Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

We study the circumstellar environment of the carbon-rich star R Scl using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI instruments VINCI and MIDI. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Data are interpreted using a self-consistent dynamic model. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 $\mu$m. The model agrees well with the time-dependent flux data at 8.5 $\mu$m, whereas it is too faint at 11.3 and 12.5 $\mu$m. The VINCI visibilities are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data

Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910

As a result of the variability survey in Chi Persei and NGC6910, the number of Beta Cep stars that are members of these two open clusters is increased to twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational properties, in particular the frequency spectra, of Beta Cep stars in both clusters and explain the differences in terms of the global parameters of the clusters. We also indicate that the more complicated pattern of the variability among B type stars in Chi Persei is very likely caused by higher rotational velocities of stars in this cluster. We conclude that the sample of pulsating stars in the two open clusters constitutes a very good starting point for the ensemble asteroseismology of Beta Cep-type stars and maybe also for other B-type pulsators.Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife, Lanzarote, Feb 2010, submitte

Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars

CONTEXT: Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. AIMS: To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. METHODS: An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC884. Fifteen different instruments collected almost 77500 CCD images in 1286 hours. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 micromag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field.Comment: Accepted for publication in Astronomy and Astrophysics, 21 pages, 14 figures, 4 tables. The full appendix is available at http://www.ster.kuleuven.be/~sophies/Appendix.pdf (74 MB, 169 pages, 343 figures, 1 table

ENERGY DEPENDENCE OF THE POLARISATION IN ZERO-FIELD QUANTUM BEATS IN Hβ EMISSION

Des battements quantiques à champ nul ont été observés sur la raie Hβ (4 861 Å) de l'hydrogène formé par l'interaction faisceau-lame de protons de 110 à 250 kev. Nous en avons déduit les sections efficaces d'excitation relatives dans les sous-niveaux magnétiques des états 4s, 4p et 4d en fonction de l'énergie. La polarisation Il-I⊥ moyennée sur les oscillations est négative. A l'occasion de ces mesures, nous avons observé l'influence de l'environnement immédiat de la feuille de carbone. Cela s'est manifesté par une polarisation moyenne pouvant devenir positive et par des oscillations d'amplitude plus importante. Nous pouvons attribuer ce phénomène aux potentiels électriques dus aux courants circulant dans la feuille et son support. Nous avons mis en évidence l'importance de la nature de la surface du support et de la résistivité de la feuille.Zero-field quantum beats were observed in the Hβ emission (4 861 Å) from hydrogen atoms formed by beam-foil interaction of 110 to 250 kev proton beams. The energy dependence of the relative excitation cross sections for the magnetic substates of the 4s, 4p and 4d levels was derived. The polarisation Il-I ⊥ averaged over the oscillations is negative. Along with these measurements, we observed the influence of the immediate neighbourhood of the carbon foil. This was seen by increased oscillation amplitudes and an averaged polarisation which might become positive. We attribute this phenomenon to electrical potentials due to currents circulating in the foil and its holder. The importance of the surface nature of the holder and the resistivity of the foil was pointed out

The ongoing 2005 -- 2006 campaign on beta Cephei stars in NGC 6910 and chi Persei (NGC 884)

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