Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “IA”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

REDDENING AND EXTINCTION TOWARD THE GALACTIC BULGE FROM OGLE-III: THE INNER MILKY WAY'S R{sub V} {approx} 2.5 EXTINCTION CURVE

We combine VI photometry from OGLE-III with VISTA Variables in The Via Lactea survey and Two Micron All Sky Survey measurements of E(J - K{sub s} ) to resolve the longstanding problem of the non-standard optical extinction toward the Galactic bulge. We show that the extinction is well fit by the relation A{sub I} = 0.7465 Multiplication-Sign E(V - I) + 1.3700 Multiplication-Sign E(J - K{sub s} ), or, equivalently, A{sub I} = 1.217 Multiplication-Sign E(V - I)(1 + 1.126 Multiplication-Sign (E(J - K{sub s} )/E(V - I) - 0.3433)). The optical and near-IR reddening law toward the inner Galaxy approximately follows an R{sub V} Almost-Equal-To 2.5 extinction curve with a dispersion {sigma}{sub R{sub V}}{approx}0.2, consistent with extragalactic investigations of the hosts of Type Ia SNe. Differential reddening is shown to be significant on scales as small as our mean field size of 6'. The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (M{sub I,RC},{sigma}{sub I,RC,0}, (V-I){sub RC,0},{sigma}{sub (V-I){sub R{sub C}}}, (J-K{sub s}){sub RC,0}) = (-0.12, 0.09, 1.06, 0.121, 0.66). Our measurements of the RC brightness, brightness dispersion, and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc. We measure an upper bound on the tilt {alpha} Almost-Equal-To 40 Degree-Sign between the bulge's major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of an N-body model oriented at {alpha} Almost-Equal-To 25 Degree-Sign . The number of RC stars suggests a total stellar mass for the Galactic bulge of {approx}2.3 Multiplication-Sign 10{sup 10} M{sub Sun} if one assumes a canonical Salpeter initial mass function (IMF), or {approx}1.6 Multiplication-Sign 10{sup 10} M{sub Sun} if one assumes a bottom-light Zoccali IMF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SEGUE 2: THE LEAST MASSIVE GALAXY

Segue 2, discovered by Belokurov et al., is a galaxy with a luminosity of only 900 L{sub Sun }. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2-a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is {sigma}{sub v} < 2.2 (2.6) km s{sup -1}, the most stringent limit for any galaxy. The corresponding limit on the mass within the three-dimensional half-light radius (46 pc) is M{sub 1/2} < 1.5 (2.1) Multiplication-Sign 10{sup 5} M{sub Sun }. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. The stars' [{alpha}/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, ([Fe/H]) = -2.22 {+-} 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo (v{sub max} < 10 km s{sup -1}), below the atomic hydrogen cooling limit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY (NuSTAR) HIGH-ENERGY X-RAY MISSION

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the {approx}10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the peak epoch of galaxy assembly in the universe (at z {approx}< 2) by surveying selected regions of the sky; (2) study the population of hard X-ray-emitting compact objects in the Galaxy by mapping the central regions of the Milky Way; (3) study the non-thermal radiation in young supernova remnants, both the hard X-ray continuum and the emission from the radioactive element {sup 44}Ti; (4) observe blazars contemporaneously with ground-based radio, optical, and TeV telescopes, as well as with Fermi and Swift, to constrain the structure of AGN jets; and (5) observe line and continuum emission from core-collapse supernovae in the Local Group, and from nearby Type Ia events, to constrain explosion models. During its baseline two-year mission, NuSTAR will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6 Degree-Sign inclination orbit, the observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of 10 yr, we anticipate proposing a guest investigator program, to begin in late 2014.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

AZIMUTHAL DENSITY VARIATIONS AROUND THE RIM OF TYCHO's SUPERNOVA REMNANT

Spitzer images of Tycho's supernova remnant in the mid-infrared reveal limb-brightened emission from the entire periphery of the shell and faint filamentary structures in the interior. As with other young remnants, this emission is produced by dust grains, warmed to {approx}100 K in the post-shock environment by collisions with energetic electrons and ions. The ratio of the 70 to 24 {mu}m fluxes is a diagnostic of the dust temperature, which in turn is a sensitive function of the plasma density. We find significant variations in the 70/24 flux ratio around the periphery of Tycho's forward shock, implying order-of-magnitude variations in density. While some of these are likely localized interactions with dense clumps of the interstellar medium (ISM), we find an overall gradient in the ambient density surrounding Tycho, with densities 3-10 times higher in the northeast than in the southwest. This large density gradient is qualitatively consistent with the variations in the proper motion of the shock observed in radio and X-ray studies. Overall, the mean ISM density around Tycho is quite low ({approx}0.1-0.2 cm{sup -3}), consistent with the lack of thermal X-ray emission observed at the forward shock. We perform two-dimensional hydrodynamic simulations of a Type Ia supernova expanding into a density gradient in the ISM, and find that the overall round shape of the remnant is still easily achievable, even for explosions into significant gradients. However, this leads to an offset of the center of the explosion from the geometric center of the remnant of up to 20%, although lower values of 10% are preferred. The best match with hydrodynamical simulations is achieved if Tycho is located at a large (3-4 kpc) distance in a medium with a mean preshock density of {approx}0.2 cm{sup -3}. Such preshock densities are obtained for highly ({approx}> 50%) porous ISM grains.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CONDITIONS FOR SUCCESSFUL HELIUM DETONATIONS IN ASTROPHYSICAL ENVIRONMENTS

Several models for Type Ia-like supernova events rely on the production of a self-sustained detonation powered by nuclear reactions. In the absence of hydrogen, the fuel that powers these detonations typically consists of either pure helium (He) or a mixture of carbon and oxygen (C/O). Studies that systematically determine the conditions required to initiate detonations in C/O material exist, but until now no analogous investigation of He matter has been conducted. We perform one-dimensional reactive hydrodynamical simulations at a variety of initial density and temperature combinations and find critical length scales for the initiation of He detonations that range between 1 and 10{sup 10} cm. A simple estimate of the length scales over which the total consumption of fuel will occur for steady-state detonations is provided by the Chapman-Jouguet (CJ) formalism. Our initiation lengths are consistently smaller than the corresponding CJ length scales by a factor of {approx}100, providing opportunities for thermonuclear explosions in a wider range of low-mass white dwarfs (WDs) than previously thought possible. We find that virialized WDs with as little mass as 0.24 M{sub Sun} can be detonated, and that even less massive WDs can be detonated if a sizable fraction of their mass is raised to a higher adiabat. That the initiation length is exceeded by the CJ length implies that certain systems may not reach nuclear statistical equilibrium within the time it takes a detonation to traverse the object. In support of this hypothesis, we demonstrate that incomplete burning will occur in the majority of He WD detonations and that {sup 40}Ca, {sup 44}Ti, or {sup 48}Cr, rather than {sup 56}Ni, is the predominant burning product for many of these events. We anticipate that a measure of the quantity of the intermediate-mass elements and {sup 56}Ni produced in a helium-rich thermonuclear explosion can potentially be used to constrain the nature of the progenitor system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

GRAVITATIONALLY UNSTABLE FLAMES: RAYLEIGH-TAYLOR STRETCHING VERSUS TURBULENT WRINKLING

In this paper, we provide support for the Rayleigh-Taylor-(RT)-based subgrid model used in full-star simulations of deflagrations in Type Ia supernovae explosions. We use the results of a parameter study of two-dimensional direct numerical simulations of an RT unstable model flame to distinguish between the two main types of subgrid models (RT or turbulence dominated) in the flamelet regime. First, we give scalings for the turbulent flame speed, the Reynolds number, the viscous scale, and the size of the burning region as the non-dimensional gravity (G) is varied. The flame speed is well predicted by an RT-based flame speed model. Next, the above scalings are used to calculate the Karlovitz number (Ka) and to discuss appropriate combustion regimes. No transition to thin reaction zones is seen at Ka = 1, although such a transition is expected by turbulence-dominated subgrid models. Finally, we confirm a basic physical premise of the RT subgrid model, namely, that the flame is fractal, and thus self-similar. By modeling the turbulent flame speed, we demonstrate that it is affected more by large-scale RT stretching than by small-scale turbulent wrinkling. In this way, the RT instability controls the flame directly from the large scales. Overall, these results support the RT subgrid model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE C-FLAME QUENCHING BY CONVECTIVE BOUNDARY MIXING IN SUPER-AGB STARS AND THE FORMATION OF HYBRID C/O/Ne WHITE DWARFS AND SN PROGENITORS

After off-center C ignition in the cores of super asymptotic giant branch (SAGB) stars, the C flame propagates all the way down to the center, trailing behind it the C-shell convective zone, and thus building a degenerate ONe core. This standard picture is obtained in stellar evolution simulations if the bottom C-shell convection boundary is assumed to be a discontinuity associated with a strict interpretation of the Schwarzschild condition for convective instability. However, this boundary is prone to additional mixing processes, such as thermohaline convection and convective boundary mixing. Using hydrodynamic simulations, we show that contrary to previous results, thermohaline mixing is too inefficient to interfere with the C-flame propagation. However, even a small amount of convective boundary mixing removes the physical conditions required for the C-flame propagation all the way to the center. This result holds even if we allow for some turbulent heat transport in the CBM region. As a result, SAGB stars build in their interiors hybrid C-O-Ne degenerate cores composed of a relatively large CO core (M{sub CO} Almost-Equal-To 0.2 M{sub Sun }) surrounded by a thick ONe zone ({Delta}M{sub ONe} {approx}> 0.85 M{sub Sun }) with another thin CO layer above. If exposed by mass loss, these cores will become hybrid C-O-Ne white dwarfs. Otherwise, the ignition of C-rich material in the central core, surrounded by the thick ONe zone, may trigger a thermonuclear supernova (SN) explosion. The quenching of the C-flame may have implications for the ignition mechanism of SN Ia in the double-degenerate merger scenario.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE GAMMA-RAY BURST HUBBLE DIAGRAM AND ITS IMPLICATIONS FOR COSMOLOGY

In this paper, we continue to build support for the proposal to use gamma-ray bursts (GRBs) as standard candles in constructing the Hubble diagram at redshifts beyond the current reach of Type Ia supernova observations. We confirm that correlations among certain spectral and light-curve features can indeed be used as luminosity indicators, and demonstrate from the most up-to-date GRB sample appropriate for this work that the {Lambda}CDM model optimized with these data is characterized by parameter values consistent with those in the concordance model. Specifically, we find that ({Omega}{sub m},{Omega}{sub {Lambda}}){approx}(0.25{sub -0.06}{sup +0.05}, 0.75{sub -0.05}{sup +0.06}), which are consistent, to within 1{sigma}, with (0.29, 0.71) obtained from the 9 yr Wilkinson Microwave Anisotropy Probe data. We also carry out a comparative analysis between {Lambda}CDM and the R{sub h} = ct universe and find that the optimal {Lambda}CDM model fits the GRB Hubble diagram with a reduced {chi}{sup 2}{sub dof}{approx}2.26, whereas the fit using R{sub h} = ct results in a {chi}{sup 2}{sub dof}{approx}2.14. In both cases, about 20% of the events lie at least 2{sigma} away from the best-fit curves, suggesting that either some contamination by non-standard GRB luminosities is unavoidable or that the errors and intrinsic scatter associated with the data are being underestimated. With these optimized fits, we use three statistical tools-the Akaike information criterion, the Kullback information criterion, and the Bayes information criterion-to show that, based on the GRB Hubble diagram, the likelihood of R{sub h} = ct being closer to the correct model is {approx}85%-96%, compared to {approx}4%-15% for {Lambda}CDM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PERSPECTIVES ON INTRACLUSTER ENRICHMENT AND THE STELLAR INITIAL MASS FUNCTION IN ELLIPTICAL GALAXIES

Stars formed in galaxy cluster potential wells must be responsible for the high level of enrichment measured in the intracluster medium (ICM); however, there is increasing tension between this truism and the parsimonious assumption that the stars in the generally old population studied optically in cluster galaxies emerged from the same formation sites at the same epochs. We construct a phenomenological cluster enrichment model to demonstrate that ICM elemental abundances are underestimated by a factor >2 for standard assumptions about the stellar population-a discrepancy we call the ''cluster elemental abundance paradox''. Recent evidence of an elliptical galaxy initial mass function (IMF) skewed to low masses deepens the paradox. We quantify the adjustments to the star formation efficiency and IMF, and Type Ia supernovae (SNIa) production efficiency, required to resolve this while being consistent with the observed ICM abundance pattern. The necessary enhancement in metal enrichment may, in principle, originate in the observed stellar population if a larger fraction of stars in the supernova-progenitor mass range form from an IMF that is either bottom-light or top-heavy, with the latter in some conflict with observed ICM abundance ratios. Other alternatives that imply more modest revisions to the IMF, mass return and remnant fractions, and primordial fraction, posit an increase in the fraction of 3-8 M{sub Sun} stars that explode as SNIa or assume that there are more stars than conventionally thought-although the latter implies a high star formation efficiency. We discuss the feasibility of these various solutions and the implications for the diversity of star formation in the universe, the process of elliptical galaxy formation, and the origin of this ''hidden'' source of ICM metal enrichment.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE FAST AND FURIOUS DECAY OF THE PECULIAR TYPE Ic SUPERNOVA 2005ek

We present extensive multi-wavelength observations of the extremely rapidly declining Type Ic supernova (SN Ic), SN 2005ek. Reaching a peak magnitude of M{sub R} = -17.3 and decaying by {approx}3 mag in the first 15 days post-maximum, SN 2005ek is among the fastest Type I supernovae observed to date. The spectra of SN 2005ek closely resemble those of normal SN Ic, but with an accelerated evolution. There is evidence for the onset of nebular features at only nine days post-maximum. Spectroscopic modeling reveals an ejecta mass of {approx}0.3 M{sub Sun} that is dominated by oxygen ({approx}80%), while the pseudo-bolometric light curve is consistent with an explosion powered by {approx}0.03 M{sub Sun} of radioactive {sup 56}Ni. Although previous rapidly evolving events (e.g., SN 1885A, SN 1939B, SN 2002bj, SN 2010X) were hypothesized to be produced by the detonation of a helium shell on a white dwarf, oxygen-dominated ejecta are difficult to reconcile with this proposed mechanism. We find that the properties of SN 2005ek are consistent with either the edge-lit double detonation of a low-mass white dwarf or the iron-core collapse of a massive star, stripped by binary interaction. However, if we assume that the strong spectroscopic similarity of SN 2005ek to other SNe Ic is an indication of a similar progenitor channel, then a white-dwarf progenitor becomes very improbable. SN 2005ek may be one of the lowest mass stripped-envelope core-collapse explosions ever observed. We find that the rate of such rapidly declining Type I events is at least 1%-3% of the normal SN Ia rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A HIGH-RESOLUTION SPECTROSCOPIC SEARCH FOR THE REMAINING DONOR FOR TYCHO'S SUPERNOVA

In this paper, we report on our analysis using Hubble Space Telescope astrometry and Keck-I HIRES spectroscopy of the central six stars of Tycho's supernova remnant (SN 1572). With these data, we measured the proper motions, radial velocities, rotational velocities, and chemical abundances of these objects. Regarding the chemical abundances, we do not confirm the unusually high [Ni/Fe] ratio previously reported for Tycho-G. Rather, we find that for all metrics in all stars, none exhibit the characteristics expected from traditional Type Ia supernova single-degenerate-scenario calculations. The only possible exception is Tycho-B, a rare, metal-poor A-type star; however, we are unable to find a suitable scenario for it. Thus, we suggest that SN 1572 cannot be explained by the standard single-degenerate model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RADIATIVE TRANSFER SIMULATIONS OF NEUTRON STAR MERGER EJECTA

Mergers of binary neutron stars (NSs) are among the most promising gravitational wave (GW) sources. Next generation GW detectors are expected to detect signals from NS mergers within about 200 Mpc. The detection of electromagnetic wave (EM) counterparts is crucial to understanding the nature of GW sources. Among the possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, predictions so far have not taken into account detailed r-process element abundances in the ejecta. We perform for the first time radiative transfer simulations of the NS merger ejecta including all the r-process elements from Ga to U. We show that the opacity of the NS merger ejecta is about κ = 10 cm{sup 2} g{sup –1}, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ∼100. As a result, the emission is fainter and lasts longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound-bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of the two NSs and a softer equation of state adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, the expected brightness of the emission is i = 22-25 AB mag, z = 21-23 AB mag, and 21-24 AB mag in the NIR JHK bands. Thus, observations with wide-field 4 m- and 8 m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE LOCATIONS OF SHORT GAMMA-RAY BURSTS AS EVIDENCE FOR COMPACT OBJECT BINARY PROGENITORS

We present a detailed investigation of Hubble Space Telescope rest-frame UV/optical observations of 22 short gamma-ray burst (GRB) host galaxies and sub-galactic environments. Utilizing the high angular resolution and depth of HST we characterize the host galaxy morphologies, measure precise projected physical and host-normalized offsets between the bursts and host centers, and calculate the locations of the bursts with respect to their host light distributions (rest-frame UV and optical). We calculate a median short GRB projected physical offset of 4.5 kpc, about 3.5 times larger than that for long GRBs, and find that ≈25% of short GRBs have offsets of ∼> 10 kpc. When compared to their host sizes, the median offset is 1.5 half-light radii (r{sub e} ), about 1.5 times larger than the values for long GRBs, core-collapse supernovae, and Type Ia supernovae. In addition, ≈20% of short GRBs having offsets of ∼> 5r{sub e} , and only ≈25% are located within 1r{sub e} . We further find that short GRBs severely under-represent their hosts' rest-frame optical and UV light, with ≈30%-45% of the bursts located in regions of their host galaxies that have no detectable stellar light, and ≈55% in the regions with no UV light. Therefore, short GRBs do not occur in regions of star formation or even stellar mass. This demonstrates that the progenitor systems of short GRBs must migrate from their birth sites to their eventual explosion sites, a signature of kicks in compact object binary systems. Utilizing the full sample of offsets, we estimate natal kick velocities of ≈20-140 km s{sup –1}. These independent lines of evidence provide the strongest support to date that short GRBs result from the merger of compact object binaries (NS-NS/NS-BH)

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

NONTHERMAL RADIATION FROM SUPERNOVA REMNANTS: EFFECTS OF MAGNETIC FIELD AMPLIFICATION AND PARTICLE ESCAPE

We explore nonlinear effects of wave-particle interactions on the diffusive shock acceleration (DSA) process in Type Ia-like supernova remnant (SNR) blast waves by implementing phenomenological models for magnetic field amplification (MFA), Alfvénic drift, and particle escape in time-dependent numerical simulations of nonlinear DSA. For typical SNR parameters, the cosmic-ray (CR) protons can be accelerated to PeV energies only if the region of amplified field ahead of the shock is extensive enough to contain the diffusion lengths of the particles of interest. Even with the help of Alfvénic drift, it remains somewhat challenging to construct a nonlinear DSA model for SNRs in which of the order of 10% of the supernova explosion energy is converted into CR energy and the magnetic field is amplified by a factor of 10 or so in the shock precursor, while, at the same time, the energy spectrum of PeV protons is steeper than E {sup –2}. To explore the influence of these physical effects on observed SNR emission, we also compute the resulting radio-to-gamma-ray spectra. Nonthermal emission spectra, especially in X-ray and gamma-ray bands, depend on the time-dependent evolution of the CR injection process, MFA, and particle escape, as well as the shock dynamic evolution. This result comes from the fact that the high-energy end of the CR spectrum is composed of particles that are injected in the very early stages of the blast wave evolution. Thus, it is crucial to better understand the plasma wave-particle interactions associated with collisionless shocks in detailed modeling of nonthermal radiation from SNRs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE METALLICITY DEPENDENCE OF THE CEPHEID P – L RELATION IN M101

The impact of metallicity on the Cepheid period-luminosity (P – L) relation is investigated using Hubble Space Telescope Advanced Camera for Surveys V and I images of M101. Variations in the reddening-free Wesenheit parameter (W), which is employed as a proxy for luminosity, are examined as a function of the radial distance from the center of M101 (and thus metallicity). We determine that there is no dependence of the slope on metallicity. However, the intercept is found to depend on metallicity by γ{sub VI} = –0.33 ± 0.12 mag dex{sup –1} and γ{sub VI} = –0.71 ± 0.17 mag dex{sup –1} using 2σ and 3σ rejection criteria, respectively. Sigma-clipping impacts the derived metallicity dependence, and the 2σ criterion applied likely mitigates blending, particularly in the crowded inner regions of M101. A metallicity-corrected distance for M101 is obtained from 619 Cepheids (μ = 28.96 ± 0.11), a result that agrees with the recently determined SN Ia distance. The metallicity effects described can be bypassed by working at near and mid-infrared wavelengths (e.g., the Carnegie Hubble Program)

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

COSMOLOGICAL SIMULATIONS OF INTERGALACTIC MEDIUM EVOLUTION. I. TEST OF THE SUBGRID CHEMICAL ENRICHMENT MODEL

We present a one-zone galactic chemical enrichment model that takes into account the contribution of stellar winds from massive stars under the effect of rotation, Type II supernovae, hypernovae, stellar winds from low- and intermediate-mass stars, and Type Ia supernovae. This enrichment model will be implemented in a galactic model designed to be used as a subgrid treatment for galaxy evolution and outflow generation in large-scale cosmological simulations, in order to study the evolution of the intergalactic medium. We test our enrichment prescription by comparing its predictions with the metallicity distribution function and the abundance patterns of 14 chemical elements observed in the Milky Way stars. To do so, we combine the effect of many stellar populations created from the star formation history of the Galaxy in the solar neighborhood. For each stellar population, we keep track of its specific mass, initial metallicity, and age. We follow the time evolution of every population in order to respect the time delay between the various stellar phases. Our model is able to reproduce the observed abundances of C, O, Na, Mg, Al, S, and Ca. For Si, Cr, Mn, Ni, Cu, and Zn, the fits are still reasonable, but improvements are needed. We marginally reproduce the nitrogen abundance in very low metallicity stars. Overall, our results are consistent with the predicted abundance ratios seen in previous studies of the enrichment history of the Milky Way. We have demonstrated that our semi-analytic one-zone model, which cannot deal with spatial information such as the metallicity gradient, can nevertheless successfully reproduce the global Galactic enrichment evolution obtained by more complex models, at a fraction of the computational cost. This model is therefore suitable for a subgrid treatment of chemical enrichment in large-scale cosmological simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

An improved multipole approximation for self-gravity and its importance for core-collapse supernova simulations

Self-gravity computation by multipole expansion is a common approach in problems such as core-collapse and Type Ia supernovae, where single large condensations of mass must be treated. The standard formulation of multipole self-gravity in arbitrary coordinate systems suffers from two significant sources of error, which we correct in the formulation presented in this article. The first source of error is due to the numerical approximation that effectively places grid cell mass at the central point of the cell, then computes the gravitational potential at that point, resulting in a convergence failure of the multipole expansion. We describe a new scheme that avoids this problem by computing gravitational potential at cell faces. The second source of error is due to sub-optimal choice of location for the expansion center, which results in angular power at high multipole l values in the gravitational field, requiring a high—and expensive—value of multipole cutoff l {sub max}. By introducing a global measure of angular power in the gravitational field, we show that the optimal coordinate for the expansion is the square-density-weighted mean location. We subject our new multipole self-gravity algorithm, implemented in the FLASH simulation framework, to two rigorous test problems: MacLaurin spheroids for which exact analytic solutions are known, and core-collapse supernovae. We show that key observables of the core-collapse simulations, particularly shock expansion, proto-neutron star motion, and momentum conservation, are extremely sensitive to the accuracy of the multipole gravity, and the accuracy of their computation is greatly improved by our reformulated solver.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

The close binary properties of massive stars in the Milky Way and low-metallicity Magellanic Clouds

In order to understand the rates and properties of Type Ia and Type Ib/c supernovae, X-ray binaries, gravitational wave sources, and gamma-ray bursts as a function of galactic environment and cosmic age, it is imperative that we measure how the close binary properties of O- and B-type stars vary with metallicity. We have studied eclipsing binaries with early B main-sequence primaries in three galaxies with different metallicities: the Large and Small Magellanic Clouds (LMC and SMC, respectively) and the Milky Way (MW). The observed fractions of early B stars that exhibit deep eclipses 0.25 < Δm (mag) < 0.65 and orbital periods 2 < P (days) < 20 in the MW, LMC, and SMC span a narrow range of (0.7-1.0)%, which is a model-independent result. After correcting for geometrical selection effects and incompleteness toward low-mass companions, we find for early B stars in all three environments (1) a close binary fraction of (22 ± 5)% across orbital periods 2 < P (days) < 20 and mass ratios q = M {sub 2}/M {sub 1} > 0.1, (2) an intrinsic orbital period distribution slightly skewed toward shorter periods relative to a distribution that is uniform in log P, (3) a mass-ratio distribution weighted toward low-mass companions, and (4) a small, nearly negligible excess fraction of twins with q > 0.9. Our fitted parameters derived for the MW eclipsing binaries match the properties inferred from nearby, early-type spectroscopic binaries, which further validates our results. There are no statistically significant trends with metallicity, demonstrating that the close binary properties of massive stars do not vary across metallicities –0.7 < log(Z/Z {sub ☉}) < 0.0 beyond the measured uncertainties.

79 ASTRONOMY AND ASTROPHYSICS↗