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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.

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At least 523 records · Page 29
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DOE OSTI2017

Model for how an accretion disk drives astrophysical jets and sheds angular momentum

Clumps of ions and neutrals in the weakly ionized plasma in an accretion disk are shown to follow trajectories analogous to those of fictitious 'metaparticles' having a charge to mass ratio reduced from that of an ion by the ionization fraction. A certain class of meta-particles have zero-canonical angular momentum and so spiral in towards the star. Accumulation of these meta-particles establishes a radial electric field that drives the electric current that flows in bidirectional astrophysical jets lying along the disk axis and provides forces that drive the jets. In conclusion, the entire process converts gravitational potential energy into jet energy while absorbing angular momentum from accreting material and shedding this angular momentum at near infinite radius.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Taming astrophysical bias in direct dark matter searches

We explore systematic biases in the identification of dark matter in future direct detection experiments and compare the reconstructed dark matter properties when assuming a self-consistent dark matter distribution function and the standard Maxwellian velocity distribution. We find that the systematic bias on the dark matter mass and cross-section determination arising from wrong assumptions for its distribution function is of order ∼ 1σ. A much larger systematic bias can arise if wrong assumptions are made on the underlying Milky Way mass model. However, in both cases the bias is substantially mitigated by marginalizing over galactic model parameters. We additionally show that the velocity distribution can be reconstructed in an unbiased manner for typical dark matter parameters. Our results highlight both the robustness of the dark matter mass and cross-section determination using the standard Maxwellian velocity distribution and the importance of accounting for astrophysical uncertainties in a statistically consistent fashion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Dynamics of astrophysical objects against the cosmological background

In this paper, we consider dynamical behavior of astrophysical objects such as galaxies and dwarf galaxies taking into account both the gravitational attraction between them and the cosmological expansion of the Universe. First, we obtain the general system of equations and apply them to some abstract systems of galaxies. Then we investigate the collision between the Milky Way and Andromeda in future. Here, we distinguish two models. For the first one, we do not take into account the influence of the Intra-Group Matter (IGrM). In this case, we demonstrate that for currently known parameters of this system the collision is hardly plausible because of the angular momentum. These galaxies will approach the minimum distance of about 290 Kpc in 4.44 Gyr from present, and then begin to run away irreversibly from each other. For the second model, we take into account the dynamical friction due to the IGrM. Here, we find a characteristic value of the IGrM particle velocity dispersion σ-tilde = 2.306. For σ-tilde ≤ 2.306, the merger will take place, but for the bigger values of σ-tilde the merger can be problematic. If the temperature of the IGrM particles is 10{sup 5} K, then this characteristic value of σ-tilde corresponds to the IGrM particle mass 17 MeV. Therefore, for the IGrM particles with masses less than 17 MeV the merger becomes problematic. We also define the region in the vicinity of our Local Group where the formation of the Hubble flows starts. For such processes, the zero-acceleration surface (where the gravitational attraction is balanced by the cosmological accelerated expansion) plays the crucial role. We show that such surface is absent for the Local Group. Instead, we find two points and one circle with zero acceleration. Nevertheless, there is a nearly closed area around the MW and M31 where the absolute value of the acceleration is approximately equal to zero. The Hubble flows are formed outside of this area.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Astrophysical tests of modified gravity: the morphology and kinematics of dwarf galaxies

This paper is the third in a series on tests of gravity using observations of stars and nearby dwarf galaxies. We carry out four distinct tests using published data on the kinematics and morphology of dwarf galaxies, motivated by the theoretical work of Hui et al. (2009) and Jain and Vanderplas (2011). In a wide class of gravity theories a scalar field couples to matter and provides an attractive fifth force. Due to their different self-gravity, stars and gas may respond differently to the scalar force leading to several observable deviations from standard gravity. HI gas, red giant stars and main sequence stars can be displaced relative to each other, and the stellar disk can display warps or asymmetric rotation curves aligned with external potential gradients. To distinguish the effects of modified gravity from standard astrophysical phenomena, we use a control sample of galaxies that are expected to be screened from the fifth force. In all cases we find no significant deviation from the null hypothesis of general relativity. The limits obtained from dwarf galaxies are not yet competitive with the limits from cepheids obtained in our first paper, but can be improved to probe regions of parameter space that are inaccessible using other tests. We discuss how our methodology can be applied to new radio and optical observations of nearby galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The sensitivity of Cherenkov telescopes to dark matter and astrophysical anisotropies in the diffuse gamma-ray background

In this article, the capability of present (H.E.S.S., MAGIC, VERITAS) and planned (CTA) ground-based Cherenkov telescope systems for detecting angular anisotropies in the diffuse gamma-ray background is investigated. Following up on a study of the impact of instrumental characteristics (effective area, field of view, angular resolution, and background rejection efficiency), the first part examines the influence of different observational strategies, i.e. whether a single deep observation or a splitting over multiple shallow fields is preferred. In the second part, the sensitivity to anisotropies generated by self-annihilating dark matter is studied for different common dark matter models. We find that a relative contribution of ∼ 10% from dark matter annihilation to the extra-galactic diffuse gamma-ray background can be detected with planned configurations of CTA. In terms of the thermally-averaged self-annihilation cross section, the sensitivity of CTA corresponds to values below the thermal freeze-out expectation (σv) = 3 × 10{sup −26} cm{sup 3} s{sup −1} for dark matter particles lighter than ∼ 200 GeV. We stress the importance of constraining anisotropies from unresolved astrophysical sources with currently operating instruments already, as a novel and complementary method for investigating the properties of TeV sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Astrophysical black holes in screened modified gravity

Chameleon, environmentally dependent dilaton, and symmetron gravity are three models of modified gravity in which the effects of the additional scalar degree of freedom are screened in dense environments. They have been extensively studied in laboratory, cosmological, and astrophysical contexts. In this paper, we present a preliminary investigation into whether additional constraints can be provided by studying these scalar fields around black holes. By looking at the properties of a static, spherically symmetric black hole, we find that the presence of a non-uniform matter distribution induces a non-constant scalar profile in chameleon and dilaton, but not necessarily symmetron gravity. An order of magnitude estimate shows that the effects of these profiles on in-falling test particles will be sub-leading compared to gravitational waves and hence observationally challenging to detect.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Modified gravity inside astrophysical bodies

Many theories of modified gravity, including the well studied Horndeski models, are characterized by a screening mechanism that ensures that standard gravity is recovered near astrophysical bodies. In a recently introduced class of gravitational theories that goes beyond Horndeski, it has been found that new derivative interactions lead to a partial breaking of the Vainshtein screening mechanism inside any gravitational source, although not outside. We study the impact of this new type of deviation from standard gravity on the density profile of a spherically symmetric matter distribution, in the nonrelativistic limit. For simplicity, we consider a polytropic equation of state and derive the modifications to the standard Lane-Emden equations. We also show the existence of a universal upper bound on the amplitude of this type of modified gravity, independently of the details of the equation of state.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Dark energy and equivalence principle constraints from astrophysical tests of the stability of the fine-structure constant

Astrophysical tests of the stability of fundamental couplings, such as the fine-structure constant α, are becoming an increasingly powerful probe of new physics. Here we discuss how these measurements, combined with local atomic clock tests and Type Ia supernova and Hubble parameter data, constrain the simplest class of dynamical dark energy models where the same degree of freedom is assumed to provide both the dark energy and (through a dimensionless coupling, ζ, to the electromagnetic sector) the α variation. Specifically, current data tightly constrains a combination of ζ and the present dark energy equation of state w{sub 0}. Moreover, in these models the new degree of freedom inevitably couples to nucleons (through the α dependence of their masses) and leads to violations of the Weak Equivalence Principle. We obtain indirect bounds on the Eötvös parameter η that are typically stronger than the current direct ones. We discuss the model-dependence of our results and briefly comment on how the forthcoming generation of high-resolution ultra-stable spectrographs will enable significantly tighter constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AMS-02 antiprotons, at last! Secondary astrophysical component and immediate implications for Dark Matter

Using the updated proton and helium fluxes just released by the AMS-02 experiment we reevaluate the secondary astrophysical antiproton to proton ratio and its uncertainties, and compare it with the ratio preliminarly reported by AMS-02. We find no unambiguous evidence for a significant excess with respect to expectations. Yet, some preference for a flatter energy dependence of the diffusion coefficient (with respect to the MED benchmark often used in the literature) starts to emerge. Also, we provide a first assessment of the room left for exotic components such as Galactic Dark Matter annihilation or decay, deriving new stringent constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Do high energy astrophysical neutrinos trace star formation?

The IceCube Neutrino Observatory has provided the first map of the high energy (∼ 0.01–1 PeV) sky in neutrinos. Since neutrinos propagate undeflected, their arrival direction is an important identifier for sources of high energy particle acceleration. Reconstructed arrival directions are consistent with an extragalactic origin, with possibly a galactic component, of the neutrino flux. We present a statistical analysis of positional coincidences of the IceCube neutrinos with known astrophysical objects from several catalogs. When considering starburst galaxies with the highest flux in gamma-rays and infrared radiation, up to n=8 coincidences are found, representing an excess over the ∼4 predicted for the randomized, or ''null'' distribution. The probability that this excess is realized in the null case, the p-value, is p=0.042. This value falls to p=0.003 for a partial subset of gamma-ray-detected starburst galaxies and superbubble regions in the galactic neighborhood. Therefore, it is possible that starburst galaxies, and the typically hundreds of superbubble regions within them, might account for a portion of IceCube neutrinos. The physical plausibility of such correlation is discussed briefly.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

CRPropa 3—a public astrophysical simulation framework for propagating extraterrestrial ultra-high energy particles

We present the simulation framework CRPropa version 3 designed for efficient development of astrophysical predictions for ultra-high energy particles. Users can assemble modules of the most relevant propagation effects in galactic and extragalactic space, include their own physics modules with new features, and receive on output primary and secondary cosmic messengers including nuclei, neutrinos and photons. In extension to the propagation physics contained in a previous CRPropa version, the new version facilitates high-performance computing and comprises new physical features such as an interface for galactic propagation using lensing techniques, an improved photonuclear interaction calculation, and propagation in time dependent environments to take into account cosmic evolution effects in anisotropy studies and variable sources. First applications using highlighted features are presented as well.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Cosmic-ray propagation with DRAGON2: I. numerical solver and astrophysical ingredients

We present version 2 of the DRAGON code designed for computing realistic predictions of the CR densities in the Galaxy. The code numerically solves the interstellar CR transport equation (including inhomogeneous and anisotropic diffusion, either in space and momentum, advective transport and energy losses), under realistic conditions. The new version includes an updated numerical solver and several models for the astrophysical ingredients involved in the transport equation. Improvements in the accuracy of the numerical solution are proved against analytical solutions and in reference diffusion scenarios. The novel features implemented in the code allow to simulate the diverse scenarios proposed to reproduce the most recent measurements of local and diffuse CR fluxes, going beyond the limitations of the homogeneous galactic transport paradigm. To this end, several applications using DRAGON2 are presented as well. This new version facilitates the users to include their own physical models by means of a modular C++ structure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

H-ATLAS/GAMA: magnification bias tomography. Astrophysical constraints above ∼1 arcmin

An unambiguous manifestation of the magnification bias is the cross-correlation between two source samples with non-overlapping redshift distributions. In this work we measure and study the cross-correlation signal between a foreground sample of GAMA galaxies with spectroscopic redshifts in the range 0.2< z <0.8, and a background sample of H-ATLAS galaxies with photometric redshifts ∼>1.2. It constitutes a substantial improvement over the cross-correlation measurements made by Gonzalez-Nuevo et al. (2014) with updated catalogues and wider area (with S / N ∼> 5 below 10 arcmin and reaching S / N ∼ 20 below 30 arcsec). The better statistics allow us to split the sample in different redshift bins and to perform a tomographic analysis (with S / N ∼> 3 below 10 arcmin and reaching S / N ∼ 15 below 30 arcsec). Moreover, we implement a halo model to extract astrophysical information about the background galaxies and the deflectors that are producing the lensing link between the foreground (lenses) and background (sources) samples. In the case of the sources, we find typical mass values in agreement with previous studies: a minimum halo mass to host a central galaxy, M {sub min}∼ 10{sup 12.26} M {sub ⊙}, and a pivot halo mass to have at least one sub-halo satellite, M {sub 1∼} 10{sup 12.84} M {sub ⊙}. However, the lenses are massive galaxies or even galaxy groups/clusters, with minimum mass of M {sub min}{sup lens}∼ 10{sup 13.06} M {sub ⊙}. Above a mass of M {sub 1}{sup lens}∼ 10{sup 14.57} M {sub ⊙} they contain at least one additional satellite galaxy which contributes to the lensing effect. The tomographic analysis shows that, while M {sub 1}{sup lens} is almost redshift independent, there is a clear evolution of increase M {sub min}{sup lens} with redshift in agreement with theoretical estimations. Finally, the halo modeling allows us to identify a strong lensing contribution to the cross-correlation for angular scales below 30 arcsec. This interpretation is supported by the results of basic but effective simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Model-independent astrophysical constraints on leptophilic Dark Matter in the framework of Tsallis statistics

Here, we derive model-independent astrophysical constraints on leptophilic dark matter (DM), considering its thermal production in a supernova core and taking into account core temperature fluctuations within the framework of q-deformed Tsallis statistics. In an effective field theory approach, where the DM fermions interact with the Standard Model via dimension-six operators of either scalar-pseudoscalar, vector-axial vector, or tensor-axial tensor type, we obtain bounds on the effective cut-off scale Λ from supernova cooling and free-streaming of DM from supernova core, and from thermal relic density considerations, depending on the DM mass and the q-deformation parameter. Using Raffelt's criterion on the energy loss rate from SN1987A, we obtain a lower bound on Λ ≳ 3 (12) TeV corresponding to q = 1.0 (1.1) and an average supernova core temperature of 0T SN =3 MeV . From the optical depth criterion on the free-streaming of DM fermions from the outer 10% of the SN1987A core, the cooling bound is restricted to Λ ≳ 1 TeV . Both cooling and free-streaming bounds are insensitive to the DM mass m χ for m χ ≲ T SN , whereas for m χ >> T SN , the bounds weaken significantly due to the Boltzmann-suppression of the DM number density. We also calculate the thermal relic density of the DM particles in this setup and find that it imposes an upper bound on Λ 4 /m χ 2 , which together with the cooling/free-streaming bound significantly constrains light leptophilic DM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A SIGNIFICANT AMOUNT OF CRYSTALLINE SILICA IN RETURNED COMETARY SAMPLES: BRIDGING THE GAP BETWEEN ASTROPHYSICAL AND METEORITICAL OBSERVATIONS

Crystalline silica (SiO{sub 2}) is recurrently identified at the percent level in the infrared spectra of protoplanetary disks. By contrast, reports of crystalline silica in primitive meteorites are very unusual. This dichotomy illustrates the typical gap existing between astrophysical observations and meteoritical records of the first solids formed around young stars. The cometary samples returned by the Stardust mission in 2006 offer an opportunity to have a closer look at a silicate dust that experienced a very limited reprocessing since the accretion of the dust. Here, we provide the first extended study of silica materials in a large range of Stardust samples. We show that cristobalite is the dominant form. It was detected in 5 out of 25 samples. Crystalline silica is thus a common minor phase in Stardust samples. Furthermore, olivine is generally associated with this cristobalite, which put constraints on possible formation mechanisms. A low-temperature subsolidus solid–solid transformation of an amorphous precursor is most likely. This crystallization route favors the formation of olivine (at the expense of pyroxenes), and crystalline silica is the natural byproduct of this transformation. Conversely, direct condensation and partial melting are not expected to produce the observed mineral assemblages. Silica is preserved in cometary materials because they were less affected by thermal and aqueous alterations than their chondritic counterparts. The common occurrence of crystalline silica therefore makes the cometary material an important bridge between the IR-based mineralogy of distant protoplanetary disks and the mineralogy of the early solar system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Cosmology and astrophysics from relaxed galaxy clusters – V. Consistency with cold dark matter structure formation

This is the fifth in a series of papers studying the astrophysics and cosmology of massive, dynamically relaxed galaxy clusters. Our sample comprises 40 clusters identified as being dynamically relaxed and hot in Papers I and II of this series. Here we use constraints on cluster mass profiles from X-ray data to test some of the basic predictions of cosmological structure formation in the cold dark matter (CDM) paradigm. In addition, we present constraints on the concentration–mass relation for massive clusters, finding a power-law mass dependence with a slope of κ m = –0.16 ± 0.07, in agreement with CDM predictions. For this relaxed sample, the relation is consistent with a constant as a function of redshift (power-law slope with 1 + z of κ ζ = –0.17 ± 0.26), with an intrinsic scatter of σln c = 0.16 ± 0.03. We investigate the shape of cluster mass profiles over the radial range probed by the data (typically ~50 kpc–1 Mpc), and test for departures from the simple Navarro–Frenk–White (NFW) form, for which the logarithmic slope of the density profile tends to –1 at small radii. Specifically, we consider as alternatives the generalized NFW (GNFW) and Einasto parametrizations. For the GNFW model, we find an average value of (minus) the logarithmic inner slope of β = 1.02 ± 0.08, with an intrinsic scatter of σ β = 0.22 ± 0.07, while in the Einasto case we constrain the average shape parameter to be α = 0.29 ± 0.04 with an intrinsic scatter of σ α = 0.12 ± 0.04. Our results are thus consistent with the simple NFW model on average, but we clearly detect the presence of intrinsic, cluster-to-cluster scatter about the average.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Screened fifth forces mediated by dark matter-baryon interactions: Theory and astrophysical probes

We derive the details of a new screening mechanism where the interactions of baryons and dark matter can be screened according to the local dark matter density. In this mechanism, the value of Newton’s constant is dark matter-density dependent, allowing for the possibility that astrophysical phenomena are very different in galaxies less dense than the Milky Way. The parametrized post-Newtonian parameter γ, which quantifies the difference between kinematical and lensing probes, also depends on dark matter density. We calculate the effects of varying G on various stages of stellar evolution, focusing on observables that impact cosmology: the Cepheid period-luminosity relation and the supernova Ia magnitude-redshift relation. Other potential tests of the model are also investigated including main-sequence, post-main-sequence, and low-mass dwarf stars. Finally, we discuss how extragalactic tests of γ could provide complementary constraints.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

On Using Linux Kernel Huge Pages with FLASH, an Astrophysical Simulation Code

We present efforts at improving the performance of FLASH, a multi-scale, multi-physics simulation code principally for astrophysical applications, by using huge pages on Ookami, an HPE Apollo 80 A64FX platform. FLASH is written principally in modern Fortran and makes use of the PARAMESH library to manage a block-structured adaptive mesh. We explored options for enabling the use of huge pages with several compilers, but we were only able to successfully use huge pages when compiling with the Fujitsu compiler. As a result, the use of huge pages substantially reduced the number of translation lookaside buffer misses, but overall performance gains were marginal.

79 ASTRONOMY AND ASTROPHYSICS↗