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At least 19 recordsLinked to original sources
arXiv2008

Two new basaltic asteroids in the Outer Main Belt?

The identification of basaltic asteroids in the asteroid Main Belt and the description of their surface mineralogy is necessary to understand the diversity in the collection of basaltic meteorites. Basaltic asteroids can be identified from their visible reflectance spectra and are classified as V-type in the usual taxonomies. In this work, we report visible spectroscopic observations of two candidate V-type asteroids, (7472) Kumakiri and (10537) 1991 RY16, located in the outer Main Belt (a > 2.85 UA). These candidate have been previously identified by Roig and Gil-Hutton (2006, Icarus 183, 411) using the Sloan Digital Sky Survey colors. The spectroscopic observations have been obtained at the Calar Alto Observatory, Spain, during observational runs in November and December 2006. The spectra of these two asteroids show the steep slope shortwards of 0.70 microns and the deep absorption feature longwards of 0.75 microns that are characteristic of V-type asteroids. However, the presence of a shallow but conspicuous absorption band around 0.65 microns opens some questions about the actual mineralogy of these two asteroids. Such band has never been observed before in basaltic asteroids with the intensity we detected it. We discuss the possibility for this shallow absorption feature to be caused by the presence of chromium on the asteroid surface. Our results indicate that, together with (1459) Magnya, asteroids (7472) Kumakiri and (10537) 1991 RY16 may be the only traces of basaltic material found up to now in the outer Main Belt.

astro-ph↗
arXiv2007

The height dependence of temperature - velocity correlation in the solar photosphere

We derive correlation coefficients between temperature and line-of-sight velocity as a function of optical depth throughout the solar photosphere for the non-magnetic photosphere and a small area of enhanced magnetic activity. The maximum anticorrelation of about -0.6 between temperature and line-of-sight velocity in the non-magnetic photosphere occurs at log tau5 = -0.4. The magnetic field is another decorrelating factor along with 5-min oscillations and seeing.

astro-ph↗
arXiv2007

Terrestrial and Habitable Planet Formation in Binary and Multi-star Systems

One of the most surprising discoveries of extrasolar planets is the detection of planets in moderately close binary star systems. The Jovian-type planets in the two binaries of Gamma Cephei and GJ 86 have brought to the forefront questions on the formation of giant planets and the possibility of the existence of smaller bodies in such dynamically complex environments. The diverse dynamical characteristics of these objects have made scientists wonder to what extent the current theories of planet formation can be applied to binaries and multiple star systems. At present, the sensitivity of the detection techniques does not allow routine discovery of Earth-sized bodies in binary systems. However, with the advancement of new techniques, and with the recent launch of CoRoT and the launch of Kepler in late 2008, the detection of more planets (possibly terrestrial-class objects) in such systems is on the horizon. Theoretical studies and numerical modeling of terrestrial and habitable planet formation are, therefore, necessary to gain fundamental insights into the prospects for life in such systems and have great strategic impact on NASA science and missions.

astro-ph↗
arXiv2007

Extrasolar scale change in Newton's Law from 5D `plain' R^2-gravity

Galactic rotation curves and lack of direct observations of Dark Matter may indicate that General Relativity is not valid (on galactic scale) and should be replaced with another theory. There is the only variant of Absolute Parallelism which solutions are free of arising singularities, if D=5 (there is no room for changes). This variant does not have a Lagrangian, nor match GR: an equation of `plain' R^2-gravity (ie without R-term) is in sight instead. Arranging an expanding O_4-symmetrical solution as the basis of 5D cosmological model, and probing a universal_function of mass distribution (along very-very long the extra dimension) to place into bi-Laplace equation (R^2 gravity), one can derive the Law of Gravitation: 1/r^2 transforms to 1/r with distance (not with acceleration).

astro-ph↗
arXiv2008

Bar-Halo Friction in Galaxies III: Particle Number Requirements for Simulations

The question whether the dark matter halo density in the centers of galaxies could be changed through interactions with a rotating bar in the baryonic disk is of considerable current interest. While N-body simulations have been used to address this question, it has also been claimed that results from such simulations cannot be trusted. Based on a perturbative treatment of resonant exchanges between orbits and a rotating perturbation, Weinberg & Katz contend that N-body simulations of this process will not reveal the continuum result unless many more than the usual numbers of particles are employed. Here I report a study designed to examine their contention, finding results that show no dependence on the number of particles over the range usually employed up to that advocated by these authors. I show that my results are independent of all numerical parameters, and that field methods perform equally with grid methods in this respect. I also identify the reasons that the required particle number suggested by Weinberg & Katz is excessive.

astro-ph↗
arXiv2008

Neutrino Astronomy with High Spatial Resolution is Already Existing

By basing on observations of skeletal structures of the Sun and assuming that some of them are located inside of star, and also that a filamentary (linear) matter (whose a model earlier was put forward by B.U. Rodionov) is in basis of these internal structures the author consider possible processes of images formation of these structures inside the Sun and theirs coming out into space and also gives an elementary estimations of its parameters, which allow: i) to form their images in a flux of electronic neutrinos; ii) to carry out these images from within of the Sun into space; iii) to develop these images in form of a concomitant flux of soft x-ray, which next is recorded by telescope of soft x-ray. It is supposed the processes considered here, actually, can be accepted as future base of neutrino astronomy with high spatial resolution.

astro-ph↗
arXiv2007

Did time begin? Will time end?

Did time begin at a Big Bang? Will the present expansion of the universe last for a finite or infinite time? These questions sound philosophical but are becoming, now in the twenty-first century, central to the scientific study of cosmology. The answers, which should become clarified in the next decade or two, could have profound implications for how we see our own role in the universe. Since the original publication of Stephen Hawking's {\it A Brief History of Time} in 1988, the answers to these questions have progressed as a result of research by the community of active theoretical physicists including myself. To present the underlying ideas requires discussion of a wide range of topics in cosmology, especially the make up of the energy content of the universe. A brief summary of my conclusions, that of three different possibilities concerning the history and future of time, the least likely is the conventional wisdom (time began and will never end) and most likely is a cyclic model (time never begins or ends), is in the short final Chapter which could be read first. To understand the reasoning leading to my conclusions could encourage reading of my entire book. My hope in writing this, my first popular book, is that it will engender reflection about time. Many a non-scientist may already hold a philosophical opinion about whether time begins and ends. This book's aim is to present some recently discovered scientific facts which can focus the reader's consideration of the two short questions in my title.

astro-ph↗
arXiv2007

Lowest Landau Level of Relativistic Field Theories in a Strong Background Field

We consider gauge theories in a strong external magnetic like field. This situation can appear either in conventional four-dimensional theories, but also naturally in extra-dimensional theories and especially in brane world models. We show that in the lowest Landau level approximation, some of the coordinates become non-commutative. We find physical reasons to formal problems with non-commutative gauge theories such as the issue with SU(N) gauge symmetries. Our construction is applied to a minimal extension of the standard model. It is shown that the Higgs sector might be non-commutative whereas the remaining sectors of the standard model remain commutative. Signatures of this model at the LHC are discussed. We then discuss an application to a dark matter sector coupled to the Higgs sector of the standard model and show that here again, dark matter could be non-commutative, the standard model fields remaining commutative.

hep-ph↗
arXiv2007

Neutrino-cooled accretion and GRB variability

For accretion rates Mdot~0.1 Msun/s to a few solar mass black hole the inner part of the disk is expected to make a transition from advection dominance to neutrino cooling. This transition is characterized by sharp changes of the disk properties. I argue here that during this transition, a modest increase of the accretion rate leads to powerful enhancement of the Poynting luminosity of the GRB flow and decrease of its baryon loading. These changes of the characteristics of the GRB flow translate into changing gamma-ray spectra from the photosphere of the flow. The photospheric interpretation of the GRB emission explains the observed narrowing of GRB pulses with increasing photon energy and the luminosity-spectral peak relation within and among bursts.

astro-ph↗
arXiv2007

Radiation from Kinetic Poynting Flux Acceleration

We derive analytic formulas for the power output and critical frequency of radiation by electrons accelerated by relativistic kinetic Poynting flux, and validate these results with Particle-In-Cell plasma simulations. We find that the in-situ radiation power output and critical frequency are much below those predicted by the classical synchrotron formulae. We discuss potential astrophysical applications of these results.

astro-ph↗
arXiv2007

Bone Cancer Rates in Dinosaurs Compared with Modern Vertebrates

Data on the prevalence of bone cancer in dinosaurs is available from past radiological examination of preserved bones. We statistically test this data for consistency with rates extrapolated from information on bone cancer in modern vertebrates, and find that there is no evidence of a different rate. Thus, this test provides no support for a possible role of ionizing radiation in the K-T extinction event.

q-bio.PE↗
arXiv2007

Unimodular Relativity and the Dark Matter Problem

We introduce a modified divergence law for the energy-momentum tensor in the theory of unimodular relativity. Consequently, an additional equation for the measure field follows from the divergence of the field equations. The equations of motion are derived and the weak-field, low-velocity limit is investigated. It is found that the gravitational acceleration acquires a term that is proportional to the gradient of the mass density. We show that this term can provide the additional acceleration observed on astrophysical scales without the need for dark matter.

gr-qc↗
arXiv2007

Newtonian limit of scalar-tensor theories and galactic dynamics: isolated and interacting galaxies

We use the Newtonian limit of a general scalar-tensor theory around a background field to study astrophysical effects. The gravitational theory modifies the standard Newtonian potential by adding a Yukawa term to it, which is quantified by two theoretical parameters: $λ$, the lenghtscale of the gravitational interaction and its strength, $α$. Within this formalism we firstly present a numerical study on the formation of bars in isolated galaxies. We have found for positive $α$ that the modified gravity destabilizes the galactic discs and leads to rapid bar formation in isolated galaxies. Values of $λ$ in the range $\approx 8$ -- 14 kpc produce strongest bars in isolated models. Then, we extent this work by considering tidal effects due to interacting galaxies. We send two spirals to collide and study the bar properties of the remnant. We characterize the bar kinematical properties in terms of our parameters ($λ, α$).

astro-ph↗
arXiv2007

Supergiant Fast X-ray Transients: A common behaviour or a class of objects?

INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours. A substantial fraction of these sources are associated with OB supergiants and hence define a new class of massive X-ray binaries, which we call Supergiant Fast X-ray Transients. Characterisation of the astrophysical parameters of their counterparts is underway. So far, we have found a number of late O and early B supergiants of different luminosities at a large range of distances. Nothing in their optical properties sets them apart from classical Supergiant X-ray Binaries. On the other hand, there is now rather concluding evidence that persistent supergiant X-ray binaries also show fast outbursts. This suggests a continuum of behaviours between typical persistent supergiant systems and purely transient systems, but offers very little information about the physical causes of the outbursts.

astro-ph↗
arXiv2007

Results on Dark Matter and beta beta decay modes by DAMA at Gran Sasso

DAMA is an observatory for rare processes and it is operative deep underground at the Gran Sasso National Laboratory of the I.N.F.N. (LNGS). Here some arguments will be presented on the investigation on dark matter particles by annual modulation signature and on some of the realized double beta decay searches.

astro-ph↗
arXiv2007

What do we really know about mass loss on the AGB?

Mass loss rate formulae are derived from observations or from suites of models. For theoretical models, the following have all been identified as factors greatly influencing the atmospheric structure and mass loss rates: Pulsation with piston amplitude scaling appropriately with stellar L; dust nucleation and growth, with radiation pressure and grain-gas interactions and appropriate dependence on temperature and density; non-grey opacity with at least 51 frequency samples; non-LTE and departures from radiative equilibrium in the compressed and expanding flows; and non-equilibrium processes affecting the composition (grain formation; molecular chemistry). No one set of models yet includes all the factors known to be important. In fact, it is very difficult to construct a model that can simultaneously include these factors and be useful for computing spectra. Therefore, although theoretical model grids are needed to separate the effects of M,L,R and/or $T_{\mathrm{eff}}$ or Z on the mass loss rates, these models must be carefully checked against observations. Getting the right order of magnitude for the mass loss rate is only the first step in such a comparison, and is not sufficient to determine whether the mass loss formula is correct. However, there are observables that do test the validity of mass loss formulae as they depend directly on $d\log \dot M/d\log L$, $d\log \dot M/d\log R$, or $d\log \dot M/d\log P$.

astro-ph↗
arXiv2008

Chaotic traveling rolls in Rayleigh-Bénard convection

In this paper we investigate two-dimensional (2D) Rayleigh-Bénard convection using direct numerical simulation in Boussinesq fluids of Prandtl number $P = 6.8$ confined between thermally conducting plates. We show through the simulation that in a small range of reduced Rayleigh number $r$ ($770 < r < 890$) the 2D rolls move chaotically in a direction normal to the roll axis. The lateral shift of the rolls may lead to global flow reversal of the convective motion. The chaotic traveling rolls are observed in simulation with {\it free-slip} as well as {\it no-slip} boundary conditions on the velocity field. We show that the traveling rolls and the flow reversal are due to an interplay between the real and imaginary parts of the critical modes.

nlin.CD↗