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At least 631 records · Page 35Linked to original sources

arXiv2009

Hydrodynamics of structure formation in the early Universe

Theory and observations reveal fatal flaws in the standard LambdaCDM model. The cold dark matter hierarchical clustering paradigm predicts a gradual bottom-up growth of gravitational structures assuming linear, collisionless, ideal flows and unrealistic CDM condensations and mergers. Collisional fluid mechanics with viscosity, turbulence, and diffusion predicts a turbulent big bang and top-down viscous-gravitational fragmentation from supercluster to galaxy scales in the plasma epoch, as observed from 0.3 Gpc void sizes, 1.5 Gpc spins and Kolmogorov-fingerprint-turbulence-signatures in the CMB. Turbulence produced at expanding gravitational void boundaries causes a linear morphology of 3 Kpc fragmenting plasma-protogalaxies along vortex lines, as observed in deep HST images. After decoupling, gas-protogalaxies fragment into primordial-density, million-solar-mass clumps of earth-mass planets forming 0.3 Mpc galactic-dark-matter. White-dwarf-heated planet-atmospheres give dimmed SNe Ia events and false gamma-ray-burst luminosity distances, not dark-energy-Lambda. Quasar microlensing observations rule out no-hair black hole models and require galaxy-dark-matter to be planets-in-clumps.

astro-ph↗

arXiv2009

Constraining the Spin-Independent WIMP-Nucleon Coupling from Direct Dark Matter Detection Data

Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for Dark Matter. For understanding the properties of WIMPs and identifying them among new particles produced at colliders (hopefully in the near future), determinations of their mass and their couplings on nucleons from direct Dark Matter detection experiments are essential. Based on our method for determining the WIMP mass model-independently from experimental data, we present a way to also estimate the spin-independent (SI) WIMP-nucleon coupling by using measured recoil energies directly. This method isindependent of the as yet unknown velocity distribution of halo WIMPs. In spite of the uncertainty of the local WIMP density (of a factor of ~ 2), at least an upper limit on the SI WIMP-nucleon coupling could be given, once two (or more) experiments with different target nuclei obtain positive signals. In a background-free environment, for a WIMP mass of 100 GeV its SI coupling on nucleons could in principle be estimated with a statistical error of only ~ 15% with just 50 events from each experiment.

hep-ph↗

arXiv2008

Pulsating B and Be stars in the Magellanic Clouds

Stellar pulsations in main-sequence B-type stars are driven by the $κ$-mechanism due to the Fe-group opacity bump. The current models do not predict the presence of instability strips in the B spectral domain at very low metallicities. As the metallicity of the Magellanic Clouds has been measured to be around $Z=0.002$ for the SMC and $Z=0.007$ for the LMC, they constitute a very suitable objects to test these predictions.

astro-ph↗

arXiv2008

Polarimetry from the Ground Up

Ground-based solar polarimetry has made great progress over the last decade. Nevertheless, polarimetry is still an afterthought in most telescope and instrument designs, and most polarimeters are designed based on experience and rules of thumb rather than using more formal systems engineering approaches as is common in standard optical design efforts. Here we present the first steps in creating a set of systems engineering approaches to the design of polarimeters that makes sure that the final telescope-instrument-polarimeter system is more than the sum of its parts.

astro-ph↗

arXiv2009

Revisiting the effect of H2, HD and LiH molecules in the cooling of primordial gas

We use a non-equilibrium chemical network to revisit and study the effect of H_{2}, HD and LiH molecular cooling on a primordial element of gas. We solve both the thermal and chemical equations for a gas element with an initial temperature T\approx 1000K and a gas number density in the range n_{tot}=1-10^{4} cm^{-3}. At low densities, n_{tot}<10^{2} cm^{-3}, the gas reaches temperatures \sim 100K and the main coolant is H_{2}, but at higher densities, n_{tot}>10^{2} cm^{-3}, the HD molecule dominates the gas temperature evolution. The effect of LiH is negligible in all cases. We studied the effect of D abundance on the gas cooling. The D abundance was set initially to be in the range n_{D}/n_{H}=10^{-7}-10^{-4.5}, with n_{HD}/n_{H}={D^{+}}/n_{H}=10^{-10}. The simulations show that at n_{tot}>10^{2} cm^{-3} the HD cooling dominates the temperature evolution for D abundances greater than 10^{-5}n_{H}. This number decrease at higher densities. Furthermore, we studied the effect of electrons and ionized particules on the gas temperature. We followed the gas temperature evolution with n_{H_{+}}/n_{H}=10^{-4}-10^{-1} and n_{D^{+}}/n_{H^{+}}=10^{-5}. The gas temperature reached lower values at high ionization degree because electrons, H^{+} and D^{+} are catalizers in the formation paths of the H_{2} and HD molecules, which are the main coolers at low temperatures. Finaly, we studied the effect of an OB star, with T_{eff}=4\times 10^{4}K, would have on gas cooling. It is very difficult for a gas with n_{tot} in the range between 1-100 cm^{-3} to drop its temperature if the star is at a distance less than 100 pc.

astro-ph↗

arXiv2008

Large-amplitude intraday variability in QSO 1156+295 observed during a VLBI experiment

We present here the discovery of rapid, large amplitude intraday variability in the compact flat-spectrum radio quasar 1156+295. The detection of 40% flux density variations at 15 GHz on a timescale of only 2.7 hours was serendipitously made when the source was observed with the Very Long Baseline Array as a part of the MOJAVE survey programme on February 5, 2007. Intraday variability on timescales of a few hours or less is rare, and there exist very few sources that show large-amplitude variations on a timescale as short as what is now observed for 1156+295. The shape of the visibility function of the source changes very little during the observation, although the correlated flux density changes by 40%. This suggests that the variability occurs in a single dominant compact component. The observed variability characteristics are consistent with interstellar scintillation in nearby, highly turbulent medium. The rms amplitude of modulation at 15 GHz is unusually large and it implies a rather high scattering measure along the line-of-sight towards 1156+295.

astro-ph↗

arXiv2008

Microlensing to probe the quasar structure: spectrophotometry of Q2237+0305 and of J1131-1231

We present the main results of the first long-term spectrophotometric monitoring of the ``Einstein cross'' Q2237+0305 and of the single-epoch spectra of the lensed quasar J1131-1231. From October 2004 to December 2006, we find that two prominent microlensing events affect images A & B in Q2237+0305 while images C & D remain grossly unaffected by microlensing on a time scale of a few months. Microlensing in A & B goes with chromatic variations of the quasar continuum. We observe stronger micro-amplification in the blue than in the red part of the spectrum, as expected for continuum emission arising from a standard accretion disk. Microlensing induced variations of the CIII] emission are observed both in the integrated line intensity and profile. Finally, we also find that images C & D are about 0.1-0.3 mag redder than images A & B. The spectra of images A-B-C in J1131-1231 reveal that, in April 2003, microlensing was at work in images A and C. We find that microlensing de-amplifies the continuum emission and the Broad Line Region (BLR) in these images. Contrary to the case of Q2237+0305, we do not find evidence for chromatic microlensing of the continuum emission. On the other hand, we observe that the Balmer and MgII broad line profiles are deformed by microlensing. These deformations imply an anti-correlation between the width of the emission line and the size of the corresponding emitting region. Finally, the differential microlensing of the FeII emission suggests that the bulk of FeII is emitted in the outer parts of the BLR while another fraction of FeII is produced in a compact region.

astro-ph↗

arXiv2009

Transient outburst mechanisms in Supergiant Fast X-ray Transients

The recent discovery of a new class of recurrent and fast X-ray transient sources, the Supergiant Fast X-ray Transients, poses interesting questions on the possible mechanisms responsible for their transient X-ray emission. The association with blue supergiants, the spectral properties similar to those of accreting pulsars and the detection, in a few cases, of X-ray pulsations, confirm that these transients are High Mass X-ray Binaries. I review the different mechanisms proposed to explain their transient outbursts and the link to persistent wind accretors. I discuss the different models in light of the new observational results coming from an on-going monitoring campaign of four Supergiant Fast X-ray Transients with Swift.

astro-ph↗

arXiv2008

Range of stability of solar neutrino flux from the SAGE experiment data

We study the extent to which the SAGE experiment data indicate the permanence of the solar neutrino flux. It is shown that in the first approximation this flux is constant and its distribution function is unimodal. Using a more detailed analysis one finds out that data of the first years of experiment (1990-1992) demonstrate a time dependence which is slightly different from what was found for the subsequent years (1993-2006). The distinctive feature of the first years of experiment is a high dispersion of neutrino flux in comparison with the following epoch. We discuss possible astronomical consequences of this result.

astro-ph↗

arXiv2010

On the impact of the atmospheric drag on the LARES mission

The goal of the recently approved space-based LARES mission is to measure the general relativistic Lense-Thirring effect in the gravitational field of the spinning Earth at a repeatedly claimed 1% accuracy by combining its node Omega with those of the existing LAGEOS and LAGEOS II laser-ranged satellites. In this paper we show that, in view of the lower altitude of LARES (h=1450 km) with respect to LAGEOS and LAGEOS II (h\approx 6000 km), the cross-coupling between the effect of the atmospheric drag, both neutral and charged, on the inclination of LARES and its classical node precession due to the Earth's oblateness may induce a 3-9% year^-1 systematic bias on the total relativistic precession. Since its extraction from the data will take about 5-10 years, such a perturbing effect may degrade the total accuracy of the test, especially in view of the large uncertainties in modeling the drag force.

gr-qc↗

arXiv2008

Birth of the Universe from the Multiverse

It is fair to say that the deepest mystery in our understanding of nature is the birth of our universe. Much of the dilemma over the last decades comes from the extraordinarily small probability that the universe started with the high energy Big Bang as compared to the chance of nucleating any other event. How can Big Bang cosmology be $10^{10^{123}}$ times less likely than nucleating the present cold universe, while accumulating such exquisite agreement with astrophysical data? Why don't we see the other nucleations that, if left to chance, seem to overwhelmingly outnumber us? Here I discuss the point of view that the selection of the initial conditions can be meaningfully addressed only within the framework of the multiverse and that the reason why Big Bang inflation was preferred over other events lies in the quantum dynamics of the landscape of the initial patches. The out-of-equilibrium dynamics selected the 'survivor' universes be born at high energies and the 'terminal' universes at low energies. I briefly review the testable predictions of this theory, in particular the giant void observed in 2007. The second part focuses on the extended framework, in particular a set of postulates needed for defining the multiverse.

hep-th↗

arXiv2008

On Cosmic Rays Sources

Study of astrophysical objects with strong dipolar magnetic fields show that the spectrum of the accelerated charged particles leaving the sources has a power law form with exponent -2.5, where the exponent is calculated on purely geometrical bases and is independent on the particle species.

astro-ph↗

arXiv2008

Accreting White Dwarfs

Thermonuclear (type Ia) supernovae are explosions in accreting white dwarfs, but the exact scenario leading to these explosions is still unclear. An important step to clarify this point is to understand the behaviour of accreting white dwarfs in close binary systems. The characteristics of the white dwarf (mass, chemical composition, luminosity), the accreted material (chemical composition) and those related with the properties of the binary system (mass accretion rate), are crucial for the further evolution towards the explosion. An analysis of the outcome of accretion and the implications for the growth of the white dwarf towards the Chandrasekhar mass and its thermonuclear explosion is presented.

astro-ph↗

arXiv2008

ESA White paper: Atmospheric modeling: Setting Biomarkers in context

Motivation: ESAs goal to detect biomarkers in Earth-like exoplanets in the Habitable Zone requires theoretical groundwork that needs to be done to model the influence of different parameters on the detectable biomarkers. We need to model a wide parameter space (chemical composition, pressure, evolution, interior structure and outgassing, clouds) to generate a grid of models that inform our detection strategy as well as can help characterize the spectra of the small rocky planets detected.

astro-ph↗

arXiv2008

Quantitative Stellar Spectral Classification. IV. Application to the Open Cluster IC 2391

In this work we perform the first test of a stellar spectral classification method (Stock & Stock 1999) by applying it to early type stars. The sample of stars are the members of the open cluster IC 2391 that have high-resolution spectra available in the UVES Project of Paranal Observatory. We show that, in general, absolute magnitudes $M_V$ and intrinsic colors $(B-V)_0$ can be recovered within the uncertainties stated in the original calibration ($\sim 0.4$ for the magnitudes and $\sim 0.03$ for the colors). This accuracy allows us to estimate distances and to infer membership of individual stars to obtain an average distance to the cluster of $156\pm 24$ pc, which is in good agreement with previous reported determinations. Finally, we identify and discuss the real strengths and limitations of this method and we suggest how it can be improved for future studies.

astro-ph↗

arXiv2008

Spectral analyses of three carbon-enhanced metal-poor stars

We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong $G$ band was evident from the SDSS spectra and the weakness of the Ca {\sc ii} K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] $<-2.50$) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO$^5$BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to --0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date.

astro-ph↗

arXiv2008

The cosmic origin of carbon and manganese

[ABRIDGED] We have determined carbon abundances for 51 dwarf stars and manganese abundances for 95 dwarf stars in two distinct and well defined stellar populations - the Galactic thin and thick disks. As these two populations have different chemical histories we have been able to, through a differential abundance analysis using high-resolution spectra, constrain the formation sites for carbon and manganese in the Galactic disk(s). The analysis of carbon is based on the forbidden [C I] line at 872.7 nm which is an abundance indicator that is insensitive to errors in the stellar atmosphere parameters. Combining these data with our previously published oxygen abundances, based on the forbidden [O I] line at 630.0 nm, we can form very robust [C/O] ratios that we then used to investigate the origin of carbon and the chemical evolution of the Galactic thin and thick disks..... Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick disks have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment (i.e., SNIa and AGB stars, respectively). For manganese, when comparing our Mn abundances with O abundances for the same stars we find that the abundance trends in the stars with kinematics typical of the thick disk can be explained by metallicity dependent yields from SN II. Furthermore, the [Mn/O] versus [O/H] trend in the halo is flat. We conclude that the simplest interpretation of our data is that manganese most likely is produced in SN II and that the Mn yields for such SNae must be metallicity dependent.

astro-ph↗

arXiv2009

On a possible connection between Chandler wobble and dark matter

Chandler wobble excitation and damping, one of the open problems in geophysics, is treated as a consequence in part of the interaction between Earth and a hypothetical oblate ellipsoid made of dark matter. The physical and geometrical parameters of such an ellipsoid and the interacting torque strength is calculated in such a way to reproduce the Chandler wobble component of the polar motion in several epochs, available in the literature. It is also examined the consequences upon the geomagnetic field dynamo and generation of heat in the Earth outer core.

astro-ph↗