In recent years many sets of experimental data have been obtained on radar reflections from the moon. After analyzing the results, workers tried to determine the scattering properties of the moon at radar wavelengths, and they concluded that rough-scattering laws could be ascribed to the surface. The available evidence no longer appears to support this view.

In this paper I combine the results of a set of population synthesis models with simple Montecarlo simulations of stochastic effects in the number of stars occupying sparsely populated stellar evolutionary phases, to show that the scatter observed in the magnitudes and colors of LMC and NGC 7252 star clusters can be understood in the framework of current stellar evolution theory, without the need to introduce ad-hoc corrections (e.g. artificially increasing the number of AGB stars).

The dynamical evolution of open star clusters has been studied successfully by numerical N-body simulations. We compare in detail the theoretically predicted lifetimes with those derived from the observed age distribution of open clusters. Rare but efficient encounters between clusters and giant molecular clouds are probably responsible for the sudden disruption of many open clusters. Massive black holes from the galactic corona would, on the average, affect only old open clusters.

We are conducting a magnetic survey of a sample of about 30 spectroscopically identified Ap stars, with weak or previously undetected magnetic fields. For 28 studied stars, we have obtained 25 detections of Stokes V Zeeman signatures. Our results suggest that all Ap stars are magnetic. Further there may exist a minimum field strength for which Ap-type characteristics are produced.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Zeeman-Doppler Imaging (ZDI) is a powerful inversion method to reconstruct stellar magnetic surface fields. The reconstruction process is usually solved by translating the inverse problem into a regularized least-square or optimization problem. In this contribution we will emphasize that ZDI is an inherent non-linear problem and the corresponding regularized optimization is, like many non-linear problems, potentially prone to local minima. We show how this problem will be exacerbated by using an inadequate forward model. To facilitate a more consistent full radiative transfer driven approach to ZDI we describe a two-stage strategy that consist of a principal component analysis (PCA) based line profile reconstruction and a fast approximate polarized radiative transfer method to synthesize local Stokes profiles. Moreover, we introduce a novel statistical inversion method based on artificial neural networks (ANN) which provide a fast calculation of a first guess model and allows to incorporate better physical constraints into the inversion process.

A program is outlined, and first results described, in which fully three-dimensional, time dependent simulations of hydrodynamic turbulence are used as a basis for theoretical investigation of the physics of turbulence. The inadequacy of the treatment of turbulent convection as a diffusive process is indicated. A generalization to rotation and magnetohydrodynamics is indicated, as are connections to simulations of 3D stellar atmospheres.

Observations of plasma and magnetic field fluctuations in the solar wind provide a valuable source of information for the study of turbulence in collisionless astrophysical plasmas. Scientific data collected by various spacecraft over the last few decades has fueled steady progress in this field. Theoretical models, numerical simulations, and comparisons between theory and experiment have also contributed greatly to these advances. This review highlights some recent advances on the observational side including measurements of the anisotropy of inertial range fluctuations as revealed by the different scaling laws parallel and perpendicular to the mean magnetic field, measurements of the normalized cross-helicity spanning the entire inertial range which demonstrate that this quantity is scale invariant, and improved measurements of the spectrum of magnetic field fluctuations in the dissipation range that show a spectral break near the lengthscale of the electron gyro-radius. The theoretical implications of these results and comparisons between theory and observations are briefly summarized.

Practical measurement of the passage of time requires the notion of a repeating phenomenon. The Earth's rotation has traditionally fulfilled this requirement. To cope with the impracticality of making precise measures of the Sun's hour angle or altitude, particularly in uncooperative weather conditions, various devices have been employed, but all have been calibrated with respect to astronomical phenomena related to the Earth's rotation, or to its orbital motion with respect to the Sun. Modern requirements for timing precision coupled with an increased understanding of the variability of the Earth's rotational speed are likely to bring about a change in the traditional relationship between precise timekeeping and astronomy. The historical background of this relationship and the current definitions are reviewed to show the development of timekeeping capabilities and the growing need for precise timekeeping. Possible future developments are outlined along with their advantages and disadvantages.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The first known pulsar glitch was discovered in the Vela pulsar at both Parkes and Goldstone in March 1969. Since then the number of known glitches has grown enormously, with more than 520 glitches now known in more than 180 pulsars. Details of glitch parameters and post-glitch recoveries are described and some implications for the physics of neutron stars are discussed.

A review of the properties of Type II Cepheids and RV Tauri stars in the Magellanic Clouds is presented. In the behaviour of their light and colour curves, the RV Tauri stars appear to be a direct extension of the Type II Cepheids to longer periods. A single P – L – C relationship describes both the Type II Cepheids and RV Tauri stars in the LMC. The derived high intrinsic magnitudes for the RV Tauri variables supports the proposition that these objects are luminous stars evolving off the AGB. Preliminary analysis of the long time-series MACHO photometry indicates one star (MACHO*05:37:45.0–69:54:16) has an obvious ‘period-quadrupled’ periodicity, which is supporting evidence for a period-doubling bifurcation transition to chaotic pulsations.

The observational evidence on the discrepancy between the mass distribution in galaxies derived from HI rotation curves and that derived from the distribution of light is reviewed. In the outer parts the discrepancy is such that in some galaxies there is at least three times as much dark matter as luminous matter. This is a direct consequence of the nearly constant circular velocity far beyond the edge of the visible part of the galaxy, as derived from the motion of HI. The discrepancy is clearly present already near the edge of the visible disk (R25). In the inner regions, i.e. inside approximately 2.5 disk scale-lengths, no dark matter is required, but its presence can not be ruled out. There is no evidence for a dependence on galaxy luminosity or morphological type. These results suggest a strong coupling between luminous matter and dark matter within individual galaxies, and among galaxies as well. Finally attention is drawn to the large-scale asymmetries in the outer parts of galaxies and to possible implications for the vertical distribution of dark matter.

The recent observations of atomic oxygen and carbon in the far ultraviolet spectrum of Comet Kohoutek (1973f) (Feldman et al. 1974; Opal et al. 1974) have demonstrated the existence of these atomic species in the cometary coma. However, in order to identify the source of their origin, it is necessary to relate the observed ultraviolet flux to the atomic production rate. Assuming the only excitation mechanisms allowed are those produced by resonance scattering and fluorescence of solar ultraviolet radiation, the problem reduces to finding the emission rate factors (g-factors) as a function of the heliocentric comet velocity. Since the widths of the solar emission lines are smaller than the maximum heliocentric Doppler shift, given by

where q is the perihelion distance in A.U., it is necessary to consider the detailed multiplet structure of the transition, the solar line shape and the relaxation of excited fine structure levels.

Parallaxes are presented for nine Planetary Nebulae central stars.

Relativistic phenomena (orbital precession, Shapiro delay, and/or orbital decay) have been measured in Arecibo timing observations of four pulsar-white dwarf binaries, leading to constraints on the neutron star masses. We have detected the decay of the PSR J0751+1807 orbit due to gravitational radiation emission, the first such measurement in a binary with a low mass ratio (m2/m1 ≪ 1). Timing data constrains the mass of this pulsar to bet between 1.6 and 2.8 M⊙. Masses of the other pulsars are in marginal agreement with the canonical pulsar mass of 1.35 M⊙, but higher values are preferred.

One of the most fundamental physical parameter in astronomy is the distance to the objects we detect in the universe. For many classes of astronomical objects, accurate and proven methods have been developed to determine their distances. Such classes of objects include stars within ∼100 pc from the sun, binary stellar systems, variable stars, stellar clusters, main sequence stars, and other galaxies. It has been, however, more difficult to develop satisfactory methods to determine accurate distances to the more than 1000 planetary nebulae that have been discovered in our galaxy.

The Culgoora magnetograph (Ramsay et al., 1970) produces simultaneous filtergrams in opposite circular polarizations at a wavelength selected by a filter bandwidth 0.005 nm (Ramsay et al., 1970). In the blue wing of the 610.27 nm line of CaI, regions of magnetic fields in strong or weak plages are very obvious in one or other polarization, depending on polarity, even before subtraction; in one polarization they are bright, but almost invisible in the other. They are more difficult to discern at equal intervals from the line centre in the other wing (Figure 1). When subtractions are carried out to yield magnetograms of the same sense, the two magnetograms from opposite wings give results which appear to be much the same. An example is shown in Figure 2. Similar results are obtained over a wide range of positions in the wings of the 610.27 nm line.

Accurate characterization of the astrometric errors in the forthcoming Gaia catalogue is essential for making optimal use of the data. Using small-scale numerical simulations of the astrometric solution, we investigate the expected spatial correlation between the astrometric errors of stars as function of their angular separation. Extrapolating to the full-scale solution for the final Gaia catalogue, we find that the expected correlations are generally very small, but could reach some fraction of a percent for angular separations smaller than about one degree. The spatial correlation length is related to the size of the field of view of Gaia, while the maximum correlation coefficient is related to the mean number of stars present in the field at any time. Our scalable simulation tool (AGISLab) makes it possible to characterize the astrometric errors and correlations, e.g., as functions of position and magnitude.

We present results of a project aimed at establishing a set of 12 spectro-photometric standards over a wide wavelength range from 320 to 2500 nm. Currently no such set of standard stars covering the near-IR is available. Our strategy is to extend the useful range of existing well-established optical flux standards (Oke 1990, Hamuy et al. 1992, 1994) into the near-IR by means of integral field spectroscopy with SINFONI at the VLT combined with state-of-the-art white dwarf stellar atmospheric models (TMAP, Holberg et al. 2008). As a solid reference, we use two primary HST standard white dwarfs GD71 and GD153 and one HST secondary standard BD+17 4708. The data were collected through an ESO “Observatory Programme” over ~40 nights between February 2007 and September 2008.

The parameter-free model of diffusion in the atmospheres of HgMn stars (Michaud 1986, Michaud et al. 1979) predicts that helium should sink below the $\hbox{He\,{\sc ii}}$ ionization zone that the diffusion of other elements may take place, and that all HgMn stars should have deficits of helium in their photospheres, with a minimum deficit of 0.3 dex. In this study, the Smith & Dworetsky (1993) sample of HgMn stars and normal comparison stars is examined, and the helium abundances determined by spectrum synthesis using échelle spectra taken at Lick Observatory and the AAT. The prediction is confirmed. All HgMn stars are deficient in He by as much as 1.5 dex. Also, two HgMn stars, HR 7361 and HR 7664, show clear evidence of helium stratification.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We analyzed the relationship between the ground-based modulation of cosmic rays (CR) and corotating interaction regions (CIRs). Daily averaged data from 8 different neutron monitor (NM) stations were used, covering rigidities from Rc = 0 − 12.91 GeV. The in situ solar wind data were taken from the Advanced Composition Explorer (ACE) database, whereas the coronal hole (CH) areas were derived from the Solar X-Ray Imager onboard GOES-12. For the analysis we have chosen a period in the declining phase of solar cycle 23, covering the period 25 January–5 May 2005. During the CIR periods CR decreased typically from 0.5% to 2%. A cross-correlation analysis showed a distinct anti-correlation between the magnetic field and CR, with the correlation coefficient (r) ranging from −0.31 to −0.38 (mean: −0.36) and with the CR time delay of 2 to 3 days. Similar anti-correlations were found for the solar wind density and velocity characterized by the CR time lag of 4 and 1 day, respectively. The relationship was also established between the CR modulation and the area of the CIR-related CH with the CR time lag of 5 days after the central-meridian passage of CH.