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We discuss two new slowly pulsating B stars for which it can be convincingly shown that they are multiperiodic variables. The group now contains ten confirmed and likely multiperiodic mid-B stars with periods between 0.7 and 4.4 days. The multiperiodicity and the length of the periods point to g-mode pulsations. The very high order of the modes implies that the modes are excited in the core or in the very outer layers of the star. A recently proposed mechanism, in which both the inner and outer layers intervene, is discussed.
The photometric experiment on board of Hipparcos has discovered, among other types of variables, a large amount of new Slowly Pulsating B Stars. We have selected the fourteen brightest stars of this sample, together with five previously known Slowly Pulsating B Stars, for long-term spectroscopic and photometric monitoring. The selected stars have spectral types ranging from B 2 up to B 9 and are thus nicely spread across the instability strip. We here present the results of a preliminary analysis of our data and point out that our sample is unique in the sense that it allows us to perform seismology of massive early-type stars.
We give a progress report on an observational program intended to determine detailed chemical abundances of β Cephei stars and constant stars with similar temperature and gravity. There is some evidence that non-variable stars have a lower metal content than variables, as the recently found pulsation mechanism would suggest.
The double cluster h and x Persei is one of the richest clusters containing early-B stars, and therefore is important for observational and theoretical studies on the fundamental parameters of massive stars. The colour-magnitude diagram of the double cluster shows an important scatter (see Figure 1). It has long been known that h and x Persei are extremely rich in Be stars (Slettebak 1968). Our previous contention (Waelkens et al. 1990) that the large-amplitude variable stars we discovered are also Be stars, could be confirmed for a few objects. Rotation velocities for stars in h and x Persei are usually high, which is not surprising in view of the large fraction of Be stars.
We present Infrared Space Observatory (ISO) spectra of 14 isolated Herbig Ae/Be stars. The IR spectra were combined with photometric data from the UV to the sub-mm region. We defined two key groups, based upon the spectral shape of the IR region. The results can be summarized as follows (see also Meeus et al. 2001): (1) the continuum of the IR to sub-mm region can be reconstructed by the sum of a power-law and a cool component, which can be represented by a black body. Possible locations for these components are an optically thick, geometrically thin disc (power-law component) and an optically thin flared region (black body); (2) remarkably, some sources lack the silicate bands; (3) PAH bands are present in at least 50% of our sample; with one exception, PAHs are not present in sources which only show a power-law continuum in the IR; (4) the dust in HAEBE stars shows strong evidence for coagulation; this dust processing is unrelated to any of the central star properties.
ISO has opened new infrared windows for spectroscopy, enabling detailed studies of the composition of the dust particles present in circumstellar disks. For oxygen-rich dust, and in particular for silicates, a forest of new features has been discovered, and comparison with laboratory data has enabled the identification of most of them. Of special relevance is the detection of crystalline silicates, which present themselves as a new diagnostic for studying the formation of comets and planetesimals in the disks surrounding young and, surprisingly, also evolved stars.
Low and intermediate mass stars leave the Asymptotic Giant Branch (AGB) when the mass in their H-rich envelope is less than about 0.01 M⊙, and the high mass loss drops several orders of magnitude. The central star rapidly evolves to the left part of the HR diagram along a track of constant luminosity (e.g. Schönberner 1983). In principle the evolution of the central star to higher Teff and the expansion and cooling of the AGB remnant are easy to calculate. In practice several complicating factors arise which make it much more difficult to predict the morphology and properties of post-AGB stars, such as binarity, post-AGB mass loss and aspherical AGB mass loss. Binarity of post-AGB stars affects the morphology of the circumstellar environment, and it affects evolutionary timescales and surface chemical abundances of the components in the system. This review discusses some properties of binary post-AGB stars.
We report on the first results from observations of 28 variable B stars obtained with the new Mercator telescope (La Palma). Besides confirming the pulsational nature of known and candidate β Cephei and slowly pulsating B stars, we also present new candidate ellipsoidal variables and spotted stars.
We review the current status of our long-term monitoring project on slowly pulsating B stars that we started in the course of 1996 and that was recently completed as far as the first part of our plan is concerned. In total, we have selected 17 southern and 8 northern stars. The idea is to fully exploit our current data in the near future and to select the most interesting targets for further very-long-term follow-up monitoring. A first conclusion is that half of the southern targets turn out to be spectroscopic binaries. Some of these have circular orbits and periods of the same order of magnitude as the intrinsic pulsation period(s) of the primary. The eccentric binaries have periods ranging from 12 to 460 d. For most stars the photometric behaviour is dominated by the same frequency as the intrinsic spectroscopic variability. Multiperiodicity in the expected frequency range is found for almost all stars. Two objects, however, turn out to have only one dominant pulsation mode.
Eta Orionis is an interesting multiple system of early type stars. It consists of a visual triple system (A, B, C), a spectroscopic triple system (Aabc) and an eclipsing binary Aab. A is a triple system consisting of a 7.989 day eclipsing binary Aab and a gravitationally bound third component Ac with orbital period of about 9.5 years. All components of the pair AB have spectral types in the range 09-B3 (C. Waelkens & P. Lampens 1988). The radial velocity curve is used to constrain the orbital elements and properties of the binary Aab.
The second component Ab is a B-type star that displays rapid absorption line profile variability. Regular patterns are easily visible in the data.
We report on the first results from observations of 31 variable A and F stars, obtained with the new Mercator telescope (La Palma). Besides confirming the γ Dor nature of known bonafide and candidate γ Dor stars, we also present new candidate γ Dor stars. In addition, we found a new short-period variable star.
We review observational evidence on the interaction between stellar pulsation and evolution. We discuss to what extent observations of pulsating stars with variable amplitudes and pulsation periods have implications on our understanding of stellar structure and evolution. The probable link between mass loss and pulsation in AGB stars and in hot luminous stars appears to be the strongest way in which pulsations affect evolution. We point out the possibility that forced oscillations in the components of binaries may have important consequences on evolution, that could offer an explanation for some classes of peculiar evolved objects.
Spectra obtained with the Infrared Space Observatory ISO of the dusty disks surrounding young stars display several characteristics which suggest the presence of large amounts of cometary material during these early stages. Of special interest are crystalline silicates and hydrated silicates, which surprisingly appear to be present already in cool material surrounding young objects. Another surprise is the occurrence of cometary features in the disks surrounding some binary post-AGB stars.
DV Cam is a triple system showing the diversity of the physics of photospheres of B-type stars at a common age: an SPB star in a wide orbit around a close binary consisting of an ultra-slowly rotating helium-weak star and a much faster rotating mid-B star.
For over a decade, the structure of the inner “hole” in the transition disk around TW Hydrae has been a subject of debate. To probe the innermost regions of the protoplanetary disk, observations at the highest possible spatial resolution are required. We present new interferometric data of TW Hya from near-infrared to millimeter wavelengths. We confront existing models of the disk structure with the complete data set and develop a new, detailed radiative-transfer model. This model is characterized by: 1) a spatial separation of the largest grains from the small disk grains; and 2) a smooth inner rim structure, rather than a sharp disk edge.
Galaxy clusters are large laboratories for magnetic plasma turbulence and therefore permit us to confront our theoretical concepts of magnetogenesis with detailed observations. Magnetic turbulence in clusters can be studied via the radio-synchrotron emission from the intra-cluster medium in the form of cluster radio relics and halos. The power spectrum of turbulent magnetic fields can be examined via Faraday rotation analysis of extended radio sources. In case of the Hydra A cool core, the observed magnetic spectrum can be understood in terms of a turbulence-mediated feedback loop between gas cooling and the jet activity of the central galaxy. Finally, methods to measure higher-order statistics of the magnetic field using Stokes-parameter correlations are discussed, which permit us to determine the power spectrum of the magnetic tension force. This fourth-order statistical quantity offers a way to discriminate between different magnetic turbulence scenarios and different field structures using radio polarimetric observations.
From recent high-accuracy transit timings measurements, we discard the 5 M⊕ planet recently proposed by Ribas et al. (2008). Thanks to a combined radial-velocity and transit timings overview we also define a mass/period domain in which a secondary planet may be found in the system. We also show that timings obtained until now, although not sufficient to remove degeneracies on mass and period, can still restrict the parameter space of the potential secondary planet.
The determination of stellar abundances is the most direct
observational test for astronuclear physics. The reliability of
such determinations depends on the quality of the observational
data, but also critically on a correct understanding of all
atmospheric processes as well as on fundamental atomic and
molecular data. We illustrate this different aspects with several
recent examples which served as a general introduction to the
"round table" on this subject.
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