A mechanism to create circumstellar envelopes in Be stars is proposed. The mechanism assumes a mass-loaded wind produced by ablated discrete stellar ejecta. The discrete mass ejections are suggested by the long-term spectrophotometric behaviours of Be stars and by their short and long-lived light outbursts discovered recently.

We present a review on relativistic effects and best estimates of the relativistic PPN parameter γ obtained by analysis of data from the International VLBI Service for Geodesy and Astrometry (IVS). Relativistic implications are also considered in view of the upcoming new generation VLBI System: VLBI2010.

According to the multiple winds model a planetary nebula forms as the result of the interaction of a fast wind from the central star with the superwind that had previously been emitted by the progenitor star. The basic theory which deals with the spherically symmetrical case is briefly summarised. Various improvements are then considered in turn. A better history is clearly needed of the way that the central star becomes hotter, it is unrealistic to make the assumption that the superwind is spherically symmetrical, and finally there are likely to be important instabilities at some of the interfaces in the PN, notably that between the shocked superwind and the HII layer. These changes in the theoretical description produce a better understanding of the conditions in the outer parts of a PN and of the nature of its general shape, and they should lead to an explanation for the occurrence of high speed motions, and of highly ionized species and high excitation spectral lines.

The distribution of the column density of HI over the sky shows strongly organized features of large angular extent. In particular, there is (1) a conspicuous set of arching concentric filaments extending more than 80° on either side of ℓ = 330°, and (2) a prominent extended region of low column density above ℓ ∼ +40° and centered on ℓ ∼ 130°. The changes is column density in the b-direction in this region of low NH is almost step-like at ℓ ∼ +40°.

In this section I briefly describe my efforts to improve the atomic and molecular line data. This work is described in more detail in Kurucz (1992a). In subsequent sections I briefly describe three methods for computing opacity and the models and spectra that result from using them.

My model calculations in the 1970s used the distribution-function line opacity computed by Kurucz (1979a,b) from the line data of Kurucz & Peytremann (1975). We had computed gf values for 1.7 million atomic lines for sequences up through nickel using scaled-Thomas-Fermi-Dirac wavefunctions and eigenvectors determined from least squares Slater parameter fits to the observed energy levels. We also collected all published data on g f values and included them in the line list whenever they appeared to be more reliable than the computed data (that work is ongoing, but I am running behind).

The relative abundances of the radionuclides in the solar system at the time of its birth are crucial arbiters for competing hypotheses regarding the birth environment of the Sun. The presence of short-lived radionuclides, as evidenced by their decay products in meteorites, has been used to suggest that particular, sometimes exotic, stellar sources were proximal to the Sun’s birth environment. The recent confirmation of neutron star - neutron star (NS-NS) mergers and associated kilonovae as potentially dominant sources of r-process nuclides can be tested in the case of the solar birth environment using the relative abundances of the longer-lived nuclides. Critical analysis of the 15 radionuclides and their stable partners for which abundances and production ratios are well known suggests that the Sun formed in a typical massive star-forming region (SFR). The apparent overabundances of short-lived radionuclides (e.g. 26Al, 41Ca, 36Cl) in the early solar system appears to be an artifact of a heretofore under-appreciation for the important influences of enrichment by Wolf-Rayet winds in SFRs. The long-lived nuclides (e.g. 238U, 244Pu, 247Cr, 129I) are consistent with an average time interval between production events of 108 years, seemingly too short to be the products of NS-NS mergers alone. The relative abundances of all of these nuclides can be explained by their mean decay lifetimes and an average residence time in the ISM of ∼200 Myr. This residence time evidenced by the radionuclides is consistent with the average lifetime of dust in the ISM and the timescale for converting molecular cloud mass to stars.

We have made a new deep 6 cm survey with the VLA in order to extend the radio source count to a lower flux density and to decrease the limits on the small scale fluctuations in the microwave background. The survey region covers ~10-5 ster and is nearly coincident with the survey previously reported by Fomalont et al. 1984a and 1984b. Both the observing time and bandwidth were increased by a factor of two over the previous survey to give a factor of two improvement in sensitivity.

The use of spacecraft for studying the physical properties of the asteroid belt can be approached in several ways. Certainly the simplest approach is to send a spacecraft into the asteroid belt and measure the effects of the environment encountered; this has the advantage of not requiring a vehicle to be targeted to any particular destination. With such an approach, properties may be obtained for those classes of objects that are populous enough to provide a significant number of encounters within the measurement range of the spacecraft. Unfortunately, the large asteroids do not constitute such a class of objects; the probability of an undirected spacecraft passing within measurement range of an asteroid having a diameter of 1 or more km is negligible.

We present a new method of phenomenologically modelling light curves of variable stars using Principal Component Analysis techniques that provide a realistic description of the variability with a minimum of free parameters. Examples of this method are demonstrated for the magnetic CP stars HD 90044 and HD 125248.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The paper provides an overview on the results of the analyses of spectro-polarimetric observations of white dwarfs, subdwarfs, and central stars of planetary nebulae. It will also discuss the question of the origin of the magnetic fields in white dwarfs.

The workshop on Historical Time-Domain Astronomy (TDA) was attended by a near-capacity gathering of ~30 people. From information provided in turn by those present, an up-to-date overview was created of available plate archives, progress in their digitization, the extent of actual processing of those data, and plans for data distribution. Several recommendations were made for prioritising the processing and distribution of historical TDA data.

We present a multi-wavelength (NIR to radio) and multi-scale (1 AU to 10 pc) study of the S255–S257 complex of young high-mass (proto)stars. The complex consists of two evolved HII regions and a molecular gas filament in which new generations of high mass stars form. Four distinct regions are identified within this dusty filament: a young NIR/optical source cluster, a massive protostar binary, a (sub)millimetre continuum and molecular clump in global collapse and a reservoir of cold gas. Interestingly, the massive binary protostellar system is detected through methanol maser and mid-IR emission at the interface between the NIR cluster and the cold gas filament. The collapsing clump is located to the north of the NIR cluster and hosts a young high-mass star associated with an outflow that is observed in mid-IR, methanol maser and radio emission. We interpret this anatomy as the possible result of triggered star formation, starting with the formation of two HII regions, followed by the compression of a molecular gas filament in which a first generation of high-mass stars forms (the NIR cluster), which then triggers the formation of high mass protostars in its near environment (the massive protostellar binary). The global collapse of the northern clump might be due to both the expansion of the HII regions that squashes the filament. In conclusion, we witness the formation of four generations of clusters of high-mass stars in S255–S257.

IR imaging, K–band spectroscopy and X-ray data of the stellar cluster in M 17 suggests three categories of massive young objects: I) “Naked” early-type O-stars in the cluster center that exhibit X-ray emission and normal optical/IR-spectra; they do not show signs for any circumstellar material. II) Late O-type stars, that show X-ray emission and H/He absorption lines in their optical/IR-spectra. There is evidence for extended circumstellar material in the form of elongated L–band nebulae, circumstellar MIR emission from regions of about 10.000 AU in diameter and filaments of He I $1.083\,\mu$m. III) Massive stars without X-ray emission, K–band spectra with emission lines of H and He and in some cases with CO band head emission. They are heavily enshrouded and also show elongated L–band nebulae, circumstellar MIR emission from regions of about 4.000 AU and ionized He envelopes.

Obviously, M 17 harbors several generations of massive stars: Only the oldest “Category I”, located in the cluster center, is free of circumstellar material. “Category II” and “III” comprise younger dust-enshrouded objects of a second generation embedded in the interface of the HII region and the adjacent molecular cloud. Due to the presence of flattened dusty disks and perpendicular gaseous outflows, accretion seems to be important for the formation of stars up to type O8.

A population of X-ray emitting sources with($L_{\rm bol} \sim 10^4\,$L$_\odot$) shows CO band head absorption; one source was found to have variable CO absorption as known from FU Orionis stars.

New data on the 20.000 AU accretion disk in M 17 reveals a collimated H$_2$$2.122\,\mu$m jet with three distinct emission knots that emerges from the disk center. The broad H$\alpha$ and Ca II emission lines from the bipolar nebula display blue-shifted absorption originating from accretion disk-driven outflows with velocities of $\pm 500$ km/s.

Polarimetric observations demonstrated that all comets with significant values of circular polarization show predominantly left–handed circularly polarized light. We discuss the presence of homochiral organics in cometary materials as a source of the observed circular polarization. We have studied the effect of chirality on light–scattering properties of cometary dust considering particles that possess optical activity. Our investigations show that the cometary dust may include optically active materials which can be prebiological homochiral organics.

The projection factor used in the Baade-Wesselink method of determining the distance of Cepheids makes the link between stellar physics and the cosmological distance scale. A coherent picture of this physical quantity is now provided based on several approaches. We present the latest news on the expected projection factor for different kinds of pulsating stars in the Hertzsprung-Russell diagram.

Observations of the longitudinal component of the photospheric magnetic fields near sunspots imply that surges and Ellerman bombs occur at neutral points in the magnetic fields (i.e. |B|=0) in the chromosphere. The neutral points appear above satellite sunspots, which are defined as polarity reversals (in B||) near the edges of large-spot penumbrae. A series of magnetograph observations shows a point of satellite polarity vanishing during a period of almost continuous surge activity. The lost magnetic energy is comparable to the energy release evidenced by the surge.

Comparing with solar cycles 21-23, the level of solar activity in the current cycle is very low. So far, there have been only five SARs and 45 X class flares. The monthly smoothed total solar irradiance decreased sharply by 0.09% from the maximum of cycle 23 to the minima between cycles 23 and 24. In this contribution, we present new studies on SARs in Cycle 24. The SARs in the current cycle have relatively smaller flare index (Iflare) and composite vector field index (Icom) comparing with the SARs in cycles 22 and 23. There is a clearly linear relationship between Iflare and Icom. The emphasis of this contribution is put on the similarity and different behaviors of vector magnetic fields of the SARs in the current solar cycle and the previous ones. We try to get a satisfactory account for the general characteristics and relatively lower level of solar flare activity in Cycle 24.

Applying a criterion previously derived by the author, the stability of stellar systems with an isotropic distribution velocity distribution and Emden's polytropic density law is demonstrated for the exponent n = 3/2.

V889 Herculis is one of the brightest single early-G type stars, a young Sun, that is rotating fast enough (Prot =1.337 days) for mapping its surface by Doppler Imaging. The 10 FOCES spectra collected between 13-16 Aug 2006 at Calar Alto Observatory allowed us to reconstruct one single Doppler image for two mapping lines. The Fe i-6411 and Ca i-6439 maps, in a good agreement, revealed an asymmetric polar cap and several weaker features at lower latitudes. Applying the sheared-image method with our Doppler reconstruction we perform an investigation to detect surface differential rotation (DR). The resulting DR parameter, δΩ/Ω≈0.009 of solar type, is compared to previous studies which reported either much stronger shear or comparably weak DR, or just preferred rigid rotation. Theoretical aspects are also considered and discussed.