The sky-averaged (global) 21-cm signal is a very promising probe of the Cosmic Dawn, when the first luminous sources were formed and started to shine in a substantially neutral intergalactic medium. I here report on the status and early result of the Large-Aperture Experiment to Detect the Dark Age that focuses on observations of the global 21-cm signal in the 16 ≲ z ≲ 30 range.

We have found two extreme GPS sources with peak frequencies of 20 and 30 GHz. The sources are both associated with X-ray bright, low redshift (z < 1) quasars. We have undertaken a search for similar sources using the Green Bank 5 GHz, NVSS 1.4 GHz surveys and follow-up observations with the Ryle Telescope at 15 GHz and the VLA up to 43 GHz.

More than 300 000 artificial debris particles with diameter larger than 1 cm are orbiting the Earth. The space debris population is similar to the asteroid belt, since it is subject to a process of high-velocity mutual collisions that affects the long-term evolution of its size distribution. The near–Earth space can be divided in three major regions where orbital debris is of concern: Low Earth Orbits (LEOs), below about 2000 km, Geosynchronous Orbits (GEOs), at an altitude of about 36000 km and the Medium Earth Orbits (MEOs) in between. The issues are in principle the same in the three regions, nevertheless they require different approaches and solutions. The space debris are composed by several different populations according to their source and their orbital region. A description of the nature and dynamics of the different populations in the low, medium and high orbital regimes is given. The impact risk posed by these debris is then briefly outlined.

The long term evolution of the whole debris population can be studied with computer models allowing the simulation of all the known source and sinks mechanisms. One of these codes is described and the evolution of the debris environment, over the next 100 years, under different traffic scenarios is shown, pointing out the possible measures to mitigate the growth of the orbital debris population.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Magnetic clouds (MCs) are objects of extreme importance in the heliosphere. They have a major role on releasing magnetic helicity from the Sun (with crucial consequences on the solar dynamo), they are the hugest transient object in the interplanetary medium, and the main actors for the Sun-Earth coupling. The comparison between models and observations is beginning to clarify several open questions on MCs, such as their internal magnetic configuration and their interaction with the ambient solar wind. Due to the decay of the solar wind pressure with the distance to the Sun, MCs are typically in expansion. However, their detailed and local expansion properties depend on their environment plasma properties. On the other hand, while it is well known that the solar cycle determines several properties of the heliosphere, the effects of the cycle on MC properties are not so well understood. In this work we review two major properties of MCs: (i) their expansion, and (ii) the magnetic flux and helicity that they transport through the interplanetary medium. We find that the amount of magnetic flux and helicity released via MCs during the last solar minimum (years 2007-2009) was significantly lower than in the previous one (years 1995-1997). Moreover, both MC size and mean velocity are in phase with the solar cycle while the expansion rate is weakly variable and has no relationship with the cycle.

The planetary ephemerides are used to examine different suggested forms of the gravitational equations of motion which could possibly cause the observed Pioneer Anomaly. It is shown that most of the forms would be unacceptable, including that generally assumed – a constant acceleration directed toward the Sun. The tests show that three other forms could not exist within 10 au's of the Sun. Only one suggested form would be compatible with the Pioneer Anomaly affecting Saturn or any other more inward planet. Additional planetary observations in the future may possibly eliminate this form also.

Core-collapse supernova explosions are driven by a central engine that converts a small fraction of the gravitational binding energy released during core collapse to outgoing kinetic energy. The suspected mode for this energy conversion is the neutrino mechanism, where a fraction of the neutrinos emitted from the newly formed protoneutron star are absorbed by and heat the matter behind the supernova shock. Accurate neutrino-matter interaction terms are crucial for simulating these explosions. In this proceedings for IAUS 331, SN 1987A, 30 years later, we explore several corrections to the neutrino-nucleon scattering opacity and demonstrate the effect on the dynamics of the core-collapse supernova central engine via two dimensional neutrino-radiation-hydrodynamics simulations. Our results reveal that the explosion properties are sensitive to corrections to the neutral-current scattering cross section at the 10-20% level, but only for densities at or above ~1012 g cm−3.

The electronic spectroscopy of various polycyclic aromatic hydrocarbon (PAH) molecules has been studied in the laboratory at low temperatures using both molecular beam and matrix isolation spectroscopy techniques. While molecular beam spectra can be readily compared to astronomical observations, the band positions measured in Ne and Ar matrices are extrapolated to obtain rather good estimates for the same transitions in the gas phase. Absolute absorption cross sections are determined for gas-phase and matrix spectra by comparing them with calibrated solution spectra. All laboratory results are analyzed and discussed in view of the role that PAHs can play as carriers of the diffuse interstellar bands (DIBs). Our studies suggest that regular neutral PAHs are not responsible for any of the known strong DIBs.

It has been shown in several independent investigations that the multiple-shell phenomenon is prevalent in planetary nebulae. Despite the common classification, the multiple shell planetary nebulae are a heterogeneous group of objects, as testified by the wide variety of their morphologies and physical structures. There are two types of double-shell structures that are seen frequently: one has an inner shell expanding supersonically into a faint, subsonically expanding halo, and the other has a bright attached envelope co-expanding with the inner shell. The physical structures and relative elemental abundances in the shells are reviewed, and their possible formation mechanisms are discussed.

Long–baseline optical interferometry has revolutionised binary star astronomy. This paper reviews some recent binary star work done using the Mark III Stellar Interferometer. For the first time, many spectroscopic binaries, eclipsing binaries, triple systems, and binaries with X-ray sources have had their visual orbit, diameter of the primary component, and color index determined. Also, accurate stellar masses, distances, radii, and effective temperatures have been derived. A preliminary distance to the Hyades cluster has also been determined. In addition, preliminary observations provide evidence of a third body in the binary star system ζ Her, which may resolve an 85-year old discrepancy in astronomy.

This paper describes a photographic technique for recording solar spectra. The technique, which we call spectra-spectroheliography, requires the same amount of observing time as conventional spectroheliography but yields much more information. Data reduction techniques have been developed to use this information for the construction of spectroheliograms and other contour maps. Parameters investigated by this method include the vector magnetic field, line-of-sight velocity, continuum intensity, and line strength. Results indicate (1) good correlation between bright regions in spectroheliograms and small vertical magnetic fields; (2) poorer correlation between larger magnetic fields and bright regions; (3) a feature in the continuum resembling a pore but having no vertical magnetic field; and (4) a gradient in the vertical magnetic field of 12 kG/arc s.

The hypothesis relating mass extinctions of life on Earth to impacts of comets whose flux is partly modulated by the dynamics of the Milky Way Galaxy contains a number of postulates that can be tested by geologic evidence and statistical analyses. In an increasing number of cases, geologic evidence for impact (widespread impact debris and/or large impact craters) is found at times of mass extinction events, and the record of dated impact craters has been found to show a significant correlation with mass extinctions. Statistical analyses suggest that mass extinction events exhibit a periodic component of about 26 to 30 Myr, and periodicities of 30± 0.5 Myr and 35 ±2 Myr have been extracted from sets of well-dated impact craters. The evidence is consistent with periodic or quasi-periodic showers of impactors, probably Oort Cloud comets, with an approximately 30-Myr cycle. The best explanation for these proposed quasi-periodic comet showers involves the Sun’s vertical oscillation through the galactic disk, which may have a similar cycle time between crossings of the galactic plane.

In this paper we review some of the evidence that the hosts of powerful high redshift (1 ≲ z ≲ 5) radio galaxies (HzRGs) are the progenitors of present day brightest cluster galaxies (BCGs). On the basis of HST imaging we argue that the scenario describing the formation of BCGs consists of at least two important stages. At z > 2 a significant fraction of the stellar mass of a BCG is formed during a massive burst of star-formation. By z ~ 1, well developed massive ellipticals are already observed and from then until the present epoch, the total mass in stars of the BCG will further grow by a factor of order 3, mainly through the accretion of cluster ellipticals.

The time during which the temperature of a terrestrial-like planet remains in the liquid water range during the fading of its parent brown dwarf is calculated as a function of brown dwarf mass and planetary semimajor axis using recent models for brown dwarf evolution and two criteria for habitable zone width. Durations of habitability range from 0.5–2 Gyr at a brown dwarf mass of 0.03 M⊙ to 2–10 Gyr at a brown dwarf mass of 0.07 M⊙ for planets within a few Roche radii.

Results from observations of the polar magnetic fields made in 1968 with the Mount Wilson magnetograph are reported. The field was of south polarity near the heliographic north pole and of north polarity near the south pole. The inversion line of the field was at latitude +70° in the north hemisphere and −55° in the south, which coincides with the position of the polar prominence zones at that time. Observations made simultaneously at the Crimean Astrophysical Observatory are in good agreement with the Mt. Wilson data and confirm the latitude variation of the field.

A few hundred of Hale's plates from 1914, which he had used to determine the Sun's general magnetic field, have been remeasured. The plates were automatically scanned with a digitized microphotometer, and the treatment of the data was made with a computer. While the visual measurements by Van Maanen had given a strong general field similar to that of a dipole, the computer reductions did not reveal any significant field (stronger than 5 G) at any latitude.

Gamma-ray emission is the most direct diagnostic of energetic ions and relativistic electrons in solar flares. Analysis of solar flare gamma-ray data has shown: (i) ion acceleration is a major consequence of flare energy release, as the total flare energy in accelerated particles appears to be equipartitioned between ≳ 1 MeV/nucleon ions and ≳ 20 keV electrons, and amounts to an important fraction of the total energy release; (ii) there are flares for which over 50% of the energy is in a particles and heavier ions; (iii) in both impulsive and gradual flares, the particles that interact at the Sun and produce gamma rays are essentially always accelerated by the same mechanism that operates in impulsive flares, probably stochastic acceleration through gyroresonant wave particle interaction; and (iv) gamma-ray spectroscopy can provide new information on solar abundances, for example the site of the FIP-bias onset and the photospheric 3He abundance. We propose a new technique for the investigation of mass motion and mixing in the solar atmosphere: the observations of gamma-ray lines from long-term radioactivity produced by flare accelerated particles.

The spectrophotometric behavior of the Be/shell stars seems to be different when their second Balmer discontinuity is in emission from that when it is in absorption. We have analysed here a particular case in which the second Balmer jump is in absorption. In this note we compare the results of a BCD analysis concerning the variations of the Be-Shell star 88 Her (HD162732), to the results of the UBV photometry.

We present polarization measurements obtained in 1970 in the green coronal line with a new coronameter located at the Pic du Midi. The analysis of these data has been conducted with the theory given by the writer in 1964 and 1965. It appears that magnetic field orientations in the Corona can be deduced from the above measurements. First results showing large scale magnetic structures are presented.

When viewed separately in the two polarities, the large-scale pattern of magnetic fields on the Sun gives an appearance reminiscent of a very large cellular structure. These structures and their relation to ‘active longitudes’ are discussed.

“The properties of radio sources in clusters” presupposes that we know something about radio sources out of clusters, or that we even know whether a radio source is “in” or “out” of a cluster. Thus we are faced with the problem of defining what we mean by a cluster. Most of us use Abell’s catalogue of RICH clusters and assume that we are really “in” a cluster. However, most radio sources are identified with faint, distant objects and it is often difficult to know whether the remark “galaxy in a group” or “galaxy in a cluster” indicates a cluster such as the Coma Cluster, a cluster similar to an “open” Zwicky cluster, or a group of galaxies which may be gravitationally bound.

This uncertainty must not be forgotten, and in the following discussion, we will try to limit the effects of this by concentrating on catalogued clusters; ignoring most distant radio galaxies, many of which may be in rich clusters; and also by neglecting quasars, some or all of which may be in clusters.

Adopting non-radial pulsation (nrp) the line profile variability lpv of several absorption lines of different ions was modeled for ω (28) CMa. The parameters suggested by Baade (1982) to characterise the pulsation could be confirmed. The pulsation period in the inertial frame was found to be negative, i.e. an actually retrograde mode appears prograde due to the rapid stellar rotation. The line profiles could be reproduced in detail including structures like spikes and ramps. They were identified as velocity effect due to the latitudinal component of the nrp velocity field υϑ in combination with low inclination i. In spite of small photometric variations (Štefl et al. 1999) the lpv of the absorption lines can be explained by nrp entirely.