Astrometric binaries are both a gold mine and a nightmare. They are a gold mine because they are sometimes the unique source of orbital inclination for spectroscopic binaries, thus making it possible for astrophysicists to get some clues about the mass of the often invisible secondary. However, this is an ideal situation in the sense that one benefits from the additional knowledge that it is a binary for which some orbital parameters are somehow secured (e.g. the orbital period). On the other hand, binaries are a nightmare, especially when their binary nature is not established yet. Indeed, in such cases, depending on the time interval covered by the observations compared to the orbital period, either the parallax or the proper motion can be severely biased if the successive positions of the binary are modelled assuming it is a single star. With large survey campaigns sometimes monitoring some stars for the first time ever, it is therefore crucial to design robust reduction pipelines in which such troublesome objects are quickly identified and either removed or processed accordingly. Finally, even if an object is known not to be a single star, the binary model might turn out not to be the most appropriate for describing the observations. These different situations will be covered.

I review the results obtained by long-baseline interferometry at infrared wavelengths on the innermost regions around young stars. These observations directly probe the location of the dust and gas in the disks. The characteristic sizes of these regions are found larger than previously thought. These results have motivated in part a new class of models of the inner disk structure. However the precise understanding of the origin of these low visibilities is still in debate. Mid-infrared observations have probed disk emission over a larger range of scales revealing mineralogy gradients in the disk. Recent spectrally resolved observations allow the dust and gas to be studied separately. The few results show that the Brackett gamma emission can find its origin either in a wind or in a magnetosphere but there are no definitive answers yet. In a number of cases, the very high spatial resolution seems to reveal very close companions. In any case, these results provide crucial information on the structure and physical properties of disks surrounding young stars especially as initial conditions for planet formation.

External destruction of protoplanetary discs acts mainly due to two mechanisms: gravitational drag by stellar encounters and evaporation by stellar winds and radiation. It is an important question whether any of these mechanisms is important in the stellar evolution process. We focus on the effect of stellar encounters and investigate if there are any observables that could trace this mechanism in young stellar clusters. An analysis of observational data of the Orion Nebula Cluster (ONC) and accompanying n-body simulations both provide evidence for encounters of star-disc systems in the ONC, eventually leading to substantial disk disruption.

The Arecibo Galaxy Environment Survey is a blind neutral hydrogen survey using the ALFA multibeam receiver at Arecibo Observatory to reach unprecedented sensitivities in a number of selected fields in the local Universe. When completed the survey will cover 200 square degrees out to a distance of at least 270 Mpc. If a population of gas-rich dark galaxies exists, then this survey is in a prime position to uncover that population.

So far 20 square degrees have been covered in the regions of Abell 1367, the Virgo Cluster, the NGC 7332/9 galaxy pair and the isolated galaxy NGC 1156. Over 200 sources have been found, including a number that have no obvious optical counterparts. We discuss here the potential of AGES for uncovering more such objects and the characteristics of the dark sources identified to date.

The quiet solar chromosphere in regions with negligible magnetic field is believed to be heated by acoustic waves. But their energy flux, measured in the upper photosphere with the Transition Region And Coronal Explorer (TRACE), has been found to be insufficient to account for the radiative emission from the chromosphere. Wedemeyer-Böhm et al. (2007) and Cuntz et al. (2007), employing a 3D hydrodynamical model by Wedemeyer et al. (2004), have proposed that the spatial resolution of TRACE is inadequate to resolve intensity fluctuations that occur on small spatial scales. This paper accepts the principle of spatial averaging by TRACE as a qualitative explanation for the low acoustic flux but finds that the hydrodynamical model is too much simplified in the treatment of radiative energy exchange to provide a quantitative measure of the suppression of the fluctuations. The heating mechanism of the chromosphere thus remains an open question.

Most of the Commission's three 90-minute time slots at the General Assembly were devoted to a series of 20 and 50 minute presentations, informally termed “Time and Astronomy”. The first part of the meeting was dedicated to time and general relativity. The second part of the session was dedicated to pulsar timing.

We study colliding winds in the superluminous binary η Carinae by performing three-dimensional, Smoothed Particle Hydrodynamics (SPH) simulations. For simplicity, we assume both winds to be isothermal. We also assume that wind particles coast without any net external forces. We find that the lower density, faster wind from the secondary carves out a spiral cavity in the higher density, slower wind from the primary. Because of the phase-dependent orbital motion, the cavity is very thin on the periastron side, whereas it occupies a large volume on the apastron side. The model X-ray light curve using the simulated density structure fits very well with the observed light curve for a viewing angle of i = 54° and φ = 36°, where i is the inclination angle and φ is the azimuth from apastron.

We present the first observation of planet-induced stellar X-ray activity, identified for the HD 179949 system, using Chandra / ACIS-S. The HD 179949 system consists of a close-in giant planet orbiting an F9 V star. Previous ground-based observations already showed enhancements in Ca II K in phase with the planetary orbit. We find an ∼30% increase in the X-ray flux over quiescent levels coincident with the phase of the Ca II enhancements. There is also a trend for the emission to be hotter at increased fluxes, confirmed by modeling, showing the enhancement at ∼1 keV compared to ∼0.4 keV for the background star.

Recent observational and theoretical advances concerning astronomical masers in star forming regions are reviewed. Major masing species are considered individually and in combination. Key results are summarized with emphasis on present science and future prospects.

Spiral, fast-rotating galaxies like the Milky Way are the most common type in the Universe. One of the most pressing challenges faced by current models of galaxy formation is the origin of their angular momentum and disk. According to the standard tidal-torque theory the galactic spin is originated by tidal interactions between dark halos around galaxies and neighboring structures in the expanding Universe. We use a large cosmological N-body simulation to study the origin of possible correlations between the merging history and spin of cold dark matter halos. In particular, we examine claims that remnants of major mergers tend to have higher-than-average spins, and find that the effect is driven largely by unrelaxed systems: equilibrium dark matter halos show no significant correlation between spin and merging history. Out-of-equilibrium halos have, on average, higher spin than relaxed systems, suggesting that the virialization process leads to a net decrease in the value of the spin parameter. We present also high-resolution N-body/SPH cosmological simulations including cold gas and dark matter to investigate the processes by which gas loses its angular momentum during the protogalactic collapse phase, leading to simulated disk galaxies that are too compact with respect to the observations. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at redshift 2. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction by small infalling substructures plays a minor role, in contrast to previous claims.

Multi-wavelength studies of energetic solar flares with seismic emissions have revealed interesting common features that may help us to identify the correlations of flare signatures from the inner to the outer solar atmosphere and, to develop diagnostic techniques to aid in the sun quake detection. In our study, we make use the relation between the microwave and the hard X-ray emissions associated with such flares to propose a scenario for the ignition of seismic transients from flares. We explore the mechanisms of energy transport to the photosphere, such us back-warming or direct particle impacts.

Observations show that for massive stars the binary frequency seems to be higher than for lower mass stars in young dense clusters. This suggests that in clusters like the ONC different mechanisms are at work in the formation of high-mass binary or multiple systems than for low-mass stars. We investigate the stellar dynamics in young dense clusters to determine the role of capture in binary formation in high-mass stars. It turns out that in contrast to lower mass stars capture is a frequent process for massive stars. However, this does not necessarily lead to long lasting binary systems but is often of transient nature. Nevertheless, capture processes could account for 15-25% of the observed ‘binaries’ of the OB-stars (75%) in Orion.

Infrared molecular spectroscopy is a key tool for the observation of gas in the innermost region of disks around T Tauri stars. In this contribution, we examine how infrared spectroscopy of CO can be used to study the inner truncation region of disks around T Tauri stars. The inferred inner gas radii for T Tauri star disks are compared to the inner dust radii of disks, to the expectations of models for disk truncation, and to the orbital distribution of short-period extra-solar planets.

We investigate the hierarchical build-up of stars in bulges within the standard Λ-cold dark matter scenario. By separating the population into stars born during starbursts that accompany the formation of spheroids in major mergers (starburst component), and stars that are previously formed in discs of progenitor galaxies (quiescent component) and added to the spheroid by dynamical interaction. Our results are summarised as follows: bulges that form early have larger starburst fraction and hence should be smaller than their counter parts that form later. The quiescent fraction in bulges is an increasing function of bulge mass, becoming constant at Mq/Mbul ~ 0.8, mainly due to the infall of satellite galaxies that contribute disc stars to the bulge. Minor mergers are an order of magnitude more frequent than major mergers and must play a significant role in the evolution of bulges. Above the critical mass Mc ~ 3 × 1010 M⊙ most of the stars in the universe are in spheroids, which at high redshift are exclusively elliptical galaxies and at low redshifts partly bulges. Due to the enhanced evolution of galaxies ending up in high density environments, the starburst fraction and the surface mass densities of bulges below Mc should be enhanced with respect to field galaxies. Dissipation during the formation of massive bulges in present day early-type spirals is less important than for the formation of present day elliptical galaxies of the same mass thereby explaining the possible difference in phase-space densities between spiral galaxies and elliptical galaxies.

The most recent stellar models have shown that the faster a massive star spins, the more its nuclear yields, mass-loss rate and lifetime are different from the standard model. One thus needs to know the rotation rate of massive stars to trace their evolutionary tracks adequately. In Wolf-Rayet (WR) stars, the direct measurement of the rotational velocity is impossible, since their continuum emission is formed in the dense wind that hides the hydrostatic, stellar surface. Here, we present a technique to derive the rotation rates of WR stars from a periodic wind phenomenon, the corotating interaction regions (CIR). For five WR stars, a first estimate of the rotation rates has been deduced from the CIR periods.

Three SuperWASP transiting planet candidates were observed through R or I filters using the 1-meter telescope and CCD camera of Yunnan Observatory from 2006 to 2007. The relative photometric data were corrected for the systematic errors by means of Tamuz et al. (2005) and Collier Cameron et al. (2006)'s algorithms. The resulting light curves demonstrate that one of three targets is a potential exoplanet candidate, which is worthy to perform further follow-up observation to clarify.

General characteristics of methanol (CH3OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact HII regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified HII regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region (hcHII) with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable amplification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.

Division II provides a forum for astronomers studying a wide range of problems related to the structure, radiation and activity of the Sun, and its interaction with the Earth and the rest of the solar system.

The main goal of the astrometry of solar system objects is to build dynamical models of their motions to understand their evolution, to determine physical parameters and to build accurate ephemerides for the preparation and the exploitation of space missions. For many objects, the ground-based observations are still very important because radar or observations from space probes are not available. More, the need of observations on a long period of time makes the ground-based observations necessary. The solar system objects have very different characteristics and the increase of the astrometric accuracy will depend on the objects and on their physical characteristics. The purpose of this communication is to show how to get the best astrometric accuracy.

Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M 81 group reveal a hitherto hidden population of extremely low mass (~ 105 M⊙) HI clouds with no obvious optical counterparts. We have searched 10 fields in the M 81 group totalling 2.2 square degree, both targeting known dwarf spheroidal galaxies and blank fields around the central triplet. Our observations show that the new population of low–mass HI clouds appears to be confined to a region toward the south–east of the central triplet (at distances of ~ 100 kpc from M 81). Possible explanations for these free–floating HI clouds are that they are related to the dSphs found to the South–East of M 81, that they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they are of primordial nature and provide fresh, unenriched material falling into the M 81 group, or that they are tidal debris from the 3–body interaction involving M 81–M 82–NGC 3077. Based on circumstantial evidence, we currently favour the latter explanation.