Astrophysical and cosmological considerations on a string dilaton of a least coupling

String-loop effects may generate very weak matter couplings for a (massless) dilaton. We examine limits on the shift of such a dilaton toward its present equilibrium value from big-bang nucleosynthesis and the binary pulsar. On the other hand, the approach of the dilaton toward its present value can be realized very early in the universe in a quick and efficient way if an inflationary period is present. We comment briefly on some implications.     

Searching for multifractal scaling in gamma ray burst time series

We present a multifractal analysis of GRB time series used to quantify the variability of the light curve. We discuss the advantages and disadvantages of multifractal analysis, including sensitivity to noise, and burst intensity, and present results for a few sample bursts.     

Prospects for solar flare X-ray polarimetry

The history of solar flare X-ray polarimetry is reviewed and it is shown that as yet, there is no experimental evidence for such polarization. The present experimental limits are at the level of a few percent but these results may be biased by a large thermal component at low energies which may decrease the apparent polarization. To avoid this difficulty it will be necessary to make observations at higher energies where thermal emission is less important.The theoretical estimates of the polarization expected in the solar flare are also reviewed. The best present theoretical estimates are in the range of a few percent and are consistent with the present experimental limits.In this paper we discuss a new satellite instrument that has sufficient sensitivity at high energies to detect the polarization that is predicted by the present theories. The instrument sensitivity for a moderate (M class) event approaches polarization levels of 1% in each of 7 energy bins spanning the 10 to 100 keV range for integration times as short as 10 s. Comparable results can be obtained for an X class flare in 1 s.Presidential Young Investigator.     

Solar Wind Electron Parameters from Ulysses/URAP Quasi-Thermal Noise Measurements at Solar Maximum

We present the solar wind plasma parameters obtained from the Ulysses spacecraft during its second pole-to-pole fast latitude scan near the 2001 solar maximum. We study the solar wind properties from the electron density and core temperature measurements made by the radio receiver on Ulysses using the method of quasi-thermal noise spectroscopy. We analyze these parameters as functions of heliographic latitude and distance. We present their histograms normalized to 1 AU and find a bimodal distribution for the electron core temperature. The cooler population can be associated with the fast wind flow emanating from coronal holes present at various latitudes. We discuss a slight north/south asymmetry found for the electron density. Finally, we compare the present results to those obtained during the 1996 solar minimum and 1991 solar maximum.     

The outbursts of dwarf novae

We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results.     

Chaotic behaviour of solar radio flux

The presence of a chaotic attractor is investigated in time series of 10.7 cm solar flux. The correlation dimension and the Kolomogorov entropy have been calculated for the time period 1964–1984. The values found for the Kolmogorov entropy show that chaos is indeed present. The correlation dimension found for high solar activity is 3.3 and for low solar activity is 4.5, indicating that a low-dimensionsion chaotic attractor is present in the time series analysed.     

Massive stars in the UV

We emphasize in this paper the importance of the UV range for our knowledge of massive stars and the fundamental role played by past and present space-based UV capabilities (IUE, HST, FUSE and others). Based on a review of the work developed in the last years and the state of the art situation for quantitative spectroscopy of massive stars, we present crucial advances which could be addressed by hypothetical future space-based UV missions. Advantages and unique data that these missions could provide are explained in the context of our present knowledge and theories on massive stars in the Milky Way and nearby galaxies. It is argued that these studies are our key to a correct interpretation of observations of more distant objects.     

Intracrystalline lipids within sulfates from the Haughton Impact Structure—Implications for survival of lipids on Mars

Lipids can be present within gypsum as intracrystalline inclusions if they become incorporated within the mineral as is it precipitates. The lipids that comprise these inclusions are protected against alteration or destruction by an external oxidising chemical environment because a protective mineral matrix surrounds them. Sulfate minerals are abundant on the surface of Mars and were present in the samples that were analysed by the Viking landers. The quantities of secondary intracrystalline fossil-lipids that are present in samples of gypsum and gypsum-rich soils from the Haughton Impact Structure, Devon Island, Canadian High Arctic are sufficient to suggest that if a similar concentration of fossil lipids was present in the sulfate-rich samples analysed by the Viking Landers then they could have been detected. Possible reasons why a secondary fossil-lipid signature was not detected include a poor rate of conversion during pyrolysis, exposure of intracrystalline lipids during periods of weathering to oxidative martian diagenesis, a low level of biological productivity or an absence of a source for lipids on the surface of Mars. Polycyclic aromatic hydrocarbons of meteoritic origin, and terpane biomarkers such as hopanes and steranes, are not present in the Haughton gypsum in sufficient quantities to have been readily detected.     

Delta-v requirements for earth co-orbital rendezvous missions

Earth co-orbital asteroids present advantages as potential targets for future asteroid rendezvous missions. Their prolonged proximity to Earth facilitates communication, while their Earth-like orbits mean a steady flux of solar power and no significant periodic heating and cooling of the spacecraft throughout the course of the mission. Theoretical studies show that low-inclination co-orbital orbits are more stable than high-inclination orbits. As inclination is the most significant indicator of low delta-v rendezvous orbits, there is the potential for a large population of easily accessible asteroids, with favorable engineering requirements. This study first looks at phase-independent rendezvous orbits to a large number of objects, then looks in more detail at the phase-dependent orbits to the most favorable objects. While rendezvous orbits to co-orbital objects do not have a low delta-v necessarily, some objects present energy requirements significantly less than previous rendezvous missions. Currently we find no ideal co-orbital asteroids for rendezvous missions, although theoretical Earth Trojans present very low-energy requirements for rendezvous.     

A search for astrophysical burst signals at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) has confirmed the standard solar model and neutrino oscillations through the observation of neutrinos from the solar core. In this paper we present a search for neutrinos associated with sources other than the solar core, such as gamma-ray bursts and solar flares. We present a new method for looking for temporal coincidences between neutrino events and astrophysical bursts of widely varying intensity. No correlations were found between neutrinos detected in SNO and such astrophysical sources.     

Rotational distortion of polytropes by analytic approximation

This paper modifies the first-order perturbation theory of Chandrasekhar, for rotational distortion of polytropes. Comparison with numerical integrations by other authors demonstrates that the present analytic theory is as accurate as other published first-order theories. The present theory is in a form permitting rapid calculation of boundary shapes as a function of the rotation parameter,v, and the polytrope index,n. Results are presented for a critically rotating polytrope, for the casen=3.     

An iterative method for constructing equilibrium phase models of stellar systems

We present a new method for constructing equilibrium phase models for stellar systems, which we call the iterative method. It relies on constrained, or guided evolution, so that the equilibrium solution has a number of desired parameters and/or constraints. This method is very powerful, to a large extent due to its simplicity. It can be used for mass distributions with an arbitrary geometry and a large variety of kinematical constraints. We present several examples illustrating it. Applications of this method include the creation of initial conditions for N -body simulations and the modelling of galaxies from their photometric and kinematic observations.     

A new model for the structure of the DACs regions in the Oe and Be stellar atmospheres

As it is already known, the spectra of many Oe and Be stars present Discrete Absorption Components (DACs) which, because of their profiles' width as well as the values of the expansion / contraction velocities, they create a complicated profile of the main spectral lines. This fact is interpreted by the existence of two or more independent layers of matter, in the region where the main spectral lines are formed. Such a structure is responsible for the formation of a series of satellite components (DACs) for each main spectral line. In this paper we present a first approximation to a mathematical model reproducing the complex profile of the spectral lines of Oe and Be stars that present DACs. This model presupposes that the regions, where these spectral lines are formed, are not continuous but consist of a number of independent absorbing density layers of matter, followed by an emission region and an external general absorption region. When we fit the spectral lines that present DACs, with this model, we can calculate the values of the apparent rotation and expansion / contraction velocities of the regions where the DACs are formed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reduced ambiguity calibration for LOFAR

Interferometric calibration always yields non unique solutions. It is therefore essential to remove these ambiguities before the solutions could be used in any further modeling of the sky, the instrument or propagation effects such as the ionosphere. We present a method for LOFAR calibration which does not yield a unitary ambiguity, especially under ionospheric distortions. We also present exact ambiguities we get in our solutions, in closed form. Casting this as an optimization problem, we also present conditions for this approach to work. The proposed method enables us to use the solutions obtained via calibration for further modeling of instrumental and propagation effects. We provide extensive simulation results on the performance of our method. Moreover, we also give cases where due to degeneracy, this method fails to perform as expected and in such cases, we suggest exploiting diversity in time, space and frequency.     

The spatial distribution of pulsars and the spiral structure of the Galaxy

We have looked for and found a possible spatial correlation between the present pulsar distribution and the estimated locations of the spiral arms at earlier epochs. Through a detailed statistical analysis we find a significant correlation between the present distribution of pulsars and the mass distribution (in the spiral arms) expected about 60 Myr ago for a corotation resonance radius of 14kpc. We discuss the implications of this correlation for the minimum mass of the progenitors of pulsars. Interpreting the spread in the locations of pulsars with respect to the past locations of the spiral arms as predominantly due to their space velocities, we derive an average velocity for the pulsar population.     

Detection of superimposed periodic signals using wavelets

In this paper we present a wavelet-based algorithm that is able to detect superimposed periodic signals in data with low signal-to-noise ratio. In this context, the results given by classical period determination methods depend strongly on the intrinsic characteristics of each periodic signal, like amplitude or profile. It is then difficult to detect the different periods present in the data set. The results given by the wavelet-based method for period determination we present here are independent of the characteristics of the signals.     

Cosmic acceleration and extra dimensions: constraints on modifications of the Friedmann equation

An alternative to dark energy as an explanation for the present phase of accelerated expansion of the Universe is that the Friedmann equation is modified, e.g. by extra dimensional gravity, on large scales. We explore a natural parametrization of a general modified Friedmann equation, and find that the present supernova Type Ia and cosmic microwave background data prefer a correction of the form 1/ H to the Friedmann equation over a cosmological constant.     

Kinematics of Giant HII Regions in Nearby Galaxies

We present optical high-spectral-resolution long-slit observationsof massive star formation regions in the nearby galaxies M101, NGC2403, and M33 obtained with the WHT. We have constructed velocity,line dispersion and intensity maps using between 6 and 12 parallelslits in each case. We will present the data for seven regions:NGC 5455, NGC 5461, NGC 5471 in M101; three regions in NGC 2403;and NGC 595 in M33. We will also present a detailed study of thedifferent profiles found across the regions, paying specialattention to the presence of supersonic profiles and multiplecomponents.