spectral and temporal properties consistent with observed gamma-ray bursts.

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arXiv2001

The r-Process in Neutrino-Driven Winds from Nascent, "Compact" Neutron Stars of Core-Collapse Supernovae

We present calculations of r-process nucleosynthesis in neutrino-driven winds from the nascent neutron stars of core-collapse supernovae. A full dynamical reaction network for both the alpha-rich freezeout and the subsequent r-process is employed. The physical properties of the neutrino-heated ejecta are deduced from a general relativistic model in which spherical symmetry and steady flow are assumed. Our results suggest that proto-neutron stars with a large compaction ratio provide the most robust physical conditions for the r-process. The third peak of the r-process is well reproduced in the winds from these ``compact'' proto-neutron stars even for a moderate entropy, \sim 100-200 N_A k, and a neutrino luminosity as high as \sim 10^{52} ergs s^{-1}. This is due to the short dynamical timescale of material in the wind. As a result, the overproduction of nuclei with A \lesssim 120 is diminished (although some overproduction of nuclei with A \approx 90 is still evident). The abundances of the r-process elements per event is significantly higher than in previous studies. The total-integrated nucleosynthesis yields are in good agreement with the solar r-process abundance pattern. Our results have confirmed that the neutrino-driven wind scenario is still a promising site in which to form the solar r-process abundances. However, our best results seem to imply both a rather soft neutron-star equation of state and a massive proto-neutron star which is difficult to achieve with standard core-collapse models. We propose that the most favorable conditions perhaps require that a massive supernova progenitor forms a massive proto-neutron star by accretion after a failed initial neutrino burst.

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arXiv2001

Bardeen-Petterson Effect and Quasi-Periodic Oscillations in X-Ray Binaries

The Bardeen-Petterson effect around a rapidly-rotating compact object causes a tilted accretion disk to warp into the equatorial plane of the rotating body. Viscous forces cause the accretion flow to divide into two distinct regions - an inner aligned accretion disk and an outer tilted accretion disk. The transition between these two occurs at a characteristic radius that depends on the mass and angular momentum of the central object and possibly on the accretion rate through the disk. We propose that accreting material passing through the transition region may generate quasi-periodic brightness oscillations (QPOs) such as have been observed in a number of X-ray binaries. We show that this effect may be present in the black-hole X-ray binary GRO J1655-40. We also argue that the QPO frequency range predicted by this model is consistent with observed QPO frequencies in both black-hole and neutron-star low-mass X-ray binaries.

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arXiv2000

Impact of Few-Body Reactions on Explosive Nucleosynthesis: The Big-Bang and Supernovae

We study the few body reactions in explosive nucleosynthesis of light-to-heavy mass elements in various astrophysical conditions. In neutrino-driven winds of gravitational core collapse of SNeII, several light nuclei as well as heavy unstable nuclei paly significant roles in producing r-process elements. We discuss how the three-body reaction $^4$He$(αn, γ)^9$Be, whose cross section is poorly known, plays very critical role. We make note that several other reactions in the r-process are relevant to a Big-Bang nucleosynthesis. We discuss briefly a Galactic chemical evolution.

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arXiv2000

Gamma-Ray Bursts via Pair Plasma Fireballs from Heated Neutron Stars

In this paper we model the emission from a relativistically expanding electron-positron pair plasma fireball originating near the surface of a heated neutron star. This pair fireball is deposited via the annihilation of neutrino pairs emanating from the surface of the hot neutron star. The heating of neutron stars may occur in close neutron star binary systems near their last stable orbit. We model the relativistic expansion and subsequent emission of the plasma and find 10^51 to 10^52 ergs in gamma-rays are produced with spectral and temporal properties consistent with observed gamma-ray bursts.

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arXiv1999

Binary Neutron Stars Systems: Irrotational Quasi-Equilibrium Sequences

We report on numerical results from an independent formalism to describe the quasi-equilibrium structure of nonsynchronous binary neutron stars in general relativity. This is an important independent test of controversial numerical hydrodynamic simulations which suggested that nonsynchronous neutron stars in a close binary can experience compression and even collapse prior to the last stable circular orbit. We show that the interior density indeed increases as irrotational binary neutron stars approach their last orbits for particular values of the compaction ratio. The observed compression is however at a significantly reduced level.

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arXiv1996

Halo White Dwarfs and the Hot Intergalactic Medium

We present a schematic model for the formation of baryonic galactic halos and hot gas in the Local Group and the intergalactic medium. We follow the dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales, including: protogalactic clouds, galactic halos, and the Local Group itself. Within this hierarchy, the Galaxy is built via mergers of protogalactic fragments. We find that early bursts of star formation lead to a large population of remnants (mostly white dwarfs), which would reside presently in the halo and contribute to the dark component observed in the microlensing experiments. The hot, metal-rich gas from early starbursts and merging evaporates from the clouds and is eventually incorporated into the intergalactic medium. The model thus suggests that most microlensing objects could be white dwarfs ($m \sim 0.5 \msol$), which comprise a significant fraction of the halo mass. Furthermore, the Local Group could have a component of metal-rich hot gas similar to, although less than, that observed in larger clusters. We discuss the known constraints on such a scenario and show that all local observations can be satisfied with present data in this model. The best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.

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arXiv1996

Stochastic Isocurvature Baryon Fluctuations, Baryon Diffusion, and Primordial Nucleosynthesis

We examine effects on primordial nucleosynthesis from a truly random spatial distribution in the baryon-to-photon ratio ($η$). We generate stochastic fluctuation spectra characterized by different spectral indices and root-mean-square fluctuation amplitudes. For the first time we explicitly calculate the effects of baryon diffusion on the nucleosynthesis yields of such stochastic fluctuations. We also consider the collapse instability of large-mass-scale inhomogeneities. Our results are generally applicable to any primordial mechanism producing fluctuations in $η$ which can be characterized by a spectral index. In particular, these results apply to primordial isocurvature baryon fluctuation (PIB) models. The amplitudes of scale-invariant baryon fluctuations are found to be severely constrained by primordial nucleosynthesis. However, when the $η$ distribution is characterized by decreasing fluctuation amplitudes with increasing length scale, surprisingly large fluctuation amplitudes on the baryon diffusion scale are allowed.

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arXiv1994

Statistical Constraints on the Inflation Effective Potential from the COBE DMR Results

We explore constraints on various forms for the effective potential during inflation based upon a statistical comparison between inflation-generated fluctuations in the cosmic microwave background temperature and the COBE DMR results. Fits to the first year 53 A + B x 90 A + B cross correlation function using an effective potential of the form V(phi) = lambda*phi^x/x!, yield upper limits of x le 107 and x le 397 at the 1 sigma and 2 sigma confidence levels, while the same analysis produces a power law of index of n = 1.10 ^{+0.47}_{-0.64} which would imply an underestimate of x le 63 from the simple analytic relation between x and n . We also quantify new limits on the parameters for polynomial effective potentials. This work highlights the importance of a careful statistical treatment when seeking constraints on the inflation-generating effective potential.

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