We study the interaction of relativistic jets with their environment, using 3-dimen- sional relativistic particle-in-cell simulations for two cases of jet composition: (i) electron-proton (e− − p+) and (ii) electron-positron (e±) plasmas containing helical magnetic fields. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability and the Mushroom instability. We have found that these kinetic instabilities are suppressed and new types of instabilities can grow. For the e− − p+ jet, a recollimation-like instability occurs and jet electrons are strongly perturbed, whereas for the e± jet, a recollimation-like instability occurs at early times followed by kinetic instability and the general structure is similar to a simulation without a helical magnetic field. We plan to perform further simulations using much larger systems to confirm these new findings.

Despite many years of effort, observational studies have not found a strong correlation between the presence of any proposed fueling mechanism and low-luminosity AGN. After a discussion of the mass requirements for fueling, I summarize this observational work and provide a number of hypotheses for why the nature of AGN fueling has remained unresolved. In particular, I stress the potential importance of the increasing number of candidate fueling mechanisms with decreasing mass accretion rate, the relevant spatial scales for different fueling mechanisms, and the lifetime of an individual episode of nuclear accretion. The episodic AGN lifetime is a particularly relevant complication if it is comparable to or shorter than the time that the responsible fueling mechanisms are observationally detectable. I conclude with a number of relatively accessible areas for future investigation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

As a preparation to the HIPPARCOS mission, a large observing programme on NLTT stars (propermotion > 0.18 ″/yr) was started in Genevaphotometry. The original programme consists of 10047 stars brighter than mR = 11.5, or mR = 12.5 if of colour class m. Among them, 7813 targets could be included in the HIPPARCOS programme, selected according to their observability and internal priorities in favour of large parallaxe stars (photometric distances < 100 pc) and high-velocity stars. The bulk of new nearby, halo, mild-metal poor and SMR stars in the HIP Catalogue originates from this proposal (N° 139). No less than 208 new nearby stars with π ≥ 40 mas were discovered south of δ +10°, the closest has π(HIP)= 182 mas. Radial velocities were obtained with CORAVEL at OHP and ESO. Most aspects of the early evolution of the Galaxy may be addressed with this sample. Here we discuss, as examples, the ages of the thick disk and of the galactic bulge.

To constrain models of dark energy, a precise measurement of the Hubble constant, H0, provides a powerful complement to observations of the cosmic microwave background. Recent, precise measurements of H0 have been based on the ‘extragalactic distance ladder,’ primarily using observations of Cepheid variables and Type Ia supernovae as standard candles. In the past, these methods have been limited by systematic errors, so independent methods of measuring H0 are of high value. Direct geometric distance measurements to circumnuclear H2O megamasers in the Hubble flow provide a promising new method to determine H0. The Megamaser Cosmology Project (MCP) is a systematic effort to discover suitable H2O megamasers and determine their distances, with the aim of measuring H0 to a few percent. Based on observations of megamasers in UGC 3789 and NGC 6264, and preliminary results from Mrk 1419, the MCP has so far measured H0 = 68.0 ± 4.8 km s−1 Mpc−1. This measurement will improve as distances to additional galaxies are incorporated. With the Green Bank Telescope, we recently discovered three more excellent candidates for distance measurements, and we are currently acquiring data to measure their distances.

The observed rate and spatial distribution of galactic novae can be explained with the concept of two nova populations, the disc novae of faster outburst development, and the slower bulge novae. Similar populations seem to exist in nearby galaxies. The space density of galactic novae depends strongly on the recurrence time, which is unknown and may lie between 500 and 20 000 years. If the recurrence time is long, the population of quiescent novae must be identified; the hibernation scenario may or may not be a solution.

The Australia Telescope Compact Array (ATCA) has been used to make the first full polarization observations of 6.7 GHz methanol masers. Linear polarization was detected towards all four sources observed, at levels between a few and 10%, while none of the sources show circular polarization stronger than approximately 1.5%. Linear polarization appears to be more common in the 6.7 GHz methanol maser transition than it is for the 12.2 GHz transition, consistent with the hypothesis that the 6.7 GHz masers are more saturated.

Photography of the southern hemisphere for the Second Cape Photographic Catalogue (CPC2) was commenced in 1962 by R H Stoy, HM Astronomer at the Cape. During the following ten years nearly 6000 plates, each having two exposures and covering 4° × 4° with fourfold corner to centre overlap were obtained. All plates have been measured on the GALAXY measuring machine at the Royal Greenwich Observatory under the general direction of W Nicholson. A preliminary catalogue giving positions and magnitudes of more than 50 000 stars in the Cape Astrographic zone (−40° to −52°) has already been published (Nicholson et al. 1984), together with details of the programme. Measurement of the plates, initial identification of star images appearing on all overlapping plates, and photographic photometry have been carried out at RGO; the subseqent astrometric data reduction is being performed at Hamburg Observatory.

In this contribution we present a few selected examples of how the latest generation of space-based instrumentation — NASA's Chandra X-ray Observatory and the Far-Ultraviolet Spectroscopic Explorer (FUSE) — are finally answering old questions about the influence of massive star feedback on the warm and hot phases of the ISM and IGM. In particular, we discuss the physical origin of the soft thermal X-ray emission in the halos of star-forming and starburst galaxies, its relationship to extra-planar Hα emission, and plasma diagnostics using FUSE observations of O vi absorption and emission.

Using the variable baseline of the Mark III Optical Interferometer (Shao et al. 1988) with lengths ranging from 3 to 31.5 m, we measured the fringe visibility of stars in three 25 nm bandpass filters, at wavelengths between 800 nm and 450 nm. This setup gives fringe spacings ranging from 55 mas to 3 mas.

255 stars brighter than fifth magnitude and north of –10° declination were selected from the Batten et al. (1989) catalog of spectroscopic binaries. We compiled values for the orbital axes from the McAlister & Hartkopf (1988) catalog. If unknown (in the majority of cases), they were estimated from the masses predicted from the spectral types, the periods (if known), and the distances to the binaries. Priority was given to the binaries with orbital axes between 3 mas and 40 mas. Observational procedures and the calibration of the data are described by Armstrong et al. (1992). Statistical and calibration uncertainties are in the range of 1% (red channel) to 10% (blue channel).

The complex structure of nuclear disks of normal spiral galaxies is illustrated with the example of five galaxies, observed at the 6m telescope. The problem of gravitational stability of nuclear disks is briefly discussed.

We summarize results of a detailed imaging program of the molecular absorbing cloud at z = 0.88582 toward the “Einstein ring” radio source PKS 1830–211.

The discovery of Main Belt Comets (MBCs) has raised many questions regarding the origin and activation mechanism of these objects. Results of a study of the dynamics of these bodies suggest that MBCs were formed in-situ as the remnants of the break-up of large icy asteroids. Simulations show that similar to the asteroids in the main belt, MBCs with orbital eccentricities smaller than 0.2 and inclinations lower than 25° have stable orbits implying that many MBCs with initially larger eccentricities and inclinations might have been scattered to other regions of the asteroid belt. Among scattered MBCs, approximately 20% reach the region of terrestrial planets where they might have contributed to the accumulation of water on Earth. Simulations also show that collisions among MBCs and small objects could have played an important role in triggering the cometary activity of these bodies. Such collisions might have exposed sub-surface water ice which sublimated and created thin atmospheres and tails around MBCs. This paper discusses the results of numerical studies of the dynamics of MBCs and their implications for the origin of these objects. The results of a large numerical modeling of the collisions of m-sized bodies with km-sized asteroids in the outer part of the asteroid belt are also presented and the viability of the collision-triggering activation scenario is discussed.

We discuss different physical aspects that may influence pulsation stability in rapidly oscillating Ap stars. We pay particular attention to the role of the magnetic field, atomic diffusion and winds and point out some of the important uncertainties involved in present studies of pulsation stability in these stars. We argue that presently the models that best reproduce the observations are those with convection suppressed, diffusion of helium, and no winds. With these models, in the region of the HR diagram where most roAp stars have been found, we predict the excitation of high order modes and the damping of low order modes, in accord with observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Recent observational data on the volatile fraction of comets are confronted with a model based on the fractional condensation, in the 80-100 °K range, of a higher-temperature equilibrium obtained from a solar mixture, more or less depleted in oxygen and in hydrogen. It is possible to almost duplicate the observational data, only by assuming that the solar ratio of C/0 is at least as large as 0.66 and that the hydrogen was drastically depleted by an unknown process in the primitive solar nebula. Although none of these two assumptions is at variance with present knowledge, the latter is sufficiently exotic to propose a simpler explanation, namely that comets could be made of interstellar grains relatively unprocessed by heat.

The observational data on flare stars observed in star clusters and associations as well as in the solar vicinity (the UV Ceti type stars) are discussed. The analysis of these data show that they constitute one common class of objects possessing flare activity and the differences between them are conditioned by the age differences. The stage of flare activity is an evolutionary stage, one of the earliest stages of evolution passed by all red dwarf stars. It comes before the end of their T Tau stage of evolution. The UV Ceti type flare stars in the solar vicinity seem to be the population of the general galactic field, which were formed in the systems, already desintegrated. Most probably the stellar flares are the result of the release of the the surplus energy having intra-stellar origin.

Based both on observations and simulations, recent works propose that the speed of the spiral pattern in disk galaxies may decrease with increasing radius; the implications are that patterns are actually short-lived, and that the azimuthal color/age gradients across spiral arms predicted by density wave theory could not be produced. We, however, have consistently found such gradients, and measured spiral pattern speeds by comparing the observations with stellar population synthesis models (González & Graham 1996; Martínez-García et al. 2009a, b; Martínez-García & González-Lópezlira 2011). Here, we summarize our previous results in non-barred and weakly barred spirals, together with six new, as yet unpublished, objects. On the other hand, we have indeed found a trend whereby pattern speeds at smaller radii are larger than expected from a model that assumes purely circular orbits (cf. Figure 1), likely due to the effect of spiral shocks on the orbits of newborn stars. The results suggest that spirals may behave as steady long-lived patterns.

The 22-year oscillations of the Earth rotation due to several geophysical processes in the core-mantle system, oceans, atmosphere and geomagnetic field are excited mainly by 22-year cycles of the solar activity. These geophysical processes produce their own oscillations of the Earth rotation with different periods around 22 years. The direct and indirect influence of the solar activity on 22-year cycles of the Earth rotation are separated from the core effects and corresponding amplitudes are estimated by means of two approaches. The first, direct approach uses extended time series of Wolf's numbers with 22-year cycles, determined by sign alternation of even sunspot cycles. A linear regression between 22-year cycles of UT1 and solar activity is determined and this regression model is used to calculate the UT1 response to the 22-year cycles of the solar activity. The second, indirect approach uses 22-year oscillation of the mean sea level, caused by water evaporation due to variations of the total solar irradiance. The influence of the mean sea level variations on the Earth rotation is calculated by means of an empirical model of global water redistribution. The core-mantle effects on the 22-year UT1 variations are determined by excluding the UT1 response to the solar activity and core angular momentum due to the geomagnetic field variations, according to the solutions from the Special Bureau of the Core (SBC).

It is well established that activity and rotation diminishes during the life of sun-like main sequence (~F7-K2V) stars. Indeed, the evolution of rotation and activity among these stars appears to be so deterministic that their rotation/activity diagnostics are often utilized as estimators of stellar age. A primary motivation for the recent interest in improving the ages of solar-type field dwarfs is in understanding the evolution of debris disks and planetary systems. Reliable isochronal age-dating for field, solar-type main sequence stars is very difficult given the observational uncertainties and multi-Gyr timescales for significant structural evolution. Observationally, significant databases of activity/rotation diagnostics exist for field solar-type field dwarfs (mainly from chromospheric and X-ray activity surveys). But how well can we empirically age-date solar-type field stars using activity/rotation diagnostics? Here I summarize some recent results for F7-K2 dwarfs from an analysis by Mamajek & Hillenbrand (2008), including an improved “gyrochronology” [Period(color, age)] calibration, improved chromospheric (R′HK) and X-ray (log(LX/Lbol)) activity vs. rotation (via Rossby number) relations, and a chromospheric vs. X-ray activity relation that spans four orders of magnitude in log(LX/Lbol). Combining these relations, one can produce predicted chromospheric and X-ray activity isochrones as a function of color and age for solar type dwarfs.

Precise white dwarf gravitational redshifts can only be obtained utilizing systems of known distance and space velocity. Binaries with known orbits such as 40 Eri B are rare, and although common proper motion pairs have proven highly useful(Wegner 1973: Koester 1987: Wegner & Reid 1987), there are some problems in their interpretation. Another approach is to employ white dwarfs in open clusters; they not only have known systemic velocities, but also provide information on the progenitors of the white dwarfs. Of the nearby galactic clusters, the Hyades currently give the best information for achieving accurate gravitational redshifts: the members are relatively bright and nearby and their kinematics are well known.

The IAU framework for relativistic reference systems is based upon the work by Brumberg and Kopeikin and by Damour, Soffel and Xu (DSX). We begin with a brief introduction into the DSX-formalism. After that the various IAU Resolutions concerning relativistic astronomical reference systems are discussed. Finally, it is indicated how the expansion of the universe can be considered in the BCRS.